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

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

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

The data showthe 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 Modernahave 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.

In fact, according to Dr. Gaston De Serres of the Insitut national de sant publique du Qubec, 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 dosecould 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 andthe chair of the CIQ, said nothing in the data suggests first-doseimmunity weakens withtime, and that future research could show the preventive benefits of large-scale one-dose vaccination areeven higher than its clinical effectiveness.

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

Brousseau emphasized the numbers don't mean a second dose is no longerrequired. 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 doseinto as many arms as possible over the next two months.

Although De Serres says there is some evidence thePfizer-BioNTech and Moderna vaccinesprotect againstthe infectious variants discovered in the United Kingdom, South Africa and Brazil, new research from Pfizer indicates a booster may be needed to ward offthe South Africa variantbecause of a lowered immune response.

For the time being, De Serressaid, Quebec's data suggeststaying the course.

The results come on the heels of De Serresand 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 Medicinepublished on Wednesday. Their analysisfound an even higher level of protection from one dose after 14 days, roughly 92per cent.

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

He said De Serres and Skowronski have discussed their findings with theexpert 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.