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A RN draws the vaccine into a syringe at the Saskatoon Tribal Council run vaccination clinic inside SaskTel centre in Saskatoon on April 15.Kayle Neis/The Canadian Press

Canadian scientists say there isn’t yet enough evidence that healthy adults need a COVID-19 booster shot and that rolling out third vaccine doses too quickly will do more harm than good.

The issue of third doses has become more heated in recent weeks as new research shows that protection against COVID-19 infection starts to slip in some vaccinated individuals over time. In light of the new evidence, Canada’s National Advisory Committee on Immunization now recommends that immunocompromised adults and seniors living in long-term care settings receive a third dose of a COVID-19 vaccine to ensure they have better protection.

Some provinces have expanded third-dose eligibility beyond those groups – for instance, Alberta says anyone 75 or older can receive a third dose, while Manitoba is now offering them to health care workers.

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But for healthy adults, experts say there’s no need to roll out third doses yet, even as some vaccine makers start to lobby for them. In fact, new data from Toronto’s Institute for Clinical Evaluative Sciences (ICES) show that nearly eight months after being vaccinated, protection against infection and severe outcomes is still very high in non-immunocompromised adults.

The study, which has not yet been published but was presented to a group of physicians and scientists last week, looked at people 16 and older in Ontario who had been vaccinated against COVID-19.

The findings show that, on average, an individual had about 81-per-cent protection against any symptomatic infection of COVID-19 about eight months after their second dose of an mRNA vaccine. Protection against severe outcomes remained well over 90 per cent. When researchers analyzed how well the vaccines protected against the Delta variant, they found similarly strong results.

While the findings also found high levels of protection against infection and severe illness in people who received two doses of AstraZeneca or one dose of AstraZeneca and one mRNA vaccine, those vaccines weren’t offered until March and more time will be needed for follow-up to see how those individuals fare, said study lead Jeff Kwong.

Dr. Kwong, who is a senior scientist at ICES, highlighted the fact that even in seniors 70 and older living in retirement homes, protection against infection and severe disease remained high about eight months after they were vaccinated. Seniors in retirement homes are typically more frail than those living in the community, so that high level of protection is a very positive sign, Dr. Kwong said.

Overall, he said, the vaccines are still holding up well in healthy adults and protection against severe illness is very high, he said.

“The incremental benefit of the boosters is not worth it,” Dr. Kwong said, adding that if and when research shows immunity levels waning in a substantial way, that will be the time to consider an extra shot.

Until the evidence supporting a third dose is there, it’s a good idea to hold off as long as possible, said Allison McGeer, an infectious-diseases specialist and microbiologist at Toronto’s Mount Sinai Hospital.

While some countries, such as Israel, have started offering booster shots to all who are eligible, Dr. McGeer said there are a few key differences. Israel had widespread access to the vaccine about three months earlier than Canada, and also has higher transmission of COVID-19 than many parts of Canada. So that may mean Canada will also start to use boosters in the coming months, particularly if transmission of the virus spikes and evidence shows immunity levels in vaccinated individuals start to drop substantially.

Dr. McGeer highlighted the fact that giving booster shots too liberally in Canada will further hamper efforts to get first doses to people living in low-income countries. Less than 5 per cent of Africans, for instance, are fully vaccinated.

“If I take my third dose now, I’m taking a dose away from a health care worker in Nigeria – let’s be really clear,” Dr. McGeer said. “If I have a right to a vaccine when vaccinations in Africa are in single digits, even in health care workers, I don’t think that’s something we should expect our governments or our public-health departments to buy into.”

Dr. McGeer said using boosters too early could also complicate efforts to respond to increased transmission of COVID-19 in the months ahead. So instead of prematurely offering third doses when healthy adults still have a strong immune response, it makes more sense to stretch out the protective benefits of the vaccine as long as possible to help keep new waves of the virus at bay, she said.

But some of the current recommendations may have to change if new evidence emerges, Dr. McGeer said. If there are a growing number of outbreaks at long-term care facilities in the weeks after Thanksgiving, it might be necessary to think about third doses for long-term care employees to give an added layer of protection to seniors.

Some health experts have also been calling for third doses for Indigenous communities. Dr. Kwong said that may be appropriate, given that many Indigenous communities have higher rates of chronic health conditions, which could leave individuals more susceptible to severe COVID-19 infections.

It’s also important to consider safety when thinking about any third doses. For instance, mRNA vaccines have been linked to rare cases of heart inflammation in young men, so any recommendations to roll out boosters in that population need to be considered very carefully, Dr. McGeer said.

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