Quebec's bet on delaying second dose of COVID-19 vaccine is paying off, early data show

Preliminary results show the current COVID-19 vaccines are 80 per cent effective after a single dose. Protection builds after two weeks in health-care workers and after three weeks in older, more vulnerable long-term care residents.

Preliminary results show strong immunity in health-care workers and long-term care residents after one dose

A medical worker loads a syringe with a dose of the Moderna COVID-19 vaccine in Los Angeles this week. Preliminary research data in Quebec show a single dose reduces the likelihood of contracting the disease by 80 per cent after 14 to 21 days. (Apu Gomes/AFP/Getty Images)

When the Quebec government confirmed in mid-January it would delay the second of two doses of the COVID-19 vaccines for up to 90 days, a debate erupted among scientists over the wisdom of the decision.

Now there is data to settle, or at least tamp down, the argument. Based on preliminary figures compiled by the Comité sur l'immunisation pour le Québec, it appears to have been the right move.

The data show the vaccines to be 80 per cent effective after 14 days in younger vaccinated populations (primarily health-care workers) and after three weeks among the residents of CHSLDs, who tend to be much older and sicker.

In other words, a vaccinated person has an 80 per cent lower likelihood of contracting COVID-19 than an unvaccinated person between two and three weeks after receiving the first shot.

Both Pfizer-BioNTech and Moderna have issued protocols on administering a second dose at a precise interval (21 and 28 days, respectively) based on clinical studies. That's also the basis on which they've been approved by Health Canada.

But Quebec's early numbers, which the CIQ said align with results observed in British Columbia and Israel, suggest it's eminently defensible to delay the booster shot longer in the context of a vaccine shortage.

Gaston De Serres, an epidemiologist at the Insitut national de santé publique du Québec, says single-dose immunity from the COVID-19 vaccine can reach 80 per cent effectiveness after 14 days. (CBC News)

In fact, according to Dr. Gaston De Serres of the Insitut national de santé publique du Québec, who sits on the CIQ and co-authored the study made public on Thursday, it was never much of a gamble given the drug companies' published clinical results and the existing scholarship on immune responses from vaccines.

"It wasn't a very big bet, with everything we know about how vaccines work. We know that's how it is. Why would these vaccines be so different?" said De Serres.

"Some people were worried because there isn't a lot of evidence as to how long protection lasts … to go outside of what is recommended by the manufacturer always looks risky, but again, the data the committee based its recommendations on were based not only on clinical trials but the overall knowledge of how the vaccines we've been working with for years work."

Pfizer's clinical trial data indicated the first dose could lead to 90 per cent effectiveness, and while the real-world number observed in Quebec falls short of that, it's still "very encouraging" in his eyes and indicates "there is no particular urgency" to administer second doses at the moment.

In fact, De Serres said, waiting could make boosters even more effective.

Dr. Nicholas Brousseau, an INSPQ researcher and the chair of the CIQ, said nothing in the data suggests first-dose immunity weakens with time, and that future research could show the preventive benefits of large-scale one-dose vaccination are even higher than its clinical effectiveness.

"It's very good news," he said.

Brousseau emphasized the numbers don't mean a second dose is no longer required. It absolutely is, he said, just not immediately.

He also cautioned that the rise of variant strains could prompt a re-think in the current strategy, which is to get one dose into as many arms as possible over the next two months.

Although De Serres says there is some evidence the Pfizer-BioNTech and Moderna vaccines protect against the infectious variants discovered in the United Kingdom, South Africa and Brazil, new research from Pfizer indicates a booster may be needed to ward off the South Africa variant because of a lowered immune response.

For the time being, De Serres said, Quebec's data suggest staying the course.

The results come on the heels of De Serres and one of his B.C. counterparts, Dr. Danuta Skowronski, presenting their examination of U.S. data in a letter to the New England Journal of Medicine published on Wednesday. Their analysis found an even higher level of protection from one dose after 14 days, roughly 92 per cent.

In Ottawa, the federal government's deputy chief public health officer, Dr. Howard Njoo, who initially expressed reservations concerning the lack of data supporting the Quebec approach, described the evidence as compelling.

He said De Serres and Skowronski have discussed their findings with the expert advisory committee that makes policy recommendations for national vaccination guidelines.

"The questions asked by the authors, I think, are legitimate ones," Njoo said, pointing out that the potential shortfall in effectiveness by only administering one dose in the short term could be offset by the benefit of vaccinating a larger number of Canadians,

"All of that information is being looked at very carefully … those deliberations and discussions are very alive and ongoing right now," he said.

with files from The Canadian Press and Lauren McCallum


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