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Coronavirus: Experts say fourth doses not needed yet

Many jurisdictions have extended fourth-dose boosters to their most vulnerable populations as the roll-out of the third-dose COVID-19 vaccine in Canada accelerates, leading some to wonder if we will all need a another injection to protect us from the virus in the near future.

Prime Minister Justin Trudeau said on Monday the country will have enough third and fourth doses for all eligible Canadians – if or when they are needed – with contracts signed until 2024 with Pfizer-BioNTech and Moderna.

Toronto-based microbiologist and infectious disease consultant Dr. Allison McGeer said Canada is not at the point where fourth doses are needed for the general population.

But will we get there somewhere down the road? May be.

“I don’t know where that point is,” she said. “We know that with other vaccines, it sometimes takes three doses or four doses to achieve a prolonged and stable effect.

“But really, the answer is we’ll just have to see how the protection goes and decide on that basis if there’s value in further doses.”

Canada’s National Advisory Committee on Immunization (NACI) has previously recommended that people with moderate or severe immunosuppression receive a fourth dose at least six months after their third injection.

Many jurisdictions began offering fourth doses to people with compromised immune systems last month or earlier this month, including British Columbia, Saskatchewan and Manitoba, while Ontario recently added nursing home residents. long term to this eligibility list.

Newfoundland and Labrador’s Chief Medical Officer of Health said on Wednesday that the province has also started offering a fourth dose to people with compromised immune systems.

McGeer said the measures follow evidence that antibody levels tend to decline more quickly in older people, adding that long-term care residents also face the downside of being in high-pressure environments. risk of spread.

“As you get older and more fragile, you respond less well to vaccines. It’s a general truth,” McGeer said.

“So in this group of people, in the midst of Omicron, there’s a good argument that giving them a fourth dose to get their levels to where the others are with a third dose will offer more protection.”

Israel is offering fourth doses to healthcare workers and anyone over 60 as the country grapples with skyrocketing COVID-19 cases.

Hospitalization rates remain low in Israel, although data shows that weekly intensive care admissions have increased recently.

McGeer said data from Israel suggests protection may wane in older people around three to four months after their third dose.

Many provinces in Canada initially allowed people to receive third-dose boosters six months after their second injection, but shortened that time to three months amid the rapid rise in Omicron.

McGeer said that longer intervals between doses are generally better with most vaccines.

“But… if there is a lot of (viral) activity, you might be willing to trade off a longer term of protection for a better response now,” she said.

“Faced with an Omicron wave… it seemed better to (go with a) shorter interval.”

Another factor complicating a future fourth-dose deployment is the possibility that an Omicron-specific vaccine may soon emerge.

Pfizer recently said that an updated vaccine could be ready by March, but McGeer said a new vaccine would need to be tested and likely wouldn’t be ready for the public until about June.

She added that it’s still not clear if a variant-specific jab would be helpful.

“We don’t necessarily want to give up protection (against) other variants because we don’t know what the next variant will be,” McGeer said.

Earl Brown, an immunologist at the University of Ottawa, said the development of variant-specific vaccines could be particularly difficult with the COVID-19 virus, which he described as “a moving target.”

It’s not the same as scientists tweaking the flu vaccine every year.

“If you had made a Delta vaccine, it would come in now and… you would have missed the mark because it’s not a Delta story anymore; it’s an Omicron story,” he said.

“It’s quite difficult with moving targets and so far these variations are happening quickly.”

Dr Zain Chagla, an infectious disease expert in Hamilton, said a large fourth dose is a flawed strategy, especially if it diverts supplies from low-income countries where many people have not yet received their first or second vaccine. He adds that three doses show strong protection against serious illness with Omicron.

“We thought we were clear giving two doses… things were going well (when) we were dealing with Delta and Omicron changed everything in three weeks,” he said.

“If we don’t take care of those in the rest of the world, if we don’t stop this virus from growing … It is really going to have serious effects on us.”

This report by The Canadian Press was first published on January 13, 2022.