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Is a single COVID-19 vaccine dose enough for those previously infected?

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As coronavirus vaccines continue to roll out to vulnerable populations across Canada, health officials are looking at data about the effectiveness of a single dose in preventing COVID-19 illness.

A new letter by two Canadian experts published in the New England Journal of Medicine (NEJM) last week stated that with a 92.6 per cent efficacy, the first dose of the Pfizer vaccine was “highly protective.”

Read more: Delaying second dose of coronavirus vaccines is ‘risky gamble,’ experts say

During a news conference on Feb. 18, Dr. Howard Njoo, Canada’s deputy chief public health officer, said that according to early data, the indicators are that there is a “good level of protection” after just one dose.

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France’s health authority, H.A.S., has gone one step further in recommending that everyone who has been previously infected with COVID-19 receive a single shot, instead of the two-dose regimen prescribed by vaccine makers Pfizer and Moderna.

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The recommendation made on Feb. 12 says the single booster shot should be given three to six months after COVID-19 infection.

The reasoning, according to H.A.S., is that people who have had a confirmed infection should be considered protected for at least three months by post-infection immunity, whether the disease was symptomatic or not.

“It is an interesting approach to take,” said Rowland Kao, professor of veterinary epidemiology and data science at the University of Edinburgh in Scotland.

“And you would expect that natural immunity will give you .. a more broad response (than the first dose) because it is the original virus that is causing it.”

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A spokesperson for H.A.S. told Global News that the French health minister has yet to make a decision on the recommendation. For now, France is giving two shots for both the Pfizer and Moderna vaccine.

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Amid shortages in vaccine supplies and a rush to control the pandemic, some experts say this strategy is worth considering as it could potentially save precious doses.

Read more: Johnson & Johnson one-shot vaccine is safe, prevents COVID-19, U.S. FDA says

Dr. Gerald Evans, chair of infectious diseases at Queen’s University in Kingston, Ont., said a single dose of vaccine in someone previously infected is “reasonable while we continue to have a short supply of vaccine globally.”

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Two small studies in the United States by Mount Sinai and the University of Maryland showed a single dose in people who had COVID-19 provided at least the same amount of protection as two shots in people who haven’t been infected. The data has not yet been peer-reviewed.

You could treat getting COVID-19 as like getting your first dose of vaccine,” said Dr. Zain Chagla, an infectious diseases physician at St. Joseph’s Healthcare in Hamilton.

A single dose could serve as a booster to get the “prime long-term response,” he told Global News.

“You could definitely save on vaccine supply with these mRNA vaccines by only giving those individuals a single dose moving forward.”

Some Canadian provinces have decided to delay giving the second dose, which some experts have called a “risky approach” and “a gamble.”

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Last week, New Brunswick health officials said the province will delay the second dose of the Pfizer-BioNTech vaccine for those who are considered to be at a lower risk.

In mid-January, Quebec announced that it was pushing the time between the two doses to a maximum of three months in an attempt to vaccinate more seniors faster with a first injection.

Vaccine manufacturers Pfizer and Moderna propose intervals of 21 and 28 days, respectively.

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In its recommendation for the previously infected, France’s H.A.S. says people who have proven immunosuppression, which makes them more vulnerable to severe COVID-19 illness, should be given the two doses.

It also says people who catch the virus in the days after a first dose is given should not receive a second shot within the usual timeframe, but within three to six months after infection.

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Read more: Booster shots, new clinical trials — What the COVID-19 variants could mean for vaccines

According to the data from the clinical trials, Pfizer’s vaccine, which is 95 per cent effective, can offer partial protection as early as 12 days after the first dose.

Kao said the immediate protection after the first dose and second dose is quite similar.

However, it still remains to be seen what the long-term immune response will be after the first dose.

We really don’t know how long that protection is going to last,“ said Kao.

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Data analysis by Canadian experts published in the NEJM found a 68.5 per cent vaccine efficacy beginning seven days after Pfizer’s first dose and a 92.6 percent efficacy two weeks after a single shot.

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Based on the evidence so far, Chagla says it is premature to roll out the single-dose strategy on a wide scale and that more research was needed on that front.

“If you could prove that works, you really do save a significant amount of vaccine … and you really can change your vaccine strategy almost overnight if you can implement something like that.”

— With files from Global News’ Linda Boyle

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