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Federal agency recommends 4 month interval between two-dose COVID-19 vaccines

Last Updated Mar 3, 2021 at 8:34 pm EDT

A health-care worker prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a UHN COVID-19 vaccine clinic in Toronto on Thursday, January 7, 2021. THE CANADIAN PRESS/Nathan Denette

The National Advisory Committee on Immunization says provinces can wait up to four months before giving the second doses of COVID-19 vaccines.

The federal body says based on emerging evidence of the protection provided by the first dose of a two dose series of vaccines, NACI is recommending that “in the context of limited COVID-19 vaccine supply” the number of individuals benefiting from the first dose should be maximized.

RELATED: AstraZeneca vaccine will be distributed to people aged 60-64, province confirms

That is up from the three week maximum first suggested when vaccines from Pfizer and Moderna were first approved.

However, since then, provincial governments have extended the time between doses, from 21 to 42 days in Ontario, up to 90 days in Quebec and four months in B.C.

A spokesperson for Ontario’s Minister of Health says the government welcomes the updated direction.

“This will allow Ontario to rapidly accelerate its vaccine rollout and get as many vaccines into arms as quickly as possible and, in doing so, provide more protection to more people,” the spokesperson says.


Studies from Israel and the United Kingdom showed that a single dose of the Pfizer-BioNTech vaccine significantly reduced COVID-19 infections, helping to guide British Columbia’s decision to delay the second dose of vaccines by four months.

B.C. provincial health officer Dr. Bonnie Henry has said the plan is based on research in the two countries as well as evidence collected by the BC Centre for Disease Control and in Quebec.

A study published Feb. 24 by a team at the University of Cambridge in the U.K., which has not yet been peer-reviewed, suggests that a single dose of the Pfizer-BioNTech vaccine can reduce the number of asymptomatic COVID-19 infections by 75 per cent.

The team examined thousands of COVID-19 test results of unvaccinated and vaccinated health-care staff, who showed no signs of infection, at the university hospital over a two-week period in January.

The study found that 26 out of 3,252 tests, or 0.8 per cent, from unvaccinated workers were positive. In comparison, 13 out of 3,535 tests, or 0.37 per cent, from vaccinated staff were positive less than 12 days after immunization, and four out of 1,989 tests, or 0.2 per cent, from vaccinated workers were positive more than 12 days after the shot.

The findings suggest a fourfold decrease in the risk of asymptomatic COVID-19 infection among health-care workers who were vaccinated more than 12 days earlier, or 75 per cent protection.

The U.K.’s Joint Committee on Vaccination and Immunization said in December that the second dose of either vaccine could be given up to 12 weeks after the first dose.

Another recent study in the U.K. concluded that the Oxford-AstraZeneca vaccine is more effective when the second dose is delivered later. The research, published in peer-reviewed medical journal The Lancet on Feb. 19, found the vaccine is 81 per cent effective when its second dose is given three months after the first, compared with 55 per cent efficacy after six weeks.

In Israel, researchers studied the effects of a single dose of the Pfizer-BioNTech vaccine and published their findings in The Lancet on Feb. 18, determining that it was 85 per cent effective against symptomatic COVID-19 infections.

The study authors analyzed data from 7,214 health-care workers who were vaccinated at Israel’s largest hospital, the Sheba Medical Centre. They saw an 85 per cent reduction in symptomatic infections between 15 and 28 days after the first dose.

The manufacturers, for their part, provide different guidelines for the time between doses. Pfizer-BioNTech recommends three weeks, Moderna proposes four weeks and Oxford-AstraZeneca calls for eight to 12 weeks.

But the companies shouldn’t be the ones determining the appropriate interval, said Dr. Danuta Skowronski, an epidemiology lead at the BC Centre for Disease Control.

“We shouldn’t ask the manufacturer for their blessing in doing this. That’s not their role,” she said.

“It really is squarely the responsibility of public health authorities to do that full benefit/risk analysis for their population.”

Henry has said that the manufacturers completed their clinical trials according to a short timeline in order to get the vaccines to market as quickly as possible.

Skowronski led the B.C.-based research that underpinned the province’s plan, including the finding that a first dose of Pfizer-BioNTech and Moderna vaccines reduced the risk of COVID-19 among long-term care residents and health-care workers by up to 90 per cent.

She also examined Pfizer-BioNTech’s submission to the U.S. Food and Drug Administration and found that it underestimated the efficacy of its own first dose. The manufacturer said it was 52 per cent effective, but it included data from the first two weeks of the shot, a time when vaccines are typically ineffective.

When Skowronski cut the first two weeks of data, she found it was 92 per cent effective, similar to a first dose of the Moderna vaccine.

B.C. reported 542 new COVID-19 cases on Wednesday and seven new deaths, bringing the total number of fatalities linked to the virus to 1,372 in the province. It also said there have been 18 new cases of variants of concern, for a total of 200 cases.

Horacio Bach, an adjunct professor of infectious diseases at the University of British Columbia, said the province has enough evidence to back the four-month interval, though he believes it was the first in the world to announce such a long delay between doses.

He said people who receive their first shot now will still have a reserve of antibodies four months later. Even if one assumes four months is too long to wait, people will still have enough antibodies to prevent severe illness and hospitalization, he said.

“Four months, I don’t think, is something out of the box,” he said. “It’s a decent period of time that we can wait.”

Bach pointed out, for example, that the booster for the hepatitis B vaccine is delivered after a year.

Giving as many people as possible a first shot will help prevent transmission of the disease in the province, he added.

“The more people you vaccinate, the less chance you give to the virus to infect. When the virus cannot find a host, meaning a person, it will reduce the level of infection automatically.”

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