Canada's COVID-19 Immunity Task Force co-chair explains why she now backs 2nd dose delay

The co-chair of Canada's COVID-19 Immunity Task Force says she's now "very much in favour" of delaying the second dose of COVID-19 vaccines for shots that must be administered under a two-dose regimen.

It's a relatively new reversal for Dr. Catherine Hankins, who told CBC Radio's The House that she resisted the idea of extending the interval between first and second doses as recently as January.

"I didn't feel that we had the data," Hankins told host Chris Hall. "But the population, real-world data coming in from the U.K., from Israel and even from B.C.and Quebec are convincing to me."

On Wednesday, Canada's National Advisory Committee on Immunization (NACI) recommended that the maximum interval between the two doses should be stretched to four months to increase the number of people receiving their first shot.

The committee previously recommended that the maximum interval between Pfizer-BioNTech doses should be three weeks, a number that climbed to four weeks for the Moderna vaccine and 12 weeks for the AstraZeneca-Oxford product. The newly approved Johnson & Johnson shot is a single-dose vaccine.

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The updated guidance comes from findings from two clinical trials examining the efficacy of the Pfizer and Moderna vaccines after a single dose, as well as population health data from several countries and two Canadian provinces on how well those shots performed after being administered once.

While the population studies yielded lower efficacy results after one dose than the clinical trials, NACI said the difference was expected given that vaccine effectiveness tends to be lower in the general population than it is under the controlled setting of a clinical trial.

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One dose still confers benefits and reduces transmission

Hankins said it was the population health data that convinced her to change course.

"Basically, it's showing that you have sustained protection ... for two months, in terms of reducing hospitalizations and deaths, including against this B117 variant that was first identified in the U.K.," she said.

"So you have those benefits to the individuals that get the single dose, but you have then this additional benefit that they showed in Israel, that people who get infected after they've had the vaccine ... are much less likely to transmit to other people."

Hankins, who is also a professor of public and population health at McGill University in Montreal, cautioned that the new recommendation does not mean people should forgo the second dose altogether.

"What's clear is we can offer more individuals the direct benefit that [a] single dose will bring, plus the indirect benefit of reduced transmission that will avert hospitalizations and deaths for people who don't yet have the vaccine. So to me, it's a win-win situation."

Before NACI released its guidelines, British Columbia had already moved to implement a four-month gap in administering doses. A number of other provinces have now extended their dose intervals to widen initial rollout efforts.

Science continues to evolve

The decision has its critics Canada's chief science adviser, Mona Nemer, on Monday called B.C.'s plan a "population level experiment," a comment Provincial Health Officer Dr. Bonnie Henry said was "unfortunate."

Nemer told CBC's Power & Politics that data from Pfizer and Moderna is based on first and second doses being spaced weeks apart, rather than months.

Hankins said she understands the confusion that can arise from mixed messaging,but noted thatthe evidence is clear that one dose can confer a degree of protection and help curb chains of transmission.

"I would say this is a very pragmatic public health policy decision," she said. "It'll be monitored very, very closely, both in terms of breakthrough infections, variants and how they're being dealt with, etc."

And that means scientific guidance could change again.

"I think it's important for the public to realize that we are gathering data all the time and it's helping inform decisions, and they should expect there to be changes as we go forward," Hankins said.