Why do we need vaccine booster injections?

Most individuals will need a vaccine booster injection at some point in time, although the period between additional injections varies greatly.¹ There are several reasons why, including the fact that some viruses mutate more quickly than others, which may reduce the protection offered by a first vaccination.² Also, some vaccines can enhance the body’s "immune memory" by stimulating B cells to produce antibodies (protective proteins produced in response to a virus or other foreign substance) and training T cells to help eliminate infected cells and prepare the body for a possible viral infection in the future. But whether immune memory is related to the way in which a vaccine is made, or to the specific virus itself, is still not clearly understood.3

What is known is that, for some vaccines, immunity begins to wear off after a while. When that happens, months or years after the initial injection, a "booster" dose is needed³ to increase the body’s immune response.

Despite advances in virology and immunology, why and how levels of immunity can fluctuate and decline over time is not well understood. That’s why scientists have called for more research into this area, arguing that if more was known about "vaccine durability," better vaccines could be made.3

What does this mean for the future of vaccine protection against COVID-19?

People who have received a vaccine against SARS-CoV-2 (the virus that causes COVID-19) typically have a good level of protection against serious illness from infection, but initial research suggests that this protection may start to wane over time.4-6 In the months after vaccination, it appears that antibodies start to decrease, which may make the immune response less effective against the virus in general,7 and the Delta variant in particular.8

Although this sounds concerning, the immune system can also "remember" the virus with memory B cells. These cells can quickly churn out antibodies should the virus invade.9 T cells also play their part by destroying infected cells and limiting the severity of disease.10 Vaccine boosters reactivate the immune memory and have been shown to result in a rapid rise in antibodies, bracing the body to be "ready" to combat infection over time.7,11

And as other SARS-CoV-2 variants arise, it is also important to ensure that vaccine boosters can offer cross-protection against new mutations of the virus. From studies so far, we have seen some promising findings for the Omicron variant, with much improved levels of protection against more severe cases of COVID-19.12,13

What’s the current guidance on vaccine booster injections?

The World Health Organization has said that it strongly supports urgent and broad access to current COVID-19 vaccines, including booster doses.14 In addition, many countries have already implemented the roll-out of booster doses as part of their vaccination program.

The main priority remains the same—to ensure that people worldwide are vaccinated against SARS-CoV-2—particularly groups who are at risk of developing severe disease, as current COVID-19 vaccines continue to provide high levels of protection,14 especially in light of surging infection rates.8

No one can be protected from COVID-19 until everyone is protected.

  1. Immunisation Advisory Centre. Efficacy and effectiveness. Available at: https://www.immune.org.nz/vaccines/efficiency-effectiveness [Accessed January 2022].
  2. World Health Organization. The effects of virus variants on COVID-19 vaccines. Available at: https://www.who.int/news-room/feature-stories/detail/the-effects-of-virus-variants-on-covid-19-vaccines [Accessed March 2022].
  3. Cohen J. How long do vaccines last? The surprising answers may help protect people longer. Science Magazine. Available at: https://www.sciencemag.org/news/2019/04/how-long-do-vaccines-last-surprising-answers-may-help-protect-people-longer [Accessed March 2022].
  4. Centers for Disease Control and Prevention. Understanding how vaccines work. Available at: https://www.cdc.gov/vaccines/hcp/conversations/understanding-vacc-work.html [Accessed March 2022].
  5. Sanderson K. COVID vaccines protect against Delta, but their effectiveness wanes. Nature. Available at: https://www.nature.com/articles/d41586-021-02261-8 [Accessed March 2022].
  6. Hamady, et al. Infection. 2022;50:11–25.
  7. Naaber P, et al. Lancet Reg Health Eur. 2021 Nov;10:100208.
  8. Dejnirattisai W, et al. Lancet. 2022;399:234–236.
  9. Palm AE, Henry C. Front Immunol. 2019;10:1787.
  10. Rosendahl Huber S, et al. Front Immunol. 2014;5:171.
  11. Siegrist CA. Chapter 2 - Vaccine Immunology. In: Plotkin’s Vaccines. 7th ed. Elsevier; 2018. pp 16–34.
  12. Accorsi EK, et al. JAMA. 2022;327:639–651.
  13. Centers for Disease Control and Prevention. Effectiveness of a third dose of mRNA vaccines against COVID-19–associated emergency department and urgent care encounters and hospitalizations among adults during periods of Delta and Omicron variant predominance. Available at: https://www.cdc.gov/mmwr/volumes/71/wr/mm7104e3.htm?s_cid=mm7104e3_w [Accessed March 2022].
  14. World Health Organization. Interim statement on COVID-19 vaccines in the context of the circulation of the Omicron SARS-CoV-2 variant from the WHO Technical Advisory Group on COVID-19 Vaccine Composition (TAG-CO-VAC). Available at: https://www.who.int/news/item/08-03-2022-interim-statement-on-covid-19-vaccines-in-the-context-of-the-circulation-of-the-omicron-sars-cov-2-variant-from-the-who-technical-advisory-group-on-covid-19-vaccine-composition-(tag-co-vac)-08-march-2022 [Accessed March 2022].
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