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Mechanism of Action (MOA) of the COVID-19 Vaccine Frontrunners

Written by Kausalya on

With over 32 million COVID-19 cases worldwide, vaccine may be the only long-term solution to control the pandemic. So, the scientists, research institutes, drugmakers, and governments are hastening the process of vaccine development against the new SARS CoV-2.

There are more than 175 vaccines under development process, while some of them had already delivered promising results from the initial human clinical trials and expected to deliver positive results from the late-stage clinical trials in the next two to three months.

Let’s talk about the science behind the frontrunners of the COVID-19 vaccine candidates.

Medical Animation Still Shot Showing the SARS CoV-2 Spike Protein Binding to ACE-2 Receptor
Medical Animation Still Shot Showing the SARS CoV-2 Spike Protein Binding to ACE-2 Receptor


Moderna in collaboration with the National Institutes of Health began its COVID-19 vaccine development in January, entered into human trials in March, and progressed into phase-3 trials on July 27. Moderna’s vaccine, mRNA-1273, is the first U.S. coronavirus vaccine to be tested in humans. The idea behind this vaccine is unique, new, and yet unproven. This vaccine uses mRNA, genetic material from the virus to induce the immune system to fight the virus. The genetic code in the form of mRNA is injected into the healthy human cells where the code instructs the cells to make copies of the spike proteins that imitate the outer surface of the coronavirus as if they had been infected. Then, the immune system will be able to learn about the spike proteins and develop ways to protect the person if the virus ever tries to enter the body.


AstraZeneca’s COVID-19 vaccine, AZD1222 (ChAdOx1 nCoV-19) is being developed by the Oxford University and the international pharmaceutical company AstraZeneca. AZD1222 is a viral-vector based vaccine, that uses a harmless virus to deliver the genetic material of a pathogen into cells, which then hopefully causes an immune response to the original pathogen. In the case of this vaccine, the genetic material of the SARS CoV-2 spike protein is modified into a weakened type of chimpanzee adenovirus, so that it doesn't infect humans. The injected vaccine then helps to produce spike proteins in the body which induces the immune system to generate antibodies against them. Once the immune system recognizes the spike protein, it can fight off the infection when the virus tries to enter the body.

The vaccine entered phase-I human clinical trials in the third week of April and progressed into phase-3 trials at the end of August after showing promising results in the initial phases. But, the trials were halted when a participant from the U.K. developed an unexplained neurological illness. The trials were restarted on Sep 12 after an independent committee safety review.

CanSino Biologics

CanSino Biologics in partnership with the Chinese Academy of Military Medical Sciences launched its phase-I COVID-19 vaccine (Ad5-nCoV) trials in March and published promising results in May followed by phase-II trial results in July. This vaccine uses a weakened form of a common cold virus, adenovirus type 5, which infects humans easily but does not cause the disease. The adenovirus is only used as a delivery system where it carries the genetic material that helps the cells to create spike protein of the SARS CoV-2. These cells then trigger the immune system to create antibodies that can fight off the infection.

The Chinese government approved this vaccine for military use in June considering the promising results of the initial phase trials.


COVID-19 vaccine (CoronaVac) developed by the SinoVac Biotech company entered its phase-I human clinical trials in April, and after showing promising results in the initial 2 phases, the company started phase-3 trials in Brazil and Indonesia. This vaccine uses an inactivated virus, a proven strategy that has been traditionally used for vaccine development, and such vaccines have been found to be safe and effective for influenza and polio. For this type of vaccine, the specific virus or bacteria is killed or inactivated, and its dead cells are introduced into the body. Even though the pathogen is dead, the immune system can still learn from its antigens how to fight its live versions in the future.

The Chinese government has already approved this vaccine for emergency use in July.

Sputnik V

Medical Animation Representing Sputnik V Vaccine
Medical Animation Representing Sputnik V Vaccine

The Gamelaya Research Institute, part of Russia’s Health Ministry launched clinical trials of its COVID-19 vaccine, Gam-Covid-Vac (later renamed – Sputnik V) in June. On Aug 11, the Russian President claimed it the first registered COVID-19 vaccine after the approval from the Russian health care regulator, even before phase-3 clinical trials. The vaccine entered phase-3 trials on Aug 12.

Sputnik V is based on a viral vector type, where weakened form of human adenoviruses (viruses that cause the common cold) are genetically modified to carry protein codes from SARS CoV-2 to trigger an immune response in the body. This vaccine uses two different strains of human adenoviruses (rAd26 and rAd5) for the first and second vaccination dose, to boost the effectiveness of the vaccine.

Disclaimer: The information in no way constitutes, or should be construed as medical advice. Nor is the above article an endorsement of any research findings discussed in the article an endorsement for any of the source publications.


  1. https://www.biopharmadive.com/news/coronavirus-vaccine-pipeline-types/579122/
  2. https://arstechnica.com/science/2020/07/meet-the-4-frontrunners-in-the-covid-19-vaccine-race/
  3. https://www.cnbctv18.com/healthcare/explained-is-russias-covid-19-vaccine-sputnik-v-ready-and-safe-6643071.htm
  4. https://theprint.in/health/all-you-need-to-know-about-cansinos-covid-vaccine-approved-for-military-use-in-china/451554/
  5. https://www.modernatx.com/modernas-work-potential-vaccine-against-covid-19
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