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Experts advise Canadians to consider personal risk before getting 4th COVID vaccine dose


The latest guidelines on COVID-19 vaccines from Canada’s National Advisory Committee on Immunization (NACI) strongly advise the country’s oldest residents to get their second booster shot if they haven’t done so already.

But as the highly transmissible Omicron BA.2 sub-variant continues to fuel the current wave of cases reported in Canada, members of this high-risk group who recently became infected may be left wondering how long to wait before getting their fourth jab.

“In general, it's always a good idea, for almost all our vaccines, to wait about three months after an infection before getting vaccinated,” Dr. Dawn Bowdish told in a phone interview on April 20. Bowdish is an associate professor at McMaster University in Hamilton, Ont., and a Canada Research Chair in aging and immunity.

According to guidance published by the Public Health Agency of Canada (PHAC) in early February, those infected with SARS-CoV-2 after their primary series – which refers to their first two vaccine doses – but before their third dose, should wait at least three months to get a booster shot. This applies to all Canadians aged 12 and older.

The three-month time period begins with the onset of COVID-19 symptoms, or a positive test if someone is asymptomatic. This is considering a person has already waited at least six months since their most recent vaccine dose. No explicit guidelines are provided by PHAC on how long an infected person should wait before getting their fourth dose, however.

Getting vaccinated too soon after a COVID-19 infection won’t offer an ideal amount of protection against future infections or severe outcomes, Bowdish said. After a person is infected with COVID-19, antibodies will continue circulating in their bloodstream for a period of time, explained Dr. Christopher Labos, a cardiologist and epidemiologist based in Montreal. These antibodies were produced in response to the virus. If a person were to get vaccinated while these antibody levels remain high, their body wouldn’t generate any new antibodies from the vaccine, he said.

“If you get your booster shots too soon after you've been infected, they won't actually do anything,” he told on April 20 over the phone. “So you want to wait long enough for the antibody levels to start to fall so that your booster shot actually does something, which is stimulate your immune system to keep making antibodies.”

Waiting at least three months after contracting COVID-19 can improve the efficacy of the vaccine by allowing any existing antibodies created as a result of the infection to clear out first so that the vaccine has no interference to mounting a new immune response, Bowdish said.

Waiting a few months also provides the body with time to recover from the infection, Labos said. In general, people should not be getting vaccinated during an active infection, he said, whether it was caused by COVID-19 or another illness.

“Number one, you might infect the people at the vaccine centre,” Labos said. “And two, it's going to be very hard for you to tease out the differences between the vaccine side-effects and the symptoms of your illness.

“You generally want to be in good health when you get vaccinated.”


While it may be important to give the body some time to recover from infection before getting another dose of a COVID-19 vaccine, Bowdish also cautions against waiting too long to get boosted. Several studies conducted since Omicron first appeared in November 2021 have shown a considerable waning in protection offered from vaccines about three to four months following a third dose.

According to a recent study conducted in England, the efficacy of three Pfizer vaccine doses against symptomatic disease brought about by Omicron was 67.2 per cent after two to four weeks. However, at 10 weeks post-vaccination, three doses of the vaccine were only 45.7 per cent effective against symptomatic COVID-19 illness. Another report published by the U.S. Centers for Disease Control and Prevention earlier this year revealed that vaccine effectiveness against hospitalization fell from 91 per cent at two months after receiving a third dose, to 78 per cent at four months after.

“The data is pretty clear that the farther out from whatever your last dose was ... the more chances you have of getting infected,” Bowdish said. “[After] you get your dose, then you've got a good three months where you can reasonably expect to be very highly protected, both from serious infections and also from symptomatic infections.”

This waning effectiveness of vaccines is even more pronounced in the elderly, Labos said. As people get older, they don’t generate a particularly robust immune response following exposure to COVID-19 infection or vaccines, meaning any immunity that’s generated as a result tends to wane more quickly, he said. Therefore, protection from fourth doses would provide them with a greater benefit compared to those who are younger and healthier, said Dr. David Carr, an emergency physician at the University Health Network in Toronto.

“We know that the most important risk factor for severity of illness is age,” Carr told in a phone interview on April 20. “When you look at the people who are the most vulnerable, who need to be the most careful, who are going to be the last ones to take off their masks and the first ones to get their vaccines, it only makes sense that we prioritize the elderly.”

In terms of finding the “sweet spot” for COVID-19 fourth doses after infection, Labos would recommend waiting between three and six months. However, those who are older in age should aim to be vaccinated on the shorter end of that window, closer to three months rather than six months, he said. This is provided the person has already waited the appropriate amount of time after their most recent vaccine dose. Based on NACI guidance, Canadians are advised to get their fourth dose of the vaccine at least six months after their third dose.

“You have to weigh the fact that the longer you wait, the more benefit you can derive from the booster, but the longer you wait, the more your previous immunity might wane,” he said. “So striking that balance is a little bit complicated.”

Currently in Canada, about 54.3 per cent of those aged 12 and older are vaccinated with their primary series and a booster dose. Many jurisdictions in Canada have already made fourth doses available to Canadians younger than 80 years of age.


Those who have already received three doses of the COVID-19 vaccine and have also been infected with the virus possess a form of hybrid immunity, which Bowdish described as “the best of both worlds,” as both factors are contributing to a longer-lasting immune response against COVID-19.

“Hybrid immunity does create a significant advantage,” Carr said. “It means your body's reacted to two different types of exposures, a full exposure and a partial [exposure], and there are gains from that.”

As a result, Labos said it’s possible that these people may have a sufficient amount of protection for the time being, meaning a fourth dose may not be necessary for several months.

“It is conceivable that with three doses, you'll have good immunity, six to nine months out,” Labos said. “That is not outside the realm of possibility.”

However, this immunity varies from person to person, Bowdish said, and data stemming from studies that explore hybrid immunity are limited. These are some of the reasons why it’s still advised that Canadians get their recommended vaccine doses, regardless of whether they’ve already been infected with the virus, she said.

“Some people have excellent hybrid immunity that we expect would protect them, and some people do not,” she said. “That heterogeneity makes it really hard to make recommendations, so we tend to always err on the side of protecting the most people possible, which is recommending everyone get all their doses.”

Ultimately, Carr encourages Canadians to consider their individual risk factors when determining when and whether to get their second booster shot, if they are eligible. Despite general recommendations on fourth doses, there’s no “one-size-fits-all approach,” Carr said.

“Everyone has different health conditions and risks and anxieties and risk tolerance,” said Carr. “It's not a simple answer.”

For those who are considered high-risk and primed for their fourth dose, meaning it’s been at least three months after their infection and six months since their last vaccine dose, Labos would advise getting a fourth shot.

“If you’re somebody who is eligible for a fourth [dose], I would get it because it is going to give you that extra degree of protection in a setting where there's a lot of community spread of COVID and there's very few restrictions in place for limiting its spread,” he said.

For those who are currently infected but have already had three doses of a COVID-19 vaccine, Labos suggests waiting at least three months and reassessing personal risk to determine whether or not they should get vaccinated with a fourth dose then, or put it off until the fall. By then, experts will be able to assess whether new variants have emerged and if new vaccines are available that may offer better immunity, Labos said. It will also allow for more time to observe what the long-term immunity of third and fourth doses of the COVID-19 vaccine looks like at a population level.

Carr also recommends that people speak with their family doctors about personal risk factors and ways to mitigate their chances of contracting COVID-19, he said. This includes past medical history, previous COVID-19 infections, and aspects of their lifestyle that could place them at higher risk of infection.

“They have to look at their continued exposure [to COVID-19], risk tolerance and their individual health to make those decisions, [which] they can offload with their practitioners,” Carr said.

With files from The Canadian Press and CTV's Heather Butts.


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