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Dr. Kieran Moore, Ontario's Chief Medical Officer attends a media briefing in Toronto on Nov. 29, 2021. Ontario reported more than 2,400 COVID-19 cases on Thursday.Chris Young/The Canadian Press

Ontario’s decision to rapidly accelerate its COVID-19 booster shot program to anyone 18 and over who received a second dose three months ago is a reflection of the seriousness of Omicron and how quickly the variant is upending existing policy designed to contain the virus’s spread, health experts say.

Until now, provinces have been offering booster shots at least six months after an individual’s second COVID-19 vaccine. In most parts of Canada, booster shots have only been available to people at higher risk, including seniors, front-line health care workers and people with compromised immune systems.

This month, Canada’s National Advisory Committee on Immunization issued guidance to provinces that suggested boosters should be given to people 50 and older at least six to eight months after their most recent COVID-19 vaccine. That guidance is based on research showing that immunity starts to wane over time. The guidelines further stated that people 18 to 49 could be offered a booster shot, depending on individual circumstances, such as underlying health conditions or rates of community transmission of the virus.

But the arrival of the Omicron variant last month has changed the rules, prompting officials in hard-hit Ontario, which reported more than 2,400 cases Thursday, to come up with a new game plan to stop a rapid surge in infections. That means people 18 and older can sign up for a third dose, as long as their second dose was administered at least three months earlier.

One of the main reasons experts are so concerned about Omicron is the variant has numerous mutations that allow it to evade current vaccines, meaning it can readily infect people who have received two doses of an mRNA COVID-19 vaccine. While research suggests doubly vaccinated people will still have significant protection against severe outcomes, they will be able to spread the virus to others, which will eventually lead to a surge in hospitalizations.

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Ontario’s COVID-19 Science Advisory Table wrote online this week that people who get infected with the variant pass it on to an average of four others. Cases of Omicron in the province are doubling every three days, meaning an unprecedented surge in infections is about to occur.

The good news is that research shows booster shots restore an individual’s immunity to Omicron, which is why officials in Ontario are suddenly racing to get as many into arms as they can.

This week, Canada’s Chief Public Health Officer, Theresa Tam, said the situation in Ontario is a harbinger for what’s to come in other provinces, suggesting officials elsewhere should also aggressively roll out boosters and take other steps to contain the spread.

Joanne Langley, a vaccine researcher and pediatric infectious-disease specialist at the IWK Health Centre in Halifax, said it’s not unheard of to shorten the interval for COVID-19 booster shots, pointing out that Britain has used a three-month interval throughout its campaign.

“People are using different intervals,” she said.

So far, other provinces are sticking with the six-month timeline for offering boosters.

This week, B.C. Health Minister Adrian Dix said booster shots will be available to people 18 and older once six months have passed since their previous shot. The province is sticking with that timeline because that’s what the science says, Mr. Dix said.

But the science has changed so quickly in the past few weeks that it’s incumbent on health officials to re-examine options to contain the spread, said Matthew Oughton, an attending physician in the division of infectious diseases at the Jewish General Hospital in Montreal.

“Omicron has changed the risk-benefit analysis to favour getting a third dose sooner rather than later,” Dr. Oughton said.

While stretching intervals between vaccine doses is typically preferable, as it’s linked to stronger and longer-lasting immunity, in this case, it makes sense to compress the timeline because of this emergent threat, Dr. Oughton said.

Dr. Langley in Halifax said it makes sense to change policy to adjust to the risks on the ground.

“The stakes are different,” Dr. Langley said. “With another wave staring you in the face and the risks to the population, it could be justifiable to use a shorter interval.”

Dr. Oughton added that while people who have received two doses may be more susceptible to COVID-19 infection, protection against severe illness is likely much stronger and lasts longer, particularly in young, healthy people.

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