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Answers to questions about COVID-19 booster shots

A booster shot is a vaccine given after a primary vaccine series that is intended to boost the immune response. Booster shots are not new (given for meningococcal and tetanus vaccinations, among others), nor is it surprising that they have become the center of attention at this point in the pandemic. Here are answers to key questions about booster shots.

Are booster shots really necessary?

While antibody levels have been shown to diminish over time, what remains an open question is whether reduction in antibodies is associated with breakthrough infections and, importantly, with infections causing severe COVID-19. There are a few pieces of evidence that suggest that vaccine protection against severe COVID-19 persists despite the reduction in antibody levels.

The first is that the characteristics of people with breakthrough infections have not tracked with the allocation of vaccines That is, if waning immunity was a risk for breakthrough infection, we would predict that people who got their vaccines earliest, such as older individuals and health care workers, would have been the first populations to get breakthrough infections.

Instead, what has been observed is that breakthrough infections first occurred in areas where the Delta variant was most prevalent and does not appear to be associated with timing of vaccination. This suggests that Delta may be a stronger correlate for breakthrough infection than waning immunity.

Altogether, it seems like booster shots may not be needed for the healthy general public at this point in the pandemic.

Who might benefit from a booster shot?

Certain populations may not make a robust immune response to the primary vaccine series and, therefore, may not retain immunological memory capable of protecting against severe COVID-19. Notably, these include immunocompromised individuals and older adults. These populations are more likely to benefit from booster shots sooner than others.

Canada’s National Advisory Committee on Immunization (NACI) recommends an additional mRNA vaccine dose for those with moderate to severe immunocompromise who received a primary mRNA or viral vector vaccine series, as does the Centers for Disease Control and Prevention (CDC). This includes patients receiving cancer treatment, those taking immunosuppressive medications, patients with a primary immunodeficiency disorder, transplant recipients, and persons with advanced or untreated HIV. At this time, an additional vaccine dose is not recommended for older individuals nor the general public.

Similar guidance has been issued from the European Centre for Disease Prevention and Control (ECDC), with an additional consideration for older individuals, particularly those living in long-term care facilities. Other countries and regions are also debating booster shots for vulnerable persons and entire populations.

At some point, it is likely that booster shots will be necessary, and the rollout should prioritize those at highest risk for severe COVID-19, including older adults and people with underlying conditions.

Are booster shots associated with more or different adverse events than the primary vaccine series?

Rare adverse events such as pericarditis and myocarditis have been observed after primary mRNA vaccination. These cardiac complications were more prevalent after the second shot, leading to questions about the safety of a third booster dose. Currently, there is limited information about the adverse events after a booster shot, but they appear to be similar to those reported after the primary vaccine series.

A study out of Israel, where booster shots are available to fully vaccinated people over 60 years old, inexplicably failed to report any safety outcomes. No safety concerns have been noted in the media coverage of the Israeli data. 

Interestingly, Moderna is testing a reduced dose booster shot, which may quell some of the adverse effects observed with their vaccine. In a press release, they report a similar safety profile after the third dose compared to the second shot.

Will a booster shot better protect against Delta or other variants?

Currently, the booster shots contain the same vaccine as the primary series, so they are not modified to target any specific SARS-CoV-2 variant. However, the original vaccine strains are effective at preventing hospitalization and death due to Delta and other common variants.

The newest variant of interest, Mu, has mutations that are expected to evade therapeutic and vaccine- or infection-induced antibodies, but whether these diminish the effectiveness of vaccines has yet to be established.

As we march towards endemicity, it becomes more likely that seasonal COVID vaccines targeting circulating strains will become a viable approach, much like we do for influenza each year. For now, the WHO is stressing that the priority should be to fully vaccinate the global population before administering booster shots.

For more information, see the topic COVID-19 (Novel Coronavirus) in DynaMed

Original article published on EBSCO Health Notes. Written by:

  • Vito Iacoviello, MD, Deputy Editor for Infectious Disease, Allergy, and Immunology at DynaMed; and
  • Heather D. Marshall, PhD, Public Health Content Manager at DynaMed.

Editor's note (September 28, 2021): Updated advice on a booster dose of COVID-19 vaccine in long-term care residents and seniors living in other congregate settings (September 28, 2021) can be found in the NACI rapid response statement on booster dose of COVID-19 vaccine in long-term care residents and senior living in other congregate settings.

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About the author(s)

DynaMed is a clinician-focused tool designed to facilitate efficient and evidence-based patient care. Rigorous, daily review of medical literature by physician and specialist staff ensures timely and objective analysis, synthesis and guidance. DynaMed includes drug content from Micromedex, Canadian and international guidelines, and clinical images. CMA members have access to DynaMed, a point-of-care tool, included with their membership ― a tool valued at US$399 a year.