ABSTRACT

AIM: The project aim is to briefly explain and summarise major COVID-19 findings onto one coherent document, so it would be more easily accessible for future medical students or researchers to use for analytical or research purposes. When researching about COVID-19, although there were plentiful resources going in depth into different subtopics like a potential prophylactic drug or a thorough explanation of a specific risk factor, it was difficult and time consuming to find one document that generally summarised all COVID-19 findings to a medical standard level. The project was split into 5 main objectives: explain research on the structure of COVID-19 and its origin with aid of self-created 3-D model of the virus, outline common symptoms of COVID-19 and briefly explain the reasoning behind these symptoms, list risk factors and explain why some of the major or less well known characteristics are risk factors for COVID-19, explain recent findings on prophylactic treatment with aid from a self drawn diagram highlighting targets for treatment in different stages of COVID-19 infection, and to explain and evaluate the efficacy of three major COVID-19 Vaccines : Astrazeneca, Pfizer and Moderna.

The wider purpose for this project is to allow medical students and researchers to access detailed research on the subject of COVID-19 including main topics such as symptoms, risk factors, treatment and structure from a single word document in a way that is useful, and most importantly, less time consuming. This would allow future research around COVID-19 to advance more rapidly.

INTRODUCTION- WHAT IS COVID-19?

COVID-19 is a type of coronavirus. Coronaviruses are a family of enveloped viruses with a single stranded RNA genome that causes respiratory and intestinal illnesses in humans and animals including the common cold, SARS (Severe Acute Respiratory Syndrome),and MERS (Middle East Respiratory Syndrome). They are categorised into four genera: alpha- CoV, beta CoV, gamma CoV, and delta CoV. Ranging from genome sizes of 26-32 kilobases in length, coronaviruses have the largest genomes for RNA viruses. Under an electron microscope, the viruses seem like they are surrounded by pointed structures, like a corona or crown, hence the name ‘coronavirus’. Both SARS and MERS are highly transmissible and pathogenic beta coronaviruses that emerged in humans at the beginning of the 21st century. SARS coronavirus was identified in February 2003 in Asia and was said to have first infected humans in Guangdong Province, Southern China in 2002. Thought to have originally been an animal virus, the coronavirus strain (usually found in small mammals) had seemingly mutated, enabling it to infect humans, causing the spread of the airborne virus that resulted in over 8000 cases and 750 deaths (1 in 10 of those infected died) in 2003. Its symptoms include fever, malaise, myalgia, headache, diarrhea, and rigors, with fever being the most commonly reported. The global outbreak was controlled in July 2003, through the implementation of appropriate infection control practices. MERS coronavirus was thought to be an ‘even deadlier’ viral respiratory disease. Killing 1 in 3 people infected, it was first reported in Saudi Arabia 2012. It has since spread to 27 countries causing 2494 laboratory cases of MERS-CoV infection and 858 MERS associated deaths. Those infected develop symptoms like fever, cough and shortness of breath.

In December 2019, bronchoalveolar samples were collected from a patient with pneumonia from an unknown origin in Wuhan, China. Using Illumina and nanopore sequencing, the whole genome sequence was obtained and analysed. Through virus isolation, they identified the cytopathic effects (structural changes in host cells) after 3 days; under a transmission electron microscope, they observed crown-like characteristics of particles, typical of coronaviruses. Alignment of the genome sequence indicated most resemblance (96% identity) with SARS like CoV strain – BatCoVRaTG13. Covid-19 was officially announced to be the name given to this disease on 11th February 2020. COVID-19 is highly infectious; it is now said that every infected person may infect three other people resulting in a significantly exponential rate of infection unlike that witnessed in the SARS epidemic. There have been 37,704,153 confirmed cases of COVID-19 affecting over 230 countries, with 1,079,029 confirmed deaths and numbers are still increasing as of 0ctober 2020 The most common symptoms of COVID-19 include fever, dry cough, fatigue, sore throat, loss of taste/smell, and sore throat (more information on symptoms is provided below). The virus is transmitted by aerosols (from coughs and sneezes, for example) and contact of contaminated objects. However, when compared to MERS and SARS, which were controlled after appropriate implementations such as travel restrictions, COVID-19 is evidently requiring more drastic measures and is clearly more infectious.

DISCUSSIONS:

Structure of COVID-19 virus

COVID-19 can be categorised into having three main viral activities; delivering genetic instructions in the form of RNA across membranes of uninfected human cells, allowing newly produced RNA to escape from an infected cell and to protect RNA during its journey to new uninfected cells. RNA of COVID-19 is tightly coiled, covered in a nucleocapsid protein and contained in an outer fatty, lipid membrane which contains viral proteins, spike, membrane and envelope. This membrane almost acts like a bag holding and protecting the RNA. It is essential to the virus that this membrane is sufficiently stable enough to protect the RNA from surroundings but simultaneously not be so stable that it causes the virus to lose its ability to break open inside the host cell and release RNA. The COVID-19 genome encodes for four structural proteins. (S) Spike protein, (M) Membrane protein, (E) Envelope proteins which make up the surrounding membrane, and (N) the Nucleocapsid protein which tightly packs around and protects the RNA.

To support improved understanding of the structure of the virus, an original 3-D model of a molecule of coronavirus was created (Figure 1)

The S (spike protein) is a trimeric glycoprotein (individual S proteins are arranged in groups of 3 on the outer membrane), giving the virus its distinctive crown-like structure, representative of coronaviruses. It is formed by an ectodomain, a single pass transmembrane anchor, and intracellular tail. The S protein contained in the lipid membrane allows the virus to bind to host cell surface receptors and hence mediates membrane fusion and virus entry into uninfected cells. The expression of spike glycoprotein can also mediate cell-to-cell fusion between adjacent infected and uninfected cells. [13] This formation of syncytia (single cell containing several nuclei) is said to be a strategy to allow direct spreading of the virus between cells, and in turn evades virus neutralising antibodies. S protein of COVID-19 shares around 76% amino acid identities with SARS CoV and it is reported both viruses use the same receptor for cell entry-human angiotensin- converting enzyme 2 -hACE2, (this receptor is also the same as the one found in RaTG13- a bat coronavirus closely related to SARS- CoV-2. ACE2 enzyme is significant in regulating fluid homeostasis in the body. Receptor recognition is an important indicator of viral ability and highlights a major target for vaccination and antiviral strategies. [17] The spike protein in SARS- CoV-2 contains a receptor binding domain which specifically identifies ACE2 as its receptor. In comparison to SARS CoV, the ACE2 binding ridge in SARS- CoV-2‘s receptor binding domain has a tighter, compact conformation. Additionally, the surface of ACE2 has two virus binding ‘hotspots’ because of various residue changes. These structural adaptations significantly increase ACE2’s binding affinity in SARS- CoV-2, when compared to that of SARS CoV explaining the significantly greater rate of infection of COVID-19 pandemic. [15]

The M (membrane protein) is the most abundant protein and defines the shape of the viral envelope. It is sometimes referred to as the central organizer of CoV assembly as it interacts with all other major coronaviral structural proteins: S, N, and E. The M protein adapts a region of the membrane for virus assembly, essentially sorting viral components so that they can be incorporated into virion (complete, infective form of a virus outside a host cell). Homotypic interactions between M interactions with (RNP) viral ribonucleoprotein and S glycoproteins at the budding site are the major driving forces for virion envelope formation. These interactions allow the forming of M-M networks that allow the exclusion of host membrane proteins from the viral envelope, ensuring protection of viruses’ genetic material as it travels between host cells. Furthermore, the binding of M protein to N protein stabilises the N protein- RNA complex hence promoting the completion of viral assembly. [14]

The E (Envelope protein) is a small hydrophobic essential membrane (ranging from 76 – 109 amino acids) that forms ion channels. The E protein has a short hydrophilic amino terminal domain, a long alpha helical hydrophobic transmembrane domain (TMD) and ends with a long carboxyl hydrophilic domain. It is concentrated at the site of intracellular trafficking namely the Endoplasmic Reticulum (ER) and Golgi Apparatus, where it participates in CoV assembly and budding. Although not understood in its entirety, its functions are thought to be associated with several aspects of the virus’ life cycle: assembly, budding, inflammasome activation, envelope formation and pathogenesis. It has an established role in virus assembly, inducing membrane curvature, promoting membrane rearrangement and scission.[16] During the cell’s replication cycle, the E protein is expressed inside the infected cell (although only a small fraction is incorporated into the virion envelope). The hydrophobic region of the TMD contains at least one predicted amphipathic (containing both hydrophobic and hydrophilic properties) alpha helix that reacts together to form an oligomer which in turn forms ion conductive pores in the membrane. The expression of both M and E protein itself is sufficient for virion assembly. [14]

The N (Nucleocapsid) protein is a significant antigen for SARS- CoV-2 and is highly immunogenic, being expressed abundantly during infection. It consists of two distinct RNA domains; N terminal domain (NTD) as well as C terminal domain (CTD) which are linked together by a structured linkage region. Due to these amino acids, N protein can bind with viral RNA genome and package it into a helical ribonucleocapsid (RNP) by having multiple copies of the N protein linking together to form a spiral that tightly coils and wraps around the RNA. In doing so, the molecule is more compact and is able to fit into the small virus particle and form a protein coat enveloping the RNA and protecting it from damage. Through studies, it was identified that the N Protein in SARS- CoV-2 has a high content of disordered region without bound nucleic acid, allowing it to promote short lived binding to various partners and maintain a correct conformation. Although SARS- CoV-2 N protein has a 90.52% sequence identity with SARS CoV N protein, the surface electrostatic potential characteristics between the two are clearly distinct. As it has been highlighted through various studies that the N proteins are vital for multiple significant functions during the virus’ life cycle, it is being heavily debated whether the viral nucleocapsid protein could be a potential antiviral target. [18]

Hemagglutinin esterase is a glycoprotein that viruses have to use as an invading mechanism. It serves 3 purposes; the first being to recognize the receptor on the cell surface, the second being to moderate the virus’ fusion with the cell membrane of the host cell, and to destroy the receptor on the host cell. When the virus meets a host cell, the hemagglutinin esterase recognises the host cell’s surface receptor and destroys the receptor. The destruction of this allows for the binding of the virus to the host cell as the receptor-binding determinants on the host cell’s surface (such as sialic acids) are destroyed. [13]

As mentioned, SARS- CoV-2 is spread via respiratory droplet or direct contact and hence enters the body through mucous membranes of the nose, mouth or eyes. This provides a direct pathway to the lungs and throat allowing infection of the respiratory tract. There are then two vital proteins for viral entry found in the body – ACE2 and TMPRSS2. The virus’ spike protein that contains receptor binding domain (S1 subunit), binds to ACE2 angiotensin converting enzyme 2, a receptor expressed on surface of various cells including airway epithelial cells, ciliated, type II pneumocytes and mucus producing goblet cells in the nose and cells in the cornea of the eye to enter the cell. [23] Once the virus binds to these receptors, cellular proteases like TMPRSS2 (which activate viral entry into cell) and lysosomal proteases cathepsins split the spike protein allowing another subunit- S2 to fuse with viral and lysosomal membranes. In doing so, the virus enters cells, the shell is broken, and viral RNA is injected enabling assembly of more copies of SARS- CoV-2, which are then released into the alveolus. The host cell is destroyed as a result and then these copies then go on to infect neighbouring cells. During infection, the virus may trigger the release of pro-inflammatory cytokines (e.g Interleukin-8, Interleukin-6 and TNFa) which results in tissue damage with significant vascular leakage. [25]

Symptoms

Symptoms of COVID-19 can range from asymptomatic to severe pneumonia. Most common symptoms include:

• Fever
• Continuous, dry cough

(For more than an hour/more than 3 episodes in 24 hours)

• Fatigue
• Sore throat
• Loss or change of taste/smell.
• High temperature (above 38°C)
• Difficulty in breathing [1]

On average, symptoms occur 5-6 days after infection. Other less common symptoms of infected patients include: nasal congestion, conjunctivitis, headaches, muscle/joint pain, skin rashes, nausea, diarrhea, chills and dizziness. Symptoms of serious COVID-19 include: shortness of breath, loss of appetite, confusion, persistent pain in chest, and a high temperature (above 38°C). According to the World Health Organization, around 80% of those infected recover without hospital treatment and of the 20% who do become seriously ill, 5% become critically ill and require intensive care. Complications leading to death include respiratory failure, acute respiratory distress syndrome, sepsis, septic shock, thromboembolism, and multi organ failure. In some rare cases, children develop a severe inflammatory syndrome a few weeks after infection.

Why do these symptoms occur? Scientists are still not completely sure about the answer to this( many of the listed symptoms are relatively common for viruses in general,but appear with greater intensity for COVID-19. Research is still ongoing, but the following reasons for common symptoms have been determined:

• Fever– Body is trying to fight off the infection and prevent ideal conditions for virus survival. Pyrogens, chemicals found in the bloodstream bind to receptors in the hypothalamus and in doing so increase body temperature. Interleukin 1 ( a pyrogen) is produced by macrophages when they encounter a pathogen. Therefore, fever being a symptom of COVID-19 is simply a signal that the body is attempting to fight off the infection. [29]
• Difficulty in breathing/cough- White blood cells release cytokines to encourage the immune system to kill SARS- CoV-2 infected cells. During this process, some healthy alveolar cells are also killed, debris and excess fluid is also left in the alveolus. This means there is a lack of oxygen entering the bloodstream due to fall in the number of Type 2 alveolar cells. More fluid also enters and accumulates in alveolus, limiting ability for lungs to take in oxygen therefore decreasing efficient gas exchange. Type II alveolar cells also secrete surfactant (a mixture that acts to decrease surface tension at air-liquid interface of alveoli). A fall in Type II cells and increase in the fluid (which dilutes surfactant) thereby triggers alveolar collapse and further causes difficulty in breathing. This in turn also causes ARDS- Acute Respiratory Distress Syndrome, a form of lung failure where difficulty in breathing where oxygen cannot enter the body and may require ventilator support. [30]
• Loss of sense of smell (also known as anosmia) – Research has confirmed COVID-19 does not directly affect olfactory sensory neurons, as ACE2 receptors are not identified here. Direct damage to these neurons would demonstrate a more complex range of symptoms such as parosmia or phantosmia. Due to infrequent cases of these and long lasting loss of smell not being reported, it indicates there is no permanent damage to olfactory sensory neurons caused by this coronavirus. It is rather thought the virus affects the functioning of supporting sustentacular cells, such as olfactory stem cells and microvilli cells, in olfactory epithelium. Thereby preventing function of sensory neurons as well as mucus layer meaning odours may not be able to reach olfactory neurons or transmit signals. [26] [27] [34] [35] [36]

Asymptomatic cases of COVID-19 are rare but not unheard of ; a large proportion of these cases found the patients to later progress into experiencing a moderate form of the disease. Most COVID-19 patients experience mild to moderate forms of the disease and usually recover. Around 80% of confirmed COVID-19 patients reportedly have had a ‘mild to moderate’, with 13.8% reporting ‘severe’ and 6.1% ‘critical’. With 37,704,153 confirmed cases and 1,079,029 COVID related deaths as of October 2020, the crude fatality ratio (proportion of deaths from a disease to the number of people diagnosed with disease) is 2.862%. [31]

Risk Factors

Certain risk factors cause some people to become more susceptible to severe forms of COVID-19. There are 2 categories of higher risk, namely ‘moderate risk’ and ‘high risk’.

Those who are at moderate risk and therefore are clinically vulnerable include those who:

• Are aged 70 years or older
• Have a lung condition that is not severe (e.g. asthma or emphysema)
• Have a heart disease
• Have diabetes
• Have chronic kidney disease/liver disease
• Have cancer
• Are pregnant
• Are obese ( BMI > 40)
• Have Type 2 diabetes
• Have Sickle cell disease
• Are taking medicine affecting immune system
• Have a condition that weakens the immune system

Those who are at higher risk and therefore are extremely vulnerable include those who:

• Have had an organ transplant
• Are having chemotherapy or are on antibody treatment for cancer
• Are having intense course of radiotherapy for lung cancer
• Are having targeted cancer treatments that can impact the immune system (e.g. PARP inhibitors)
• Have serious heart condition and are pregnant
• Are an adult with Down syndrome
• Are taking medicine or have a condition that makes them more susceptible to infections (e.g. immunosuppressant/ sickle cell disease)
• Have had a bone marrow or stem cell transplant in the past 6 months
• Have been diagnosed with severe lung condition (e.g. severe asthma, COPD)
• Are an adult undergoing dialysis or has severe long term kidney disease

[38]

AGE – Why is it a risk factor?

In the UK, being aged 70 or over puts an individual in the ‘moderate risk’ category and of these people, those who have underlying conditions such as hypertension, diabetes, or cardiovascular disease fall into the ‘higher risk’ category. Reported cases of COVID-19 of children under 19 seems to be relatively low and mild with only 2.4% of all total reported cases belonging to those under 19. From studies and research, it is evident mortality of COVID-19 increases with age. This may be due to the fact that the likelihood of having comorbidities (presence of additional conditions in co-occurrence with primary condition) and autoimmune disorders increases at an older age, causing older people having a higher risk of being extremely vulnerable to COVID-19.

Age is also a significant factor due to immunosenescence. As age increases, the thymus (the gland that makes white blood cells, or T lymphocytes) shrinks, leading to a decrease in T cell function. T cells are involved in directly killing infected host cells, activating other immune cells, producing cytokines, and regulating immune system response; therefore, a decrease in T cell function causes a weaker immune system (immunosenescence) and therefore a higher susceptibility to being infected. As age increases, macrophages (white blood cells that ingest antigens) also become slower, fewer white blood cells are capable of responding to new antigens, and the proportion of antibody production is relatively lower than those of a younger age. Collectively,this causes a slower, less responsive, and less effective recognition and defence to the foreign COVID-19 virus by the body. Furthermore, as more time is taken for the immune system to respond to the virus, the period of viral replication increases, allowing the virus to invade a higher proportion of host cells and cause a graver effect. [36] [37] [39]

OBESITY – Why is it a risk factor?

Having a BMI of between 30 and 40 or over makes someone clinically vulnerable to severe forms of COVID-19. The main reason behind obesity being a risk factor is an increase in likelihood of developing blood clots.

Adipose tissue in humans is described as oe of two kinds: brown (BAT) or white (WAT). Adipose tissue secretes bioactive substances;WAT helps to store energy in the form of fat and also provides insulation for organs while BAT burns energy and generates heat. In adults, the proportion of brown adipose tissue in the body corresponds with body mass index (BMI), highlighting the significance of BAT in the body’s metabolic processes. Obese individuals have a high amount of adipose tissue in their bodies, which is associated with the expression of proinflammatory factors like cytokines and chemokines. Due to this, adipose tissue tends to become inflamed in obese individuals, leading to macrophage infiltration, a significant contributor to inflammation which is also linked to endothelial dysfunction.

The endothelium is a permeable barrier lining the interior surface of blood and lymphatic vessels. It is crucial in regulating blood flow, immune function, vascular relaxation, and the exchange between bloodstream and vessels and therefore is essential for inflammatory process and wound healing. Endothelial dysfunction (prevalent in obese individuals) reduces the body’s ability to perform these roles (due to damaging of the blood vessels) and additionally results in vasodilation impairment as arteries lose the ability to effectively dilate. In turn, this restricts blood flow and increases likelihood of obese individuals to develop severe blood clots. Blood clots can easily lead to severe complications such as deep vein thrombosis and pulmonary embolism which are gravely life threatening. Many COVID-19 patients in ICU are also reportedly developing blood clots and a few having severe related complications,hence increasing the number COVID-19 related deaths. Recent data indicates risk of blood clots being significantly higher in COVID-19 patients than usually observed in ICU or hospital patients. Likelihood of experiencing blood clots due to COVID-19 is only further exacerbated by obesity. This makes obesity a major risk factor for developing serious forms and complications of COVID-19.

The reason COVID-19 causes blood clots is still unclear, although certain mechanisms are thought to be contributors to this complication. For example it is known that the COVID-19 virus uses ACE2 protein as a receptor to enter host cells. This protein is expressed by endothelial cells; as a result,it is suggested the virus could indirectly damage these cells and its surrounding tissue, causing a higher risk of blood clotting. Another line of reasoning is that the virus induces specific changes in immune system response including increased production of inflammatory molecules which could also increase risk of blood clots. [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

DIABETES- Why is it a risk factor?

There is not enough data yet to definitively state diabetes is a risk factor for COVID-19 ; however,it is well known that diabetes increases risk of developing infections in general. This is due to a few reasons; one being due to high blood sugar levels that can weaken a person’s immune system (damaging neutrophil function, which causes a negative impact on humoral immunity and the antioxidant system in addition to occasionally reduced T cell response). Another reason may be due to a gradually less efficient vascular system, meaning blood cannot deliver essential oxygen and other nutrients as efficiently to maintain healthy body tissues. Several studies in China illustrated a higher proportion of diabetic COVID-19 patients with prevalence of hypertension, cardiovascular disease, cerebrovascular disease compared to non-diabetic COVID-19 patients. Poor glycemic control associated with diabetes also impairs several aspects of a person’s immune system (both innate and adaptive). Analysis done in the UK indicated COVID-19 patients with poor glycemic control before hospital admission had a higher chance of developing severe complications and COVID-19 related death.

Type 1 diabetes, for example,is an autoimmune disease where in pancreatic beta cells that produce insulin are a target of immune attack. COVID-19’s ability to productively infect beta cells may lead to a slight impairment or worse, complete destruction, of beta cells. This would damage or destroy the body’s ability to produce insulin, further promoting deterioration of glycemic control in both pre-existing and newly diabetic patients. Furthermore, the body releases stored glucose into the blood order to provide energy required to fight off the infections. As diabetic patients already cannot produce enough (Type 1) or any (Type 2) insulin to maintain a healthy blood glucose concentration, this may result in even higher blood sugar level, potentially leading to diabetic ketoacidosis or hyperosmolar hyperglycemic state. Studies have supported this statement as there is a significantly high number of cases of diabetic COVID-19 patients developing diabetic ketoacidosis or hyperosmolar hyperglycemic state. Therefore, it is assumed having diabetes does not make a person more susceptible to getting infected by COVID-19;instead, it makes a person more likely to develop more serious complications or a graver form of the disease, if infected. [50] [51]

[52] [53] [54] [55] [56] [57] [58] [59]

Gender – Is it a risk factor?

Many studies claim being of male sex increases risk factor for severe COVID-19 outcomes. Various studies have supported the fact that SARS-CoV-2 expresses differences in the severity of COVID-19 outcomes between the two sexes. Many countries have stated a higher number of COVID-19 related deaths in men than in women; Dr. Deborah Birx (coronavirus response coordinator for the White House) stated COVID-19 related deaths in Italy were“twice as high in men at all ages”, Chinese Center for Disease Control and Prevention stated death rate for males in China was 2.8% compared to 1.7% in women, and the United States (June 2020) saw 57% of COVID-19 related deaths in males,just to name a few cases of the gender gap in COVID-19 infection.Similar patterns were observed in the previous coronavirus outbreaks of SARS and MERS. Reasons could be behavioral or biological. One reason is due to variance in genes; women have XX chromosomes, whereas men have XY. The X chromosome had been identified to contain a high density of immune related genes and regulatory elements (such as transcription regulators or activated cytokine receptors) greatly involved in adaptive and innate immune response. Therefore, women generally produce stronger innate and adaptive immune responses than men. X Chromosome Inactivation (XCI) is the transcriptional silencing of one X chromosome in female mammalian cells to balance dosage of gene expression from X chromosome between XX in females and XY in males, initiated in early embryonic development. As males carry one X chromosome per cell and therefore are haploid, any unfavorable genetic variants on the X chromosome will lead to a more significant immunological impact in males compared to females. [60] [63] [65]

The female sex hormone estrogen also plays a significant role in immunity. Estrogen has an immune enhancing effect through multiple forms: upregulating pro inflammatory cytokines, modulating B cell function/differentiation, regulating neutrophil numbers, and enhancing expression of CCR5, which induces T cell homing. [61] Testosterone does the opposite, acting as an immune suppressor inhibiting effect of immune system through upregulating anti-inflammatory cytokines and elevating Th1 response and activation of CD8 cells whilst down regulating natural killer cells response and Tumor necrosis factor alpha (TNFa). Estrogen has also been found to enhance macrophage function through regulation of phagocytic activity and cytokine induction as well as promote the expansion of regulatory T cells (Treg cells) which are vital in controlling immune response. Basal levels of immunoglobulin, antibody response, and cytotoxic T cell activity is also higher in females. These differences in sex genes and hormones explain the differential immune response between men and women. [62]

Some studies have also claimed there are higher circulating ACE2 levels in men than in women. ACE2 (angiotensin converting enzyme 2) has receptors to which SARS-CoV 2’s-spike protein attaches and binds to enter the target host cell (as explained in the ‘Structure of COVID-19” section). ACE2 is expressed across multiple tissues in the body, with highest levels reportedly in the small intestine, testis, kidney, heart, thyroid and adipose tissue. Higher ACE2 expression would mean more entry points for SARS-Cov-2 into host cells and thereby a faster replication of the virus. Higher levels of ACE2 would also explain higher susceptibility of severe COVID-19 in men than in women;however, there is not enough evidence to confirm higher ACE2 plasma concentration in men and therefore make this correlation. [66]

Ethnicity – Why is it a risk factor?

Studies have shown that people from ethnic minorities are more likely to have worse clinical outcomes from COVID-19. Studies involving 18 million people have suggested that people from Black and Asian minorities were at a higher risk of SARS-COV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) with one study suggesting Black and Asian people had twice the risk of dying compared with White people.[67] Although there is not yet a definitive answer as to why this is the case, there are several plausible reasons that may have contributed to the results found by the studies.  [68]

1. Underlying health conditions – Regardless of your ethnicity, people with underlying health conditions (for instance, heart disease) are more likely to suffer from severe COVID-19 symptoms due to compromised immune systems. However, it is also clear that a greater proportion of people in ethnic minorities are affected with underlying health issues in comparison with the proportion of people who identify as white with underlying health issues.
2. Higher exposure – It is clear from studies that people from ethnic minorities are more likely to work in areas of high exposure to Covid-19.

From this graph (Figure 2) it is clear that a larger proportion of people from ethnic minorities work in the public sector where rates of infection are higher. For instance, the transportation industry is dominated by people of ethnic minorities. Over 20% of the NHS workforce is made up of people from ethnic minorities. These job sectors are of higher exposure to the virus as they meet more people on a daily basis regardless of lockdowns or regional restrictions. [69]

Figure 2 shows the ethnic composition in each job sector and also shows the number of deaths of workers occuring in each sector per 100,000 workers

Research on different types of treatment

CATHEPSIN L INHIBITORS

The Spike protein on SARS-CoV-2 can also be activated by pH dependent endosomal cysteine protease cathepsin L (CatL), which mediates cell to cell fusion. Cathepsin L is a lysosomal endopeptidase capable of degrading many proteins including enzymes, transcription factors, and receptors by cleaving peptide bonds with aromatic and hydrophobic residues. This protease is activated by removing their predomain and prodomain during translocation from the Endoplasmic reticulum to Golgi apparatus and its gene is activated by numerous growth factors. CatL cleaves and degrades viral S1 subunit in SARS-CoV-2 and is the main protease that does so due to its acidic pH optimum. Numerous experiments have indicated the role of CatL in activation of viral entry of SARS-CoV-2 via endosome, with a significant decrease in viral entry after its inhibition. [76]

SARS-CoV-2 is thought to stimulate overexpression of Cathepsin L thereby enhancing viral entry and infection. The human body contains natural inhibitors that regulate the expression of Cathepsin L, including papain type cysteine proteases and cystatins which bind at acidic pH to CatL, dissociating it at a neutral pH. Various synthetic CatL inhibitors are also being designed and considered; this includes the likes of antimicrobial, antimalarial and immunomodulatory drugs (some of the top drug candidates are explained below). By inhibiting CatL, two major “blocks” for viral infection-host cell surface (viral entry) and inside host cell endosomes (viral release and replication)- can be blocked. [77]

Figure 3 illustrates how SARS CoV2 enters and replicates within the host cell and the main targets for prophylactic treatment of COVID-19.

Studies identified the similarity between SARS-CoV and SARS-CoV-2 (76% identical spike protein) through various analyses, including specific amino acid sequences and effects of overexpression of ACE2 cells. It was further identified that both coronaviruses utilise the same cellular proteases for viral entry- TMPRSS2 and CatB/L (a lysosomal protease). Then, the proposition arose of using serine protease inhibitor camostat mesilate as an antiviral drug to inhibit TMPRSS2 cell mediated entry of COVID-19 and hence viral replication, until appropriate vaccine/treatment was globally available. Camostat mesilate is already clinically proven and approved in Japan for human use (for pancreatic inflammation and reflux oesophagitis), so it was suggested it could be used safely as a treatment for some extent of prevention against COVID-19. It is now being clinically trialed in several countries including Denmark, Israel,and USA. The oral drug has been used for over 2 decades in Japan; it is relatively expensive but inhibits TMPRSS2 at lower concentrations and has only a few mild side effects.

Several studies found when mice were given camostat mesilate (at concentrated doses that are also clinically achievable for humans) during SARS-CoV-2 infection,mortality rates were reduced. By inhibiting proteases like TMPRSS2, camostat mesilate reduces the rate of cellular infection by SARS-CoV-2 as cell mediated viral entry by TMPRSS2 can no longer be activated. This would reduce severe prognosis of COVID-19 and consequently,morbidity and mortality rates. However, many aspects must be considered before confirmation of camostat mesilate as a preventive drug against SARS-CoV-2, one being that camostat mesilate has a short plasma half-life within the body (less than a minute). This is due to its rapid hydrolysis into GBPA 4-(4-guanidinobenzoyl-oxyphenylacetic acid), which then further hydrolyses into GBA (4-guanidinobenzoic acid), which is not a TMPRSS2 inhibitor. A limitation to consider is that the use of an oral drug to inhibit cell entry requires the need for early treatment initiation before the first stage of SARS-CoV2-infection in order to minimize cell damage and excessive cytokine production.[70] [71] [72] [73] [74] [83]

Similar to camostat mesilate, bromhexine hydrochloride was also found to have an inhibitory effect on TMPRSS2. Its combined use with chloroquine (which disrupts viral lysosomal pathway) was explored as a prophylactic treatment for COVID-19. Bromhexine hydrochloride is more widely known in Europe (compared to camostat mesilate) and is commonly used as mucolytic treatment of respiratory diseases. Studies found TMPRSS, when compared to other proteins, required lower concentration to effectively inhibit cell culture, making TMPRSS inhibition a major research possibility for COVID-19 treatment strategies. [99] [100] [101] [102] [103]

Amantadine is amongst the agents being considered as a therapeutic target for COVID-19 treatment. As a prophylactic agent for influenza A and Parkinson’s disease, it was illustrated that Amantadine could downregulate CatL mRNA expression. Amantadine hydrochloride also acts as a lysosomotropic agent, disrupting lysosomal pathways and therefore restricting viral capacity for replication. It was first approved by the US Food and Drug Administration (FDA) for prevention of influenza A virus in 1976. When influenza enters a cell, an endosome and then a proton channel is formed, carrying protons to the inside of the viral particle. Amantadine is able to cross the endosome membrane and interrupt release of virion (complete, infective form of a virus outside the host cell with a core of RNA and a capsid) into the cell. It can also enter the E channel of SARS-CoV-2 through interaction with amino acids ALA22 and PHE26, further preventing viral release of nucleus into the cell. The clinical experience of amantadine utilisation for several decades makes it a strong candidate to consider for COVID-19 prevention. Its manufacture is relatively cost effective and with a distribution system already available,the drug is readily and globally available. Current use of amantadine for other medical uses as mentioned are generally well tolerated by the digestive system and indicate minute side effect profile. [80] [81] [82] [83] [84]

Teicoplanin (CatL inhibitor)

Teicoplanin is another CatL activity inhibitor, the glycopeptide, antibacterial drug has previously been used for influenza virus, HIV, Ebola, MERS, and SARS coronaviruses and is officially approved by FDA. Recent studies have indicated that Teicoplanin inhibits SARS-CoV-2 spike pseudoviruses into the cytoplasm in a dose dependent manner. It does so through proteolysis of various polyproteins with replicase and transcription domains. The similarity in amino acid sequence identity and usage of the same receptor for cell entry (ACE2) in both SARS-CoV-2 and COVID-19 makes a stronger argument for Teicoplanin and other glycopeptides for COVID-19 prevention, as teicoplanin has been utilised for prior coronaviruses and other viruses (as mentioned above). [78] [79]

CHLOROQUINE AND HYDROXYCHLOROQUINE (CatL inhibitor)

Chloroquine and hydroxychloroquine have been used for over half a century for the treatment of malaria and rheumatological conditions. Hydroxychloroquine has been used predominantly as therapy for autoimmune diseases. Several researchers have suggested the use of these as treatments and prevention against SARS-CoV-2 . Chloroquine is known to increase endosomal pH required for virus fusion and viral release of RNA or DNA into the cytosol as well as causing inhibition of activation of Cathepsin L, consequently hindering activation of spike protein on SARS-CoV-2 that mediates cell-to-cell fusion. Hydroxychloroquine was found to promote in vitro inhibition ofSARS-CoV-2 . Numerous studies were conducted on potential drawbacks of these two drugs such as associated cardiovascular toxicity or acute poisoning in overdoses. The efficacy of hydroxychloroquine and chloroquine as treatment or prevention against COVID-19 remains unclear due to overall mixed results. On the 17th of June 2020, WHO officially announced they would be stopping their hydroxychloroquine arm (to find an effective treatment against COVID19). This decision was made utilising evidence from the Solidarity trial (an international clinical trial to find prophylactic treatment forSARS-CoV-2 ), UK’s recovery trial, and Cochrane review. Data from studies demonstrated hydroxychloroquine does not reduce mortality rate of hospitalised COVID-19 patients. [85] [86] [87] [88] [89] [90]

OTHER CATHEPSIN L INHIBITORS

Other than the inhibitors mentioned above, there are numerous other inhibitors of Cathepsin L being tested for effectiveness of prevention against COVID19. Some of these include oxocarbazate, K11777, dexamethasone (which inhibits CatL in muscle cells), and astaxantin. [91] [92]

HEPARIN

The anti-inflammatory drug Heparin is most known as an anticoagulant used in prevention of blood clot formations. Some retrospective studies highlighted the association of systemic anticoagulation with reduced mortality rates of hospitalised patients, but this was mainly seen in intubated patients. Pro-inflammatory mediators can promote inflammatory action and coagulation, which induces expression of leukocyte adhesion molecules on endothelial cells and suppresses response of fibrinolytic pathways. By reducing the activity of these mediators, uncontrolled hyper inflammation can be prevented consequently, an improvement in patient conditions could be observed. Heparin can bind to protein sites including cytokines, antithrombin III, growth factors, and more, thus reducing pro-inflammatory activity. It also can alter the protein structure of cytokines so that it no longer is able to bind to its specific receptors and trigger inflammatory activity. Hyaline membrane formation has also been observed in several severe hospitalised COVID-19 patients. This membrane is formed due to alveolar entry of inflammatory exudate. The formation of hyaline membranes can worsen lung conditions in severe COVID-19 patients as it forms a physical barrier over alveoli, increasing diffusion pathway and thereby reducing diffusion of gases across alveolar epithelium. Without inflammatory regulation due to pro-inflammatory conditions,removal of this membrane can be prevented by the body through expression of plasminogen activator inhibitor 1. [93] [94] [95] [96]

Studies have also indicated SARS-CoV-2 spike glycoprotein utilises its interactions with glycosaminoglycans for target cell/ACE2 attachment (mainly heparin sulfate, which is found on the human cell surface). Research is ongoing on whether addition of heparin can act as a ‘competitor’ against heparin sulfate, competing for viral adhesion and thereby reducing SARS-CoV-2 entry. Overall, the safety, drug dosage, and efficacy of heparin have still yet to be clearly identified through clinical trials. [97] [98]

REMDESIVIR (Viral RNA Polymerase inhibitor)

Remdesivir is an antiviral drug that is intracellularly metabolized to form substrate remdesivir triphosphate, which has an inhibitory effect on viral RNA polymerase thereby preventing viral replication of COVID-19. The drug was developed more than a decade ago for treatment of hepatitis C virus and respiratory syncytial virus, although it was proven to not be sufficiently effective. In October 2020, the US FDA authorised the use of Remdesivir in adults and adolescent patients (12 and over) with COVID-19,starting intravenous use of the drug over 5 days to patients who require invasive mechanical ventilation or oxygenation and over 10 days for those patients who required this support. Treatment with Remdesivir is associated with shorter subsequent oxygen use and mechanical ventilation for patients, therefore reducing hospital stay duration of many. It was therefore suggested that Remdesivir could prevent patients from progressing into a severe form of COVID-19 and reduce mortality rates. [104] [105] [106]

It is important to note that supplies of remdesivir are varied across the world; availability over a short time and high costs also become an issue when considering effectiveness of remdesivir as a treatment strategy forSARS-CoV-2. Furthermore, some wider studies indicated insignificant effect on remdesivir; WHO’s Solidarity – an open label study that does not include a control group receiving placebo – evaluated the effectiveness of the drug on a significantly greater, more diverse sample size than other relevant studies on remdesivir. It was found a great number of patients in the remdesivir arm of the Solidarity trial still demonstrated no benefit from the drug. This questioned the effectiveness of the drug in moderately ill patients, but studies are still ongoing on the extent of its effect on improving COVID-19 patient conditions and progression. As of November 2020, WHO announced a “conditional recommendation” against the use of remdesivir for treatment of COVID-19, meaning the evidence of risks or benefits are too uncertain to support its use. [107] [108]

LOPINAVIR-RITONAVIR

Antiviral drug combination of Lopinavir and ritonavir has been suggested for COVID-19 treatment. Lopinavir is known as a human immunodeficiency virus (HIV) type 1 aspartate protease inhibitor found to also have an inhibitory effect on the main protease ofSARS-CoV-2 , which is required for replication. Ritonavir is combined with Lopinavir to inhibit cytochrome P450,in turn increasing the drug’s plasma half-life. Lopinavir-ritonavir has also demonstrated inhibitory activity against MERS in both in vitro and animal models. However, although some studies have found Lopinavir-ritonavir to reduce risk of severe clinical outcomes, many studies have disproved this association. WHO’s Solidarity trial and Recovery trial announced after conducting studies that Lopinavir-ritonavir does not reduce severe COVID-19 outcomes in hospitalised patients as there is no solid evidence to prove so. It was also brought to discussion that the drug may be difficult to administer to patients on ventilators; the effects of giving Lopinavir-ritonavir to people on other medications also remains unclear and is another area of concern. [109] [110] [111]

FURIN INHIBITORS (DIMINAZENE AND IMATINIB)

Spike proteins on SARS-CoV-2 can be cleaved at multiple stages, one of which is a furin cleavage site- RRAR (which was unidentified in other lineage beta coronaviruses). This increases efficiency of membrane fusion activity and could explain SARS-CoV-2 ’s greater infectious capacity. Furin is a type of proprotein convertase located in the trans-Golgi network. Activated by an acidic pH, furin can cleave the S glycoprotein on SARS-CoV-2 and due to the abundance of furin proteases in respiratory tract, this cleavage could take place immediately after exit from epithelial cells, enabling rapid infection and entry to other neighbouring host cells. [115] [116] [117]

Diminazene aceturate is an antiparasitic agent and a trypanocidal drug identified to have the strongest inhibitory effect of furin amongst other competitive inhibitors of furin, mimicking at least two arginines (a natural amino acid found in the body). Additionally, diminazene also upregulates ACE2 expression,which can reduce the damaging effect of inflammatory responses, namely cytokine storms (frequently associated with hospitalised COVID-19 patients). Other potential furin inhibitor effects or molecules with high furin binding affinities are being explored (e.g., Dec-RVKR-CMK, a1-PDX, theaflavin, or epigallocatechin gallate). However, effects on proprotein-convertases like furin are generally non-specific and irreversible. Research is ongoing to explore the prospects for this drug and possibly to identify a highly specific furin inhibitor with significant beneficial effects. [118] [119] [120] [121] [122] [123]

VACCINE AND TREATMENT- DISCUSSION

AstraZeneca Vaccine: (Images from [127])

The SARS-CoV-2 virus structure has protein spikes on the surface of the protein as shown in Figure 4. To create these spikes, the virus uses genetic instructions in the form of RNA. The AstraZeneca vaccine works by taking the genetic instructions and storing it in double-stranded DNA. The DNA is then added to another virus structure called adenovirus. This can be seen in figure 5.

The vaccine is injected into the arm of the patient, where the adenovirus collides into a body cell and latches onto the proteins on the surface of the body cell. The body cell then engulfs the adenovirus, where the adenovirus travels to the body cell’s nucleus and has its DNA pushed into the nucleus – this can be seen in figure 6. The DNA contains the genetic instructions for the building of the Coronavirus protein spikes, and this is ‘read’ by the nucleus and is copied into the cell’s mRNA then released from the nucleus. The cell then translates the mRNA and starts building the coronavirus protein spikes. The spikes then migrate to the surface of the cell and protrude out. This process is illustrated by Figure 7. By then, the vaccinated cell will collide with the B-Lymphocyte cells. The B-Lymphocyte cells then produce complementary antibodies that will attack the spike proteins from the vaccinated cell as shown in Figure 8. These antibodies then proliferate and attack all the spike proteins on the vaccinated cell.

The dead virus cell debris can then be engulfed by macrophages. The debris can be spike proteins and protein fragments. These parts are then presented to T-lymphocytes. The T-Lymphocyte cells attack infected body cells with the same protein spike fragment as the protein spike fragment presented to the T-Lymphocyte by the macrophages. The T-lymphocyte cell has its protein spike receptor latch onto the infected cell’s spike protein and releases chemicals like perforin and cytotoxins as displayed in figure 9. These chemicals then destroy the infected cell.

The adenovirus structure is used as it enters cells, but the virus will not replicate inside the cell as the viral gene required to do so is removed. This means that it cannot cause COVID-19 as it does not carry sufficient genetic material to assemble the complete SARS-CoV-2 virus and as a result, it provides a safe way of creating antibodies against the virus, typically without serious side effects. As a result, if the person were to ever get the SARS-CoV-2 virus, the body’s immune system will instantly recognise the foreign cells and immediately produce the antibodies required to destroy all virus cells as shown in figure 10. The T-Lymphocytes would also have the protein fragments and will also destroy all infected cells.

The vaccine has been tested in clinical trials of over 23,000 people in the UK, Brazil and South Africa. Generally, large trials have indicated the Oxford AstraZeneca vaccine is 62% effective with more recent trials of the vaccine showing its effectiveness in preventing 73% of cases in individuals with at least one underlying health issue. Most common side effects of the Oxford AstraZeneca vaccine are tenderness, warmth, pain, redness, itching, swelling or bruises at the site of injection, fatigue, fever-like symptoms, headache nausea and muscle ache. However, these side effects occur in around 1 out of 10 people, and are generally classified as mild to moderate, usually resolving within a few days after vaccination. Furthermore, these side effects are observed commonly with numerous other vaccines and therefore are outweighed significantly by proven beneficial effects. [124] [125] [126] [128] [131] [139]

Moderna Vaccine: (Images from [149])

The Moderna vaccine works by taking the mRNA genetic coding for the spike proteins of the virus and injecting it into the body. However, the mRNA of the virus’ spike proteins is fragile and would be destroyed if it were to be directly injected into the body. As a result, the Moderna team has designed the vaccine to have lipid nanoparticles form an oil bubble as shown in Figure 11. These bubbles will house the mRNA for the spike protein.

The vaccine containing the mRNA oil bubbles is then injected into the body. The vaccine particles collide and fuse with the body cells, where the mRNA is passed through to the body cell and read by the body cell after which spike proteins are built. These spike proteins migrate to the vaccinated cell’s surface. At this point, the mRNA would have been destroyed by the vaccinated body cell. The vaccinated cell would also have some of the protein spikes broken into fragments. These fragments of protein would also rise to the surface. This whole process is illustrated in Figure 12. At this point, the body’s immune system will recognise the vaccinated cell as an intruder cell and will begin an immune response. B-Lymphocyte cells may collide with the vaccinated cell and latch onto the protein spikes. It will then produce the complementary antibodies that will attack the spike proteins from the vaccinated cell. Once the production of the antibody is complete, the B-lymphocyte cells will proliferate and release antibodies to attack all the spike proteins. These antibodies latch onto the spike proteins, making the vaccinated cell unable to infect other cells. This is because by blocking the spike proteins of the vaccinated cell, the spikes are no longer able to latch onto other cells and infect the healthy body cells. The now inactive vaccinated cell will be marked as a cell for destruction and goes under phagocytosis, where the inactive vaccinated cell is engulfed.

Another way the immune system may react is by having macrophages (antigen-presenting cells) collect the protein spikes and fragments as shown in figure 13. These can then be presented to T-Lymphocytes, where the presented T-Lymphocytes attack any virus-infected cell with the same protein fragment/spike on their surface. The way this works is by having the T-Lymphocytes’ protein spike receptor latch onto the infected cell’s spike protein and release chemicals like perforin and cytotoxins. These chemicals destroy the infected cell, as illustrated in figure 14. With these two methods combined (Antibodies and T-Lymphocytes), if the person were to become infected with the SARS-CoV-2 virus, the body’s immune system would have the antibodies and T-Lymphocytes ready to destroy infected cells and provide immunity from the virus.

[149] [128] [131] [132] [136] [137]

Pfizer Vaccine: (Image from [148])

The Pfizer vaccine works in a similar way to the Moderna vaccine. It also uses the mRNA genetic coding for the building of spike proteins and uses lipid nanoparticles to form an oil bubble in which the mRNA coding is stored as shown in figure 15. It is injected into the body where the vaccine particles collide and fuse with the body cells. The mRNA will be read by the vaccinated cell and spike proteins will be built by the vaccinated cell. These spikes will migrate to the surface, at which point the body will detect the vaccinated cell as a foreign cell. The B-lymphocytes, macrophages, and T-Lymphocytes will all respond in the same way as they would in the Moderna and AstraZeneca vaccine and immunity would be reached.  [129] [132] [128] [131] [148]

CONCLUSION (VACCINE COMPARISON)

1. Durability:

The added durability of the DNA and the protein coat of the adenovirus in the AstraZeneca vaccine means that it can be stored within a regular refrigerator, increasing ease and efficiency of storage and transport. By contrast, the Pfizer vaccine requires a special freezer that can achieve the -70°C needed for the vaccine to be preserved in storage. The Moderna vaccine would also require a freezer, but would only require conditions of -20°C – conditions, which are easier to create for the vaccine.

1. Reliability of Vaccine (Pfizer vs Moderna):

The added content of mRNA in the Moderna vaccine in comparison to the Pfizer vaccine means that the likelihood of the nanoparticles containing the mRNA staying intact is greater in the Moderna Vaccine in comparison to the Pfizer vaccine. The lower amount of mRNA in the Pfizer vaccine makes it even more critical that the nanoparticles remain intact. Damaged nanoparticles could reduce the overall effectiveness of the vaccine.

1. Cost:

The cost of the AstraZeneca vaccine is $4 per dose,$15 dollars cheaper than the Pfizer vaccine and around \$30 dollars cheaper than the Moderna vaccine.

1. Effectiveness:

The effectiveness of the Moderna vaccine is 94%, 4% greater than the effectiveness of the Pfizer vaccine and 20% more effective than the AstraZeneca vaccine.

[128] [131] [140] [148] [149]

CONCLUSION- TREATMENTS

Particularly as influenza viruses also utilise TMPRSS2 protease, studies are ongoing regarding the effectiveness of camostat mesilate in inhibiting viral entry into human epithelial cell cultures (indicating if it is completely safe and effective for prevention of COVID-19 in humans). The efficacy of bromhexine hydrochloride for treatment against COVID-19 lung dysfunction in particular is thought to only be slight; however, researchers continue to study whether it can be an effective method of prevention. The main research and considerations surrounding amantadine involve identifying a balance between a “normal” and a “high” dose risk, possible need for higher doses for resistant patients, and mitigating potential side effects- drowsiness, dizziness, orthostatic hypotension, dry mouth, constipation and peripheral edema. Teicoplanin has low toxicity, few side effects, long half-life in the blood plasma, and appropriate administration. The safety, drug dosage, and efficacy of heparin has still yet to be clearly identified through clinical trials. Trials and studies on lopinavir-ritonavir are still ongoing; for example, on whether increasing concentration of dosages could be more effective, Data on the optimum concentration of this drug is limited and varied. Whether the drug could demonstrate positive effects if administered early (first stages of COVID-19 symptoms), before the requirement for hospitalization, is also an area of ongoing research.[83] [84] [103][112] [113] [114]

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60) Panning, B. X-chromosome inactivation: the molecular basis of silencing. J Biol 7, 30. October 27, 2008. X-chromosome inactivation: the molecular basis of silencing | Journal of Biology | Full Text (biomedcentral.com)

61) Khan D, Ansar Ahmed S. The Immune System Is a Natural Target for Estrogen Action: Opposing Effects of Estrogen in Two Prototypical Autoimmune Diseases. Front Immunol. January 6, 2016. The Immune System Is a Natural Target for Estrogen Action: Opposing Effects of Estrogen in Two Prototypical Autoimmune Diseases (nih.gov)

62) Taneja V. Sex Hormones Determine Immune Response. Front Immunol August 27, 2018. Sex Hormones Determine Immune Response (nih.gov)

63) Schurz, H., Salie, M., Tromp, G. et al. The X chromosome and sex-specific effects in infectious disease susceptibility. Hum Genomics. January 8, 2019. The X chromosome and sex-specific effects in infectious disease susceptibility | Human Genomics | Full Text (biomedcentral.com)

64) Jin J-M, Bai P, He W, Wu F, Liu X-F, Han D-M, Liu S and Yang J-K . “Gender Differences in Patients With COVID-19: Focus on Severity and Mortality”. Front. Public Health April 29, 2020. Frontiers | Gender Differences in Patients With COVID-19: Focus on Severity and Mortality | Public Health (frontiersin.org)

65) Catherine Gebhard, Vera Regitz-Zagrosek,Hannelore K. Neuhauser,Rosemary Morgan et al. “Impact of sexand gender on COVID- 19 outcomes in Europe.” May 25,2020.Impact of sex and gender on COVID-19 outcomes in Europe (nih.gov)

66) Griffith DM, Sharma G, Holliday CS, Enyia OK, Valliere M, Semlow AR, et al. “Men and COVID-19: A Biopsychosocial Approach to Understanding Sex Differences in Mortality and Recommendations for Practice and Policy Interventions.” Prev Chronic Dis. July 16,2020. Men and COVID-19: A Biopsychosocial Approach to Understanding Sex Differences in Mortality and Recommendations for Practice and Policy Interventions (cdc.gov)

Ethnicity:

67) “Why have Black and South Asian people been hit hardest by COVID-19?”. Office for National Statistics. December 14 ,2020. https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/articles/whyhaveblackandsouthasianpeoplebeenhithardestbycovid19/2020-12-14

68) “Why do people from ethnic minorities suffer more from COVID-19?”. UK Research and Innovation. May 27,2020. https://coronavirusexplained.ukri.org/en/article/cad0011/

69) Clareece R. Nevill, Laura J. Gray, Christopher A. Martin, Joshua Nazareth et al.Ethnicity and clinical outcomes in COVID-19: A systematic review and meta-analysis. EClinical Medicine. December 1, 2020. https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(20)30374-6/fulltext#seccesectitle001207

Camostat Mesilate/Protease inhibitors:

70) Gunst JD, Staerke NB, Pahus MH, Kristensen LH et al. “The Impact of Camostat Mesilate on COVID-19 Infection (CamoCO-19)“. Clinical trials gov. April 30, 2021. The Impact of Camostat Mesilate on COVID-19 Infection – Full Text View – ClinicalTrials.gov

71) “The Impact of Camostat Mesilate on COVID-19 Infection”. Centerwatch. May 6, 2021.The Impact of Camostat Mesilate on COVID-19 Infection | Clinical Research Trial Listing ( Coronavirus | Coronavirus Infection | Corona Virus In fection | Covid-19 ) ( NCT04321096 ) (centerwatch.com)

/72) Hoffmann M, Hofmann-Winkler H, Smith JC, et al. Camostat mesylate inhibits SARS-CoV-2 activation by TMPRSS2-related proteases and its metabolite GBPA exerts antiviral activity. Preprint. bioRxiv. August 5, 2020. Camostat mesylate inhibits SARS-CoV-2 activation by TMPRSS2-related proteases and its metabolite GBPA exerts antiviral activity (nih.gov)

73) Peter Breining, Anne Lier Frølund, Jesper Falkesgaard Højen, Jesper Damsgaard Gunst et al. “Camostat mesylate against SARS-CoV-2 and COVID-19—Rationale, dosing and safety“BCPT. November 11, 2020. Camostat mesylate against SARS‐CoV‐2 and COVID‐19—Rationale, dosing and safety – Breining – – Basic &amp; Clinical Pharmacology &amp; Toxicology – Wiley Online Library

74) Sagawa T, Inoue KI, Takano H. “Use of protease inhibitors for the prevention of COVID-19”. Prev Med. December 14, 2020. Use of protease inhibitors for the prevention of COVID-19 (nih.gov)

Use of Lopinavir/Ritonavir:

75) Song, Y., Peng, W., Tang, D., & Dai, Y. (2020). Protease Inhibitor Use in COVID-19. SN comprehensive clinical medicine. August 14, 2020. Protease Inhibitor Use in COVID-19 (nih.gov)

Cathepsin L:

76) Adlhoch C., Alberska N., Albiger B., Alexakis L., Baka A., Broberg E., et al. (2020). Coronavirus disease 2019 (COVID-19) pandemic: increased transmission in the EU/EEA and the UK – seventh update. Frontiers | Cathepsin L in COVID-19: From Pharmacological Evidences to Genetics | Cellular and Infection Microbiology (frontiersin.org)

77) “Cathepsin L”. Heidrun Kirschke, in Handbook of Proteolytic Enzymes (Third Edition), 2013. Cathepsin L – an overview | ScienceDirect Topics

Teicoplanin:

78) Demsie DG, Gebre AK, Yimer EM, Alema NM, Araya EM, Bantie AT, Allene MD, Gebremedhin H, Yehuala Wa, Tafere C, Tadese HT, Amare B, Weldekidan E, Gebrie D. Glycopeptides as Potential Interventions for COVID-19. Biologics. October 13, 2020. https://www.dovepress.com/glycopeptides-as-potential-interventions-for-covid-19-peer-reviewed-fulltext-article-BTT

79) Junsong Zhang, Xiancai Ma, Fei Yu, Jun Liu, Fan Zou. “Teicoplanin potentially blocks the cell entry of 2019-nCoV”.February 13, 2020. https://www.biorxiv.org/content/10.1101/2020.02.05.935387v1.full

80) Anja Pišlar, Ana Mitrović, Jerica Sabotič,Urša Pečar Fonović et al. “The role of cysteine peptidases in coronavirus cell entry and replication: The therapeutic potential of cathepsin inhibitors”. November 2, 2020. The role of cysteine peptidases in coronavirus cell entry and replication: The therapeutic potential of cathepsin inhibitors (plos.org)

81) Miao-Miao Zhao, Wei-Li Yang, Fang-Yuan Yang, Li Zhang et al. “Cathepsin L plays a key role in SARS-CoV-2 infection in humans and humanized mice and is a promising target for new drug development“.medrxiv. October 27, 2020. Cathepsin L plays a key role in SARS-CoV-2 infection in humans and humanized mice and is a promising target for new drug development | medRxiv

82) Paweł Grieb PhD, Maciej Świątkiewicz PhD, Katarzyna Prus MD, Konrad Rejdak MD. “Amantadine for COVID-19”. December 17, 2020. Amantadine for COVID‐19 – Grieb – – The Journal of Clinical Pharmacology – Wiley Online Library

83) Liu, T., Luo, S., Libby, P., & Shi, G. P. (2020). Cathepsin L-selective inhibitors: A potentially promising treatment for COVID-19 patients. Pharmacology & therapeutics, 213, 107587. May 26, 2020. Cathepsin L-selective inhibitors: A potentially promising treatment for COVID-19 patients (nih.gov)

84) Gomes CP, Fernandes DE, Casimiro F, da Mata GF, Passos MT, Varela P, Mastroianni-Kirsztajn G and Pesquero JB (2020) Cathepsin L in COVID-19: From Pharmacological Evidences to Genetics. Front. Cell. Infect. Microbiol. 10:589505. December 8, 2020. Frontiers | Cathepsin L in COVID-19: From Pharmacological Evidences to Genetics | Cellular and Infection Microbiology (frontiersin.org)

Cholorquine/hydroxychloroquine:

85) April Jorge. “Hydroxychloroquine in the prevention of COVID-19 mortality”. The Lancet Rheumatology. November 5, 2020. Hydroxychloroquine in the prevention of COVID-19 mortality – The Lancet Rheumatology

86) “Coronavirus disease (COVID-19): Solidarity Trial and hydroxychloroquine”. World Health Organisation. June 19, 2020. Coronavirus disease (COVID-19): Hydroxychloroquine (who.int)

87) Nicholas J. White ,James A. Watson, Richard M. Hoglund, Xin Hui S. Chan et al. “COVID-19 prevention and treatment: A critical analysis of chloroquine and hydroxychloroquine clinical pharmacology”. Plos medicine. September 3, 2020. COVID-19 prevention and treatment: A critical analysis of chloroquine and hydroxychloroquine clinical pharmacology (plos.org)

88) Myron S. Cohen, M.D. “Hydroxychloroquine for the Prevention of Covid-19 — Searching for Evidence”. The New England Journal of Medicine. August 6, 2020. Hydroxychloroquine for the Prevention of Covid-19 — Searching for Evidence | NEJM

89) BMJ 2020;369:m2378. June 15, 2020. Hydroxychloroquine for covid-19: the end of the line? | The BMJ

90) Rami A. Al-Horani,Bani Kar and Kholoud F. Aliter. “Potential Anti-COVID-19 Therapeutics that Block the Early Stage of the Viral Life Cycle: Structures, Mechanisms, and Clinical Trials”. MDPI. July 3, 2020. IJMS | Free Full-Text | Potential Anti-COVID-19 Therapeutics that Block the Early Stage of the Viral Life Cycle: Structures, Mechanisms, and Clinical Trials | HTML (mdpi.com)

Other CATL Inibitors:

91) Parag P. Shah, Tianhua Wang, Rachel L. Kaletsky, Michael C. Myers et al. “A Small-Molecule Oxocarbazate Inhibitor of Human Cathepsin L Blocks Severe Acute Respiratory Syndrome and Ebola Pseudotype Virus Infection into Human Embryonic Kidney”. May 13, 2010. https://www.med.unc.edu/genetics/purvislab/files/2018/03/2010Shah.pdf

92) Kas Steuten, Heeyoung Kim, John C. Widen, Brett M. Babin et al. “Challenges for targeting SARS-CoV-2 proteases as a therapeutic strategy for COVID-19”. November 23, 2020. https://www.biorxiv.org/content/10.1101/2020.11.21.392753v1.full

Heparin:

93) Joseph A. Hippensteel, Wells B. LaRiviere, James F. Colbert, Christophe J. Langouët-Astrié et al.”Heparin as a therapy for COVID-19: current evidence and future possibilities“. American Physiological Society. July 20, 2020. https://journals.physiology.org/doi/full/10.1152/ajplung.00199.2020

94) Baranca Buijsers,Cansu Yanginlar, Marissa L. Maciej-Hulme, Quirijn de Mast et al.”Beneficial non-anticoagulant mechanisms underlying heparin treatment of COVID-19 patients”. EBioMedicine. August 25, 2020. https://www.thelancet.com/journals/ebiom/article/PIIS2352-3964(20)30345-5/fulltext

95)Gozzo L, Viale P, Longo L, Vitale DC and Drago F (2020) The Potential Role of Heparin in Patients With COVID-19: Beyond the Anticoagulant Effect. A Review. Front. Pharmacol. August 21, 2020. https://www.frontiersin.org/articles/10.3389/fphar.2020.01307/full

96) Jennifer Byrne.”Heparin may neutralize virus that causes COVID-19“.HemOnc.August 28,2020. https://www.healio.com/news/hematology-oncology/20200827/heparin-may-neutralize-virus-that-causes-covid19

97) van Haren, F.M.P., Page, C., Laffey, J.G. et al. Nebulised heparin as a treatment for COVID-19: scientific rationale and a call for randomised evidence. Crit Care 24, 454 (2020). https://ccforum.biomedcentral.com/articles/10.1186/s13054-020-03148-2

98) Gozzo L, Viale P, Longo L, Vitale DC, Drago F. The Potential Role of Heparin in Patients With COVID-19: Beyond the Anticoagulant Effect. A Review. Front Pharmacol. 2020;11:1307. August 21, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7472559/

Bromhexine Hydrochloride:

99) Stepanov D, Lierz P (2020) Bromhexine Hydrochloride: Potential Approach to Prevent or Treat Early Stage Novel 2019 Coronavirus Disease. J Infect Dis Epidemiol 6:135. 2020. https://www.clinmedjournals.org/articles/jide/journal-of-infectious-diseases-and-epidemiology-jide-6-135.php?jid=jide

100) Wang, Yuying MBa; Zhang, Yinghua MBb; Chen, Xia MBc; Xue, Kun MMc; Zhang, Tianjing MDd; Ren, Xiaohong MBc,∗ Evaluating the efficacy and safety of bromhexine hydrochloride tablets in treating pediatric COVID-19, Medicine: Volume 99 – Issue 37 – p e22114. September 11, 2020 https://journals.lww.com/md-journal/fulltext/2020/09110/evaluating_the_efficacy_and_safety_of_bromhexine.48.aspx

101) Ting Li, Laifang Sun, Wenwu Zhang, Chanfan Zheng, Chenchen Jiang et al. “Bromhexine Hydrochloride Tablets for the Treatment of Moderate COVID-19: An Open-Label Randomized Controlled Pilot Study“.ASCPT. September 3, 2020. https://ascpt.onlinelibrary.wiley.com/doi/full/10.1111/cts.12881

102) Wang Y, Zhang Y, Chen X, Xue K, Zhang T, Ren X. Evaluating the efficacy and safety of bromhexine hydrochloride tablets in treating pediatric COVID-19: A protocol for meta-analysis and systematic review. Medicine (Baltimore). September 11, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7489657/

103) Markus Depfenhart,Danielle de Villiers, GottfriedLemperle,MarkusMeyer and SalvatoreDiSomma. “Potential new treatment strategies for COVID-19: is there a role for bromhexine as add-on therapy?”. Research gate. May 2020. https://www.researchgate.net/publication/341666906_Potential_new_treatment_strategies_for_COVID-19_is_there_a_role_for_bromhexine_as_add-on_therapy

Remdevesivir:

104) Jeremy Hsu. “Covid-19: What now for remdesivir?“.BMJ 2020;371:m4457. November 20, 2020. https://www.bmj.com/content/371/bmj.m4457

105) Rafael Dal-Re, Rita Banzi,Sophie Georgin- Lavialle, Reecha Sofat et al. “Remdesivir for COVID-19 in Europe: will it provide value for money?”. The Lancet Respiratory Medicine. December 17, 2020. https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(20)30568-3/fulltext

106) Harrison Wein, Erin Bryant.”Final report confirms remdesivir benefits for COVID-19”.NIH Research Matters.October 20,2020. https://www.nih.gov/news-events/nih-research-matters/final-report-confirms-remdesivir-benefits-covid-19

107)COVID-19 Treatment Guidelines Panel. Coronavirus Disease 2019 (COVID-19) Treatment Guidelines. National Institutes of Health. https://www.covid19treatmentguidelines.nih.gov/antiviral-therapy/remdesivir/

108) “WHO recommends against the use of remdesivir in COVID-19 patients”. World Health Organisation. November 20, 2020. https://www.who.int/news-room/feature-stories/detail/who-recommends-against-the-use-of-remdesivir-in-covid-19-patients

Lopinavir-ritonavir:

109) “WHO discontinues hydroxychloroquine and lopinavir/ritonavir treatment arms for COVID-19”. World Health Organisation. July 4, 2020. https://www.who.int/news/item/04-07-2020-who-discontinues-hydroxychloroquine-and-lopinavir-ritonavir-treatment-arms-for-covid-19

110) Daniel Dolan, Jack Ingham, Janos Baombe. “Lopinavir–ritonavir and COVID-19”. BMJ Journals.July 2, 2020. https://emj.bmj.com/content/37/7/450.2

111)Zhina Hadisi, Tavia Walsh, Seyed Mohammad Hossein Dabiri, Amir Seyfoori et al. “Management of Coronavirus Disease 2019 (COVID-19) Pandemic: From Diagnosis to Treatment Strategies”. December 16,2020. https://onlinelibrary.wiley.com/doi/full/10.1002/adtp.202000173

112) Bin Cao, Yeming Wang,Danning Wen,Wen Liu et al. “A Trial of Lopinavir–Ritonavir in Adults Hospitalized with Severe Covid-19”.N Engl J Med 2020; 382:1787-1799 DOI: 10.1056/NEJMoa2001282. May 7, 2020. https://www.nejm.org/doi/full/10.1056/NEJMoa2001282

113) “What coronavirus treatments are around the corner?”. British Heart Foundation. February 17, 2021. https://www.bhf.org.uk/informationsupport/heart-matters-magazine/news/behind-the-headlines/coronavirus/what-coronavirus-treatments-are-around-the-corner

114) Gavriatopoulou M, Ntanasis-Stathopoulos I, Korompoki E, et al. Emerging treatment strategies for COVID-19 infection. Clin Exp Med. October 30, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7598940/

Furin Inhibitors:

115) Canrong Wu, Mengzhu Zheng, Yueying Yang, Xiaoxia Gu et al. “Furin: A Potential Therapeutic Target for COVID-19”.iScience,Volume 23, Issue 10,ISSN 2589-0042. October 23, 2020. https://www.sciencedirect.com/science/article/pii/S2589004220308348

116) Vincent Racaniello. “Furin cleavage site in the SARS-CoV-2 coronavirus glycoprotein”.Virology Blog. February 13, 2020. https://www.virology.ws/2020/02/13/furin-cleavage-site-in-the-sars-cov-2-coronavirus-glycoprotein/

117) Canrong Wu, Mengzhu Zheng,Yueying Yang,Xiaoxia Gu et al. “Furin: A Potential Therapeutic Target for COVID-19”.iScience. October 23, 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7534598/

118) Follis KE, York J, Nunberg JH. Furin cleavage of the SARS coronavirus spike glycoprotein enhances cell-cell fusion but does not affect virion entry. Virology. July 5. 2006. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7111780/

119) Xia, S., Lan, Q., Su, S. et al. The role of furin cleavage site in SARS-CoV-2 spike protein-mediated membrane fusion in the presence or absence of trypsin. Sig Transduct Target Ther 5, 92. June 12, 2020. https://www.nature.com/articles/s41392-020-0184-0

120) National Center for Biotechnology Information (2021). “Diminazene”.PubChem Compound Summary for CID 2354, Diminazene. July 18, 2021 https://pubchem.ncbi.nlm.nih.gov/compound/2354#section=Names-and-Identifiers

121) Raquel Barrantes Gallego, Francisco F R Vidigal. Diminazene for COVID-19: a missed opportunity?. Authorea. October 31, 2020. https://www.authorea.com/doi/full/10.22541/au.160417264.45598527/v1

122) Jean-Philippe Labelle. “Using Diminazene Aceturate to prevent cytokine storm caused by COVID19”.ResearchGate. June 2020. https://www.researchgate.net/publication/342009874_Using_Diminazene_Aceturate_to_prevent_cytokine_storm_caused_by_COVID19

123) Aiach M, Dreyfus G, Michaud A, Relland J, Murawski M, Leclerc M, Carpentier A. Low molecular weight (LMW) heparin derivatives in experimental extra-corporeal circulation (ECC). Haemostasis. 1984;14(4):325-32. doi: 10.1159/000215083. PMID: 6500382. https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjkrIvnwZLvAhUto3EKHXphDOgQFjAAegQIAxAD&url=https%3A%2F%2Fpubmed.ncbi.nlm.nih.gov%2F6500382%2F&usg=AOvVaw2eU9dEM6oPF5jaxBaz8zow

AstraZeneca/Pfizer/Moderna vaccine:

124) “Covid: How does the Oxford-AstraZeneca vaccine work?”. BBC. May 7, 2020. https://www.bbc.co.uk/news/health-55302595

125) “How the Oxford-AstraZeneca covid-19 vaccine was made”. BMJ Journals. BMJ 2021;372:n86. January 12, 2021. https://www.bmj.com/content/372/bmj.n86

126) “ Summary of Product Characteristics for Vaxzevria”. Gov.UK. July 16,2021. https://www.gov.uk/government/publications/regulatory-approval-of-covid-19-vaccine-astrazeneca/information-for-healthcare-professionals-on-covid-19-vaccine-astrazeneca

127) Jonathan Corum and Carl Zimmer. “How the Oxford- Astra Zeneca Vaccine works”. May 7, 2021. https://www.nytimes.com/interactive/2020/health/oxford-astrazeneca-covid-19-vaccine.html

128) Sophie Meixner.“Foetal cells are used to make the Oxford coronavirus vaccine. But they came from a foetus in 1973”. August 27,2020. https://www.abc.net.au/news/2020-08-28/oxford-coronavirus-vaccine-ethical-religious-foetal-cell/12592800

129) “COVID-19 vaccines”.Vaccine Knowledge Project.March 3, 2021. https://vk.ovg.ox.ac.uk/vk/covid-19-vaccines

130) “Understanding How COVID-19 Vaccines Work”. CDC.May 27, 2021. https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/how-they-work.html

131) “Covid: How does the Oxford-AstraZeneca vaccine work?”. BBC. May 7, 2021. https://www.bbc.co.uk/news/health-55302595

132) Dr Martin Bootman, Dr. Jon Golding, David Kingsley. “How does the human body fight a viral infection?”. OpenLearn. September 25, 2020. https://www.open.edu/openlearn/science-maths-technology/biology/how-does-the-human-body-fight-viral-infection

133) Britannica, The Editors of Encyclopaedia. “Antibody”. Encyclopedia Britannica, 27 May. 2020,

https://www.britannica.com/science/antibody

134) Bembenek ME, Burkhardt A, Ma J, Li Z, Loke HK, Wu D, Xu Q, Tayber O, Xie L, Li P, Li L. Determination of complementary antibody pairs using protein A capture with the AlphaScreen assay format. Anal Biochem. January 15, 2011. https://pubmed.ncbi.nlm.nih.gov/20868646/

135)Dr. Biology. “T-cells”. ASU – Ask A Biologist. 16 February, 2011. https://askabiologist.asu.edu/t-cell

136) Jacob S. Sherkow, Lisa Larrimore Ouellette, Nicholson Price, and Rachel Sachs.”How Does Moderna’s COVID-19 Vaccine Work, and Who Is Funding Its Development?” BillofHealth. August 27,2020. https://blog.petrieflom.law.harvard.edu/2020/08/27/moderna-covid19-vaccine-government-funding/

137) Kerry Laing, Fred Hutchinson Cancer Research Centre, Seattle, USA. “Immune responses to viruses”. British Society for immunology. https://www.immunology.org/public-information/bitesized-immunology/pathogens-and-disease/immune-responses-viruses

138) UPMC. “What’s the Difference Between Vaccines From Moderna and Pfizer?”.January 19,2021. https://share.upmc.com/2021/01/vaccines-moderna-pfizer/

139) Dr. Sanchari Sinha Dutta.”What are Adenovirus-Based Vaccines?”.News medical Life Sciences.March 10,2021.

140) Jake Smith. “How Does AstraZeneca’s COVID-19 Vaccine Compare to Pfizer’s, Moderna’s, and Johnson & Johnson’s?“.Prevention.May 3, 2021. https://www.prevention.com/health/a35118263/astrazeneca-vs-pfizer-vs-moderna-covid-19-vaccine/

149) Moderna – https://www.nytimes.com/interactive/2020/health/moderna-covid-19-vaccine.html

Diagrams/Models:

141) Niall McCarthy. “How Contagious is the Coronavirus?”.Statista.January 29, 2020. https://www.statista.com/chart/20641/average-number-infected-by-an-individual-with-diseases/

142) https://www.ahajournals.org/cms/asset/68f5c529-4929-47e9-a64e-201e84e5e6ee/1786fig01.jpg – OBESITY Risk factor Adipose Tissue:

143)Liu T, Luo S, Libby P, Shi GP. Cathepsin L-selective inhibitors: A potentially promising treatment for COVID-19 patients. Pharmacol Ther. May 26,2020. Cathepsin L-selective inhibitors: A potentially promising treatment for COVID-19 patients (nih.gov)

144) “SARS-CoV-2 Protease Assay Services”. ReactionBiology. 2021. SARS-CoV-2-Related Protease Assays | Reaction Biology

145) Christy Clutter. “The Biology and Immunology of COVID-19 Susceptibility”.American Society for Microbiology. June 5,2020. The Biology and Immunology of COVID-19 Susceptibility (asm.org)

146) CanrongWu, Mengzhu Zheng, Yueying Yang, Xiaoxia Gu et al. “Furin: A Potential Therapeutic Target for COVID-19”.iScience.October 23, 2020. Furin: A Potential Therapeutic Target for COVID-19 – S

147) Supratha Murugaraj, Martin Binoy. “Main targets for SARS CoV Treatment”. March 2020.