Mechanism of action of Covid-19 Vaccines | Sunday Observer

Mechanism of action of Covid-19 Vaccines

19 September, 2021

Spike Protein

Figure 1 Image credit: New York Times

Vaccines play a key role in reducing the spread, hospitalisation and fatality from the Covid-19 pandemic. There are 2 main types of vaccine – mRNA (e.g. Pfizer) and Viral Vector (e.g. AstraZenica, Sputnik V).

mRNA Vaccines Viral Vector Vaccines


Figure 2. Image Credit:

mRNA or messenger RNA (Figure 2), as the name suggests, is a piece of genetic material that sends a message to cells – instructing them how to make particular proteins [3].

Viral Vectors are viruses (for example Adenovirus – Figure 3), which has modified genetic material needed to make Spike Proteins [4].

Figure 3

After being injected into the body, the mRNA instruct ribosomes within cells to make copies of Covid-19 Spike Proteins (importantly not the virus itself) [3].

mRNA Vaccines do not cause Covid-19 or alternations in host DNA [3].

Once inside the body, these viral vectors release its modified genetic material into host cells. This provides them with the instructions needed to make copies of Covid-19 Spike Proteins [4].

As outlined, both types of vaccine aim to re-create Covid-19 Spike Proteins. Once this is achieved the mRNA and viral vectors are destroyed by the body.

The proteins are engulfed by immune cells (Figure 6) and presented to special types of immune cells called T-Cells [5].

T Cells help other immune responses to happen (Figure 6). They activate Cytotoxic T cells (which kill pathogens) and B Cells which produce antibodies [5].

Figure 4

Covid antibodies

When/if SARS-CoV-2 enters the body, these antibodies will recognise it’s Spike Proteins. They can block SARS-CoV-2 from binding onto host cells as it would usually do. They can also act as a target for Cytotoxic T cells which destroy virus. This immune response allows protection from Covid-19.

Dr. Bhanuka Senasinghe
MBChB (Hons), MA Clinical Research Physician – Therapy Areas: Covid-19, Influenza, RSV London, United Kingdom


1) Ciotti M, Ciccozzi M, Terrinoni A, Jiang W, Wang C, Bernardini S. The Covid-19 pandemic. Critical Reviews in Clinical Laboratory Sciences. 2020;57(6):365-388.

2) Understanding mRNA Covid-19 Vaccines [Internet]. Centers for Disease Control and Prevention. 2021 [cited 16 September 2021]. Available from: ncov/vaccines/different-vaccines/mrna.html

3) Huang Q, Zeng J, Yan J. Covid-19 mRNA vaccines. J Genet Genomics. 2021;48(2):107–14. 4) CDC. Understanding how Covid-19 vaccines work [Internet]. 2021 [cited 2021 Sep 16]. Available from: work.html

5) Chaplin DD. Overview of the immune response. J Allergy Clin Immunol. 2010;125(2 Suppl 2):S3-23.