Everything you need to know about COVID-19 boosters

Recently, Canada’s National Advisory Committee on Immunization (NACI) issued new guidelines on who needs to get a third COVID-19 vaccination soon.

The first vaccines in Canada were administered less than a year ago, so the jury is still out on the degree to which our immunity changes over time. But recent evidence suggests that—like other routine vaccinations—everyone would benefit from a third shot at some point. The question is now timing.

One caveat: there is a slight difference between booster shots and third doses. While the vaccines themselves are the same, third doses are an extension of the regular two-dose regimen for people whose immune system weren’t fully activated by the first two (just like how most people’s immune system weren’t fully activated by the very first dose of the Pfizer, Moderna or Astra-Zeneca vaccine). That includes people with certain chronic conditions or those on medications that suppress their immune system.

A booster is for people who had a fulsome response to the COVID-19 vaccine (i.e., healthy adults), but who would benefit from further protection, either because their immunity has waned or because they are at greater risk of a poor health outcome if they get sick. 

As Rachel Gutman writing in the Atlantic explained, a booster is like filling in the cracks in your immunity shield, but for those requiring a third dose, it’s still creating that shield. Third doses come at least 28 days after the second, but ideally within four to five months. Boosters on the other hand should be administered no sooner than six months, and ideally between eight months and one year.

Here’s who should get a booster sooner rather than later… 

The NACI recommends that the following Canadian should now be offered a booster:

Adults over 70

Because this age group is at higher risk if they get sick.

People who received two doses of the AstraZeneca/COVISHIELD vaccine or one dose of the Janssen vaccine

New evidence suggests that a third dose of an mRNA vaccine (like Moderna or Pfizer) could be beneficial for those who have received an adenovirus vector vaccine (like AstraZeneca or the single-dose Johnson & Johnson) which seem to have conferred a slightly lower level of immunity.

This article provides a good overview possible vaccine combinations and how much protection they offer. Long story, short: It seems to be okay to mix and match.

Adults in or from First Nations, Inuit and Métis communities

These communities tend to have poorer health outcomes because of a variety of social and economic factors that put them at greater risk. They also have a more difficult time accessing good quality healthcare if they get very sick. Offering a booster is intended help offset that.

Adults who are frontline healthcare workers who have direct in-person contact with patients and who were vaccinated with a very short interval between their first and second doses

A lot of healthcare workers were prioritized when the vaccines first became available. As a result, the timeline between their first and second doses was shrunk down to as little as four weeks. This might have helped boost antibodies in the short term, but sooner is not necessarily better for building an optimal long-term immune response (this is a good explainer on why a decrease in antibody counts doesn’t mean our immunity is disappearing).

A boost will help them build more fulsome immunity, which is necessary given the higher likelihood of exposure to the virus.

Plans still to come in Ontario and B.C.

Ultimately, however, the NACI does not make the rules, it only offers advice to the Canadian provinces, which make their own rules.

In August, the province of Ontario began offering boosters/third doses to the immunocompromised (to see who that includes, check this document) and has since expanded eligibility for boosters. 

B.C. says anyone who wants to receive a mRNA booster show will be able to receive one by this coming May.


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