Nearly 50 per cent of the Canadian population has received a third dose of COVID-19 vaccines, but fourth doses, as of now, are only recommended for certain high-risk groups.
The National Advisory Committee of Immunization (NACI), which advises the federal, provincial and territorial governments about the use of vaccines, in April 2022 strongly recommended a second booster for adults 80 and older and residents of long-term care or other congregate settings.
NACI previously recommended that people who are “moderately to severely immunocompromised” receive a fourth dose of a COVID-19 vaccine six months after getting their third shot.
For the general healthy population, there is no immediate rush to get a fourth dose, given the protection from a single boost, the low transmission rate of the virus and outdoor social interactions in the summer months, experts say.
“I don’t think that there is an absolute requirement for a fourth [shot] at this moment in time,” said Ciriaco Piccirillo, an immunologist and senior scientist at the Research Institute of McGill University Health Centre (MUHC).
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According to NACI, the intent of a booster dose is to restore protection that may have decreased over time or is no longer sufficient in individuals who initially responded adequately to a complete primary vaccine series.
Antibody levels tend to naturally decay approximately four to six months following vaccination, Piccirillo said.
“If we are boosted another time, those levels will go back up,” he told Global News.
While there is no disadvantage in getting a fourth dose now, people should consider their own individual risks and consult their doctor before deciding to get another shot, said Omar Khan, a professor of biomedical engineering and immunology at the University of Toronto.
“Really, the question one should ask yourself is: ‘How likely am I to be exposed and what is my risk level?’” he said.
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Meanwhile, a prior COVID-19 infection should also be considered when determining your vaccination status, said Michael Grant, a professor of immunology at the Memorial University of Newfoundland.
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“In every study so far, infection is at least equivalent to one dose of a vaccine,” he said during a webinar on Zoom Thursday by the COVID-19 Immunity Task Force.
“And in some conditions, certainly when it’s a severe infection, it’s inducing better immunity.”
There is limited data on the effectiveness of a fourth COVID-19 dose.
According to a study published in the New England Journal of Medicine in May, a fourth dose of COVID-19 vaccine given to people over 60 in Israel made them twice as resistant to Omicron infection as thrice-vaccinated people in the same age group.
Meanwhile, another study from Israel in January indicates a fourth dose of the coronavirus vaccine provides only limited defence against the highly transmissible Omicron variant.
The current booster shots have exactly the same formula as the first two doses, based on the original Wuhan strain of COVID-19.
There are looming questions about the value for healthy young individuals in getting an additional boost with an ancestral spike protein that may not exist anymore, Grant said.
“It will boost your antibodies somewhat, but it certainly doesn’t boost it selectively for Omicron and the new variants,” he said.
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As the virus mutates, Pfizer and Moderna are currently testing in late-stage clinical trials an updated version of their COVID-19 vaccines to better target the Omicron variant and its sub-lineages.
Both companies are hoping for their variant-adapted vaccine to be ready for approval in the fall.
This is another option to consider in the coming months.
So, should Canadians wait for the updated vaccine or get boosted now?
It’s not an either-or decision and anything you decide will depend on your individual risks, said U of T’s Khan.
“Both are wonderful options,” he said.
“Having one fourth dose of the current vaccine versus boost with the new version … you can do both. It’s not a problem.”
Piccirillo agreed, adding that the general healthy population can “probably wait it out” till the fall for whichever booster is available.
“It would probably be more strategic to have a fourth booster administered, preferably with a variant-adjusted vaccine, perhaps at the end of the summer, beginning of the next fall,” he said during the CITF webinar Thursday.
— with files from The Canadian Press and Reuters
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