Let’s Talk About Boosters

I get a lot of questions on the difference between boosters/ third doses/ annual shots. The differences are mostly semantics, and can also overlap when it comes to Covid-19 vaccines. I also talk about why we may need boosters, what that means in terms of vaccine efficacy and how we need to be mindful about who gets boosters as it pertains to global vaccine equity.

Boosters vs Series vs Annual shots

A series is typically the number of doses of vaccines that are required to mount a thorough, robust and long-acting immunity. Typically, most vaccines need 2-3 doses to achieve this. For the Covid-19 vaccines, those who are immunocompromised might need a 3-dose series, which is why for this population, the third dose is done with a 28-day gap (similar to first two doses). It is possible that ultimately the Covid-19 vaccines end up becoming a 3-dose series for everyone – we just don’t know at this point.

A booster is given when there is waning (reducing) immunity. Typically this is done at least many months to years after finishing the series. This is done to boost the Memory B and T cells. As an example, the TDaP (tetanus, diphtheria, whooping cough) vaccine booster is given 5-10 years after completing the series. For the covid-19 vaccines, booster doses, for example, are being recommended for seniors who live in congregate settings. It is not fully understood why some vaccines are effective for life, and others require regular boosters.

Annual shots are typically when immunity does not last or if we need to provide an update to the immunity. An example of this would be Influenza (flu shot) where we have different strains that are prevalent and studies show that typically immunity only lasts around 6 months. We don’t know at this point whether we will need annual doses of Covid-19 vaccines, or doses specific to new variants.

Boosters for Covid-19

What’s important to remember is that just because we are currently identifying populations that require third doses/boosters, it does NOT mean that vaccinations are not working.

1- Vaccines are still very effective in preventing severe illness, hospitalization and deaths. Currently in Ontario, only 3% of those hospitalized due to Covid-19 are fully vaccinated. 92% are unvaccinated, and the remainder have one-dose (1).

2- We don’t have long-term efficacy data of the vaccines as yet, and this is why we continue to do studies to assess how long immunity lasts. It might be that over time, we realize that 3 doses in the primary series are necessary, or longer intervals between doses are key or booster doses are needed or doses against specific variants might be required.

3- Expanding on the last part, variants such as Delta or newer variants might be able to evade the immune system and thereby reduce the effectiveness of vaccines. We know that unvaccinated people are driving variants (2), and given the large percentage of people who are still unvaccinated globally, we may see newer variants that might necessitate boosters that specifically target the variants.

4- Global equity of vaccines is critical.

Global Vaccine Equity

Booster doses for everyone (not just those who might benefit from it, such as elderly and immunocompromised) during a pandemic that is still causing widespread devastation is not only not evidence-driven, but in fact antithetical to a principle of equity, something widely mentioned yet poorly addressed throughout this pandemic.

Considering that this is a “global pandemic” wherein variants migrate and disseminate widely, this is a prudent and thoughtful strategy. The first response to variants should not be boosters. It should be serving global vaccine equity and optimizing public health measures.

I discuss this in detail in my very passionate editorial in the Toronto Star.


1- https://covid19-sciencetable.ca/wp-content/uploads/2021/09/Update-on-COVID-19-Projections_2021.09.01_English-1.pdf

2- https://www.reuters.com/article/factcheck-vaccine-variants-idUSL2N2NL1M2