National Post (National Edition)
Study warns not to delay second Pfizer dose
With Canada receiving a relative trickle of COVID-19 vaccine, federal advisers have given the green light to a stopgap measure: delaying the second dose of the shots so more people can at least get the protection of one.
Indeed, most of the vaccine seems to have been administered that way so far.
But a new mathematical modelling study by Canadian and U.S. researchers, already facing criticism from other experts, suggests the strategy has its risks.
Putting off the second dose of the Moderna shot by up to 12 weeks to allow more people to receive one dose should lessen the amount of infection, hospitalizations and deaths, the study concludes.
But doing so with the Pfizer vaccine, whose first dose is less effective, could have the opposite result — more illness than if the followup doses were put in people's arms on schedule, the research suggests.
Both vaccines have what is considered excellent, 94 to 95 per cent protective power against symptomatic COVID disease after two doses. The issue is what happens if they are not given close enough together.
“Recent spikes in cases and hospitalizations in the United Kingdom, the United States and Canada have led to a public health conundrum,” notes the paper by mathematicians and epidemiologists at York University and the University of Toronto in Canada, and Yale and University of Maryland in the U.S.
That is: “Whether to vaccinate more individuals with the first dose of available vaccines and delay the second dose.”
Their answer to the conundrum, in a paper posted online but not yet peer-reviewed, was a mixed bag, depending on the product in question.
But two key Canadian experts questioned the findings Tuesday, suggesting the authors' conclusion about the Pfizer vaccine, which accounts for the majority of shots so far in Canada, is based on a faulty premise.
The paper's statement that the Pfizer vaccine is just 52-per-cent effective with the first dose only relates to the initial two weeks following the shot; after that period the first dose is more like 90-per-cent effective, said infectious-disease specialist Dr. Caroline Quach Thanh, chair of Canada's National Advisory Council on Immunization.
“Their underlying assumptions look to be incorrect,” echoed Julie Bettinger, a vaccine-safety scientist and another member of the advisory committee.
The committee has said that health authorities can consider delaying the second dose of both vaccines to as long as six weeks, as opposed to the recommended three weeks for Pfizer, and four weeks for Moderna, if necessary. The U.S. government also recently urged that clinics not hold on to doses for second shots.
Health Canada acknowledges there is a “high level of scientific uncertainty” around the protection offered when doses are spread further apart, but urges health officials to consult the advisory committee's recommendations, said Tammy Jarbeau, a department spokeswoman.
University of Ottawa health policy expert Amir Attaran, while not commenting directly on the new paper, said he has qualms about changing the recommended gap.
“In the absence of evidence for delaying doses — and neither vaccine's clinical trials furnished such evidence — it would be speculative and imprudent to deviate from the planned dosing schedule,” said Attaran, who has a doctorate in immunology. “The immune system is an often-mysterious black box that does not behave as predicted, and so humility before the data is best.”
As of Tuesday afternoon, 984,541 doses of vaccine had been administered in Canada, most of them single doses, according to the covid19tracker.ca website. Fewer than 120,000 Canadians have received the full two doses.
The new paper expands on an existing U.S. COVID-19 virus transmission model to consider the impact of delaying second doses versus giving them at the time recommended.
Postponing by nine to 12 weeks the followup shot of the Moderna product — thus allowing more people to get one dose — would result in as many as nine fewer hospitalizations and one fewer death per 100,000 population, they say.
Delaying the Pfizer second jab beyond three weeks would result in more infection and death, the researchers concluded.
However, the authors say if the pre-existing immunity in the population was high enough, delaying the second dose of even the Pfizer shot could save lives.
But clinical trials expert Ed Mills, an adviser to the Gates Foundation and parttime McMaster University professor, was dismissive of the model, partly because there is no previous data on COVID vaccination with which to verify the accuracy of the projections.
“This is all just made up,” he said. “It's as likely to be true as if you or I ask our kids for their predictions.”