Evidence supporting the COVID-19 vaccine’s effectiveness weeks after its first injection is piling up, Canada’s top public health official says.
On Tuesday, Dr. Theresa Tam called the figures “incredible,” and said “over two months of data and beyond” showed sustained efficacy among COVID-19 vaccines even after a “significant number of weeks.”
The news comes as provinces and territories grapple with whether to lengthen the window between first and second shots.
Tam’s comments echo a number of studies that have emerged over the last few months that experts have pointed to as justification to delay the second dose in an attempt to reach herd immunity faster.
Modelling headed by Saskatchewan’s Dr. Graham Jurgens and German modeller Kyle Lackner released Tuesday estimated delaying the second dose four months would help population in high density areas reach herd immunity between five and six weeks sooner, with an average population immunity about 20 per cent higher, relative to standard three or four-week spacing.
A re-analysis of Pfizer’s clinical trials published in the New England Journal of Medicine last month, in which two Canadian researchers said the results showed a “highly protective” efficacy of 92.6 percent beginning two weeks after the first injection.
Another study analyzing the long-term effects of a single dose of the AstraZeneca vaccine found delaying the second shot by 12 weeks or more still resulted in an efficacy of 81.3 per cent.
The news comes as the National Advisory Committee on Immunization assesses the time intervals between the first and second doses of authorized vaccines in Canada, including Pfizer-BioNtech’s, Moderna’s and AstraZeneca’s, which was approved on Friday.
Tam said the committee is expected to provide recommendations this week.
Several provinces have already extended the window between the first and second doses in order to fast-track the number of Canadians receiving their first dose of the COVID-19 vaccine.
The largest thus far is in British Columbia, which announced it would increase the gap from up to 28 days to 112 on Monday.
Colin Furness, an epidemiologist with the University of Toronto, called the move “aggressive,” adding that it was both “dangerous” and “risky” to veer off from recommended timeframes.
“When the vaccines were validated or tested, they were tested according to a certain schedule,” he said. “When you lengthen it, you go into uncharted territory.”
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Without enough concrete evidence to fully understand the effects of delaying doses, Furness warned of several things that could happen.
Waiting several extra weeks to administer the second dose could lower the vaccine’s effectiveness, he said, or the body might shut down its immune response after so long and produce a completely different effect. Or it could make the vaccines even more effective than anticipated.
According to Furness, all options are possible until the vaccine’s long-term effects can be properly studied.
“We’re trying to make COVID go away. If getting everyone a single shot faster makes COVID go away faster, you actually end up with a better outcome,” he said.
“The reasoning is quite clear why to delay the second shot. The question, though, is, does that then mean that we need a third or fourth?”
The race to vaccinate Canada’s population has been ramping up, made more urgent by the emergence of COVID-19 variants across the country.
As of March 1, Tam said health officials had detected 1,351 cases of the virus variants, with 1,257 cases of the B.1.1.7 strain first discovered in the U.K. accounting for the bulk of confirmed infections. No province has been spared.
However, she added that Canada was “gaining more ground every day” with the authorization of more vaccines.
“We now have (three) different COVID-19 vaccines with unique advantages, but all contributing to the reduction of severe COVID-19 illness and death in Canada,” she said.
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