Globally, there are currently over 50 candidates for vaccines against COVID-19 in clinical trials . The following is a comparison between the three most popular candidates as of now.
1) Pfizer-BioNTech Vaccine
The Pfizer-BioNTech Vaccine is an mRNA vaccine developed by German Biotechnology company BioNTech along with the American pharmaceutical corporation Pfizer.
It makes use of mRNA coding for the viral spike protein from the surface of Sars-Cov-2. The mRNA enters human cells where it is used to make the spike protein before degenerating in the cytoplasm. The body’s immune system produces antibodies against the produced spike protein. The produced antibodies thus prevent development of COVID-19 on exposure to the virus.
The vaccine requires two doses, administered 21 days apart. Preliminary trials showed a high efficacy of about 95%. Common side effects included pain at the injection site and flu-like symptoms such as headache or mild fever. 
2) Moderna Vaccine
The Moderna Vaccine is an mRNA vaccine developed by American pharmaceutical company Moderna. Like the Pfizer-BioNTech vaccine, it makes use of the mRNA coding for the viral spike protein in order to elicit an immune response through a similar mechanism.
The Moderna Vaccine also requires two doses, administered 28 days apart. Preliminary trials showed a slightly lower efficacy than the Pfizer-BioNTech Vaccine, at about 94.1%. Common side effects of the two vaccines were found to be similar i.e. injection site pain and mild flu symptoms. 
Apart from the differences in the time between the two doses and their efficacies, the Moderna Vaccine differs from the Pfizer-BioNTech Vaccine on the basis of the recipients’ ages. The former has only been approved for people aged 18 and above while the latter has been approved for people aged 16 and above. The two vaccines also differ based on how they are administered: the Pfizer-BioNTech vaccine must be diluted before injection while this is not the case for the Moderna Vaccine.
3) Oxford-AstraZeneca Vaccine
The Oxford-AstraZeneca Vaccine is a viral vector developed by British-Swedish pharmaceutical company AstraZeneca and the University of Oxford. It makes use of adenovirus, which usually causes common cold in chimpanzees but does not cause disease in humans. Adenovirus is modified to contain the genetic sequence of the spike protein found on the outer membrane of Sars-Cov-2. When injected into the body, it triggers an immune response by producing antibodies against the spike protein, thus preventing COVID-19 from developing on exposure to the virus.
The Oxford-AstraZeneca Vaccine has a lower efficacy than its counterparts at around 70.4%, although this efficacy is significantly higher than that of the annual flu shot. The vaccine trials used two dose regimes: one in which a low dose was followed by a standard booster dose and a second in which two standard doses were administered. The former showed a higher efficacy . Common side effects of the Oxford-AstraZeneca vaccine include fatigue, headache and muscle pain .
Despite its lower efficacy, the Oxford-AstraZeneca vaccine is likely to be popular as it does not require specialized cold storage, unlike the Pfizer vaccine, but can be transported using a normal refrigerator, making it much easier to widely distribute .
 “COVID-19 Vaccines.” World Health Organization. World Health Organization. Accessed January 31, 2021. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/covid-19-vaccines.
 “Pfizer-BioNTech COVID-19 Vaccine Questions.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, January 7, 2021. https://www.cdc.gov/vaccines/covid-19/info-by-product/pfizer/pfizer-bioNTech-faqs.html.
 “Moderna COVID-19 Vaccine Questions.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, January 7, 2021. https://www.cdc.gov/vaccines/covid-19/info-by-product/moderna/moderna-faqs.html.
 “Pfizer and Moderna Vaccine Comparison.” Association of State and Territorial Health Officials, January 13, 2021. https://www.astho.org/COVID-19/Pfizer-Moderna-Vaccine-Comparison/.
 Voysey, Merryn, Sue Ann Costa Clemens, Shabir A Madhi, Lily Y Weckx, Pedro M Folegatti, Parvinder K Aley, Brian Angus, et al. 2020. “Safety and Efficacy of the ChAdOx1 NCoV-19 Vaccine (AZD1222) against SARS-CoV-2: An Interim Analysis of Four Randomised Controlled Trials in Brazil, South Africa, and the UK.” The Lancet 397 (10269). https://doi.org/10.1016/s0140-6736(20)32661-1.
 Knibbs, Jessica. 2021. “Covid Vaccine Update: Three Major Side Effects from the Coronavirus Vaccine.” Express.co.uk. January 21, 2021. https://www.express.co.uk/life-style/health/1386951/covid-vaccine-update-side-effects-fatigue-headache-muscle-pain.
 BBC News. 2021. “Covid: What Is the Oxford-AstraZeneca Vaccine?,” January 4, 2021, sec. Health. https://www.bbc.com/news/health-55302595.