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Ashleigh Tuite and David Fisman are epidemiologists and professors at the Dalla Lana School of Public Health.

Almost as soon as the safety and efficacy data for the first COVID-19 vaccines emerged, there were questions about how long the vaccines would provide protection and whether we would eventually need booster shots. The recent announcement by Pfizer-BioNTech that the company will seek emergency use authorization from the U.S. Food and Drug Administration for a third dose of their mRNA vaccine, to be administered six to 12 months after the second shot, provides a first hint at a possible timeline for COVID-19 boosters. Their request is based on data suggesting that antibodies begin to wane about six months after vaccination.

Antibodies are only one element of the body’s immune response, and protection by vaccines is often longer lasting than we’d estimate based on antibody levels alone. Nonetheless, the concept of a booster shot to ensure longer-lasting protection from infection is not without precedent. Vaccination for human papillomavirus and hepatitis B virus incorporate a third dose six months after the first, and five months after the second, to cement a more durable immune response. Boosting is recommended for other vaccines commonly used in Canadian adults, and the changeable nature of influenza virus necessitates annual vaccinations. Third doses, boosters and repeat vaccination are nothing new.

Ottawa says COVID-19 booster shots may be needed, closely monitoring variants

We think a widespread program for third doses this coming fall and winter is unlikely in Canada. The medical necessity of such a program is unclear, and the use of third doses in the broader Canadian population while most of the world’s population remains unvaccinated would be profoundly inequitable. But there are groups for whom a third dose may be beneficial: people with immune systems weakened by medications or disease, and elders in congregate living settings. Some people with weakened immune systems do not mount a strong immune response even after two doses of vaccine, so the addition of a third shot may bring protection up to the baseline level. Israel and France have approved a third dose in people with compromised immune systems, and Israel has extended boosters to adults over 60 who are at least five months past their second dose.

How long a person is protected from infection and severe illness after vaccination depends on a range of factors. Younger people typically have a more robust immune response, which may translate into longer-lasting immunity. The timing between the first and second doses may also matter. Canada’s decision to extend the interval between doses for much of the population was controversial, but we now have data indicating a stronger immune response associated with this approach. This might translate into a longer time until we need boosters, though we do not know yet if this is the case.

Extended dose intervals were not used in our priority populations (e.g., long-term care home residents). The wisdom of that approach is clear, as long-term care home residents were largely spared from the third wave in Canada, but they are now likely to require third doses sooner than other Canadians, since they were vaccinated earliest.

Direct protection of vulnerable individuals (partly through boosting) has become all the more important as more infectious viral variants, such as the highly transmissible Delta variant, means that protecting them indirectly through vaccine-derived herd immunity is likely out of reach for now. Delta’s higher transmissibility and somewhat lower vaccine effectiveness means a higher threshold for herd immunity, one that requires very high vaccine uptake in the eligible population. We think Canada will achieve vaccine-derived herd immunity against COVID-19 in the future, but this will depend on the development of safe and effective vaccines for younger children, which are not yet available.

As we attempt to vaccinate our way out of the pandemic, we need also to remember that we have other tools at our disposal, including masks, the use of ventilation improvements for this aerosol-transmitted disease, and testing, tracing and isolation of cases as case counts decline. Heading into the fall and a presumptive fourth wave, it is imperative we monitor breakthrough infections among vaccinated individuals to better characterize waning immunity and identify immune-evading variants.

As Canadians, we are in the privileged position of being able to discuss how we might use booster shots in the global context of extreme vaccine scarcity. For now, use of third shots should be restricted to those with a less robust or waning immune response who are at heightened risk of severe illness. Over the longer term, it is likely that vaccine dosing schedules, and vaccines themselves, will continue to evolve to meet challenges created by evolving population immunity and viral evolution.

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