Health

Pfizer-BioNTech vaccine shots can be given up to 6 weeks apart, WHO says

World Health Organization experts on Friday issued recommendations that the interval between administration of two doses of the Pfizer-BioNTech vaccine against the coronavirus can be extended to up to six weeks.

While WHO recommends 21-28 day interval, the goal is being complicated by supply, distribution issues

A health-care worker prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a COVID-19 vaccine clinic in Toronto on Thursday. The World Health Organization said countries need to be pragmatic with vaccine supply, and that available clinical data for the Pfizer-BioNTech vaccine suggests the interval between shots can be expanded. (Nathan Denette/The Canadian Press)

World Health Organization experts on Friday issued recommendations that the interval between administration of two doses of the Pfizer-BioNTech vaccine against the coronavirus can be extended to up to six weeks.

WHO's Strategic Advisory Group of Experts on immunization, known as SAGE, formally published its advice after a full review of that vaccine, which is the first to get emergency approval from the United Nations health agency to fight the COVID-19 pandemic. It said an interval of 21 to 28 days between the doses is recommended.

But the United Nations health agency also noted that "a number of countries face exceptional circumstances of vaccine supply constraints combined with a high disease burden," and said some have been considering delaying the administration of a second dose as a way to broaden initial coverage.

The agency said this "pragmatic approach" could be considered as a response to "exceptional epidemiological circumstances."

"WHO's recommendation at present is that the interval between doses may be extended up to 42 days, on the basis of currently available clinical trial data," it said, adding: "Should additional data become available on longer intervals between doses, revision of this recommendation will be considered."

Hard-hit Britain, for example, has decided to delay for as much as 12 weeks — and data from that expansion could help contribute to possible revisions in the WHO recommendation, said WHO spokeswoman Dr. Margaret Harris.

Dr. Isaac Bogoch, an infectious disease specialist and a member of Ontario's vaccine task force, told CBC News on Friday that, ideally, the three- to four-week schedule is followed. Bogoch brought up the fact that mutations of the virus could potentially hinder vaccine efficacy if longer intervals become the norm.

"Changing the dosing schedule shouldn't be a rescue measure for not having administered all your vaccines, so that should be the priority: as many doses in as many arms in a short a period of time as possible…," said Bogoch.

"If it's extended a little bit, it ain't the end of the world, as long as people got those two doses."

Dr. Theresa Tam, Canada's chief public health officer, said Tuesday there is some promising evidence that single doses of vaccines designed to be given in two shots are effective for a while, but that evidence is limited.

Dr. David Fisman, an epidemiologist in Toronto, told the Canadian Press he suspected the six-week window wasn't used in the COVID-19 trials because this is a public health emergency.

"For many vaccines a second dose is given at six weeks and that works nicely," he said, noting sometimes the "boost" from the second dose is actually better if it's given a little later.

WATCH l Grappling with the question of vaccine priority at long-term care facilities:

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A geriatrician and an infectious disease specialist answer questions about how COVID-19 vaccines are being distributed in long-term care homes including if the first doses should have gone to residents instead of health-care workers. 5:44

In the recommendations, WHO added that countries seeking to extend the interval should make sure that vaccinated patients can still have access to a second dose.

The agency also said it also does not recommend COVID-19 vaccination of travellers unless they face high risks or qualify as priority cases.

READ l WHO on vaccine intervals:

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WHO also said that symptomatic reinfection by the coronavirus within six months of a first infection is "rare," so people who have had the illness within the previous six months "may delay vaccination until near the end of this period."

WHO said there was currently no evidence on the need for a booster dose, and said there was no data available on the interchangeability of the Pfizer-BioNTech vaccine with other COVID-19 vaccines. It also cited a lack of evidence about whether vaccination reduces the risk of transmission of the virus to other people.

With files from CBC News and The Canadian Press

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