The long shot: Why waiting 90 days for a second vaccine dose might be a great idea

The news that Public Health will delay giving the second COVID-19 vaccine dose to some New Brunswickers for as long as 90 days seemed shocking when it was revealed at Thursday's live update.

One day later, epidemiologists, researchers and even Canada's top doctor are weighing in with new findings and factors, and suddenly it doesn't seem so shocking anymore. 

In fact, some argue, it might be a very good idea.

On Thursday, during the rollout of the province's revised vaccine plan, Chief Medical Officer of Health Dr. Jennifer Russell said the next phase, in April and May, will see a range of new groups immunized.

• Trudeau says vaccine 'big lift' is coming, as Tam warns of variant threats
• Province to delay second shot for some so more people can get at least one dose

But, she said, those at low risk of severe illness may not get the second booster shot for up to 90 days. 

The strategy is a calculated risk that contradicts the vaccine manufacturers' own recommendations — Pfizer-BioNTech and Moderna both recommend a second booster dose within 28 days of the first dose — and more than doubles the 42-day wait time some other provinces have moved to.

Russell herself acknowledged Thursday that the approach is "not perfect" and carries some unknowns.

However, she noted, in the face of limited vaccine supply and the growing threat of variants, it accomplishes a crucial goal: maximizing protection by getting first shots to as many people as possible. 

'We are facing an emergency'

That's a compelling argument in the 90-day wait time's favour, according to Ontario epidemiologist Raywat Deonandan.

"If you'd have asked me two months ago, I would have said that this is inadvisable," Deonandan said in an email Friday.

"However, we are facing an emergency. The threat of the new variants is real. If we are to either avoid a third wave, or mitigate its impact, then we should get as much immunity into as many people … as quickly as possible."

That, he said, means using "all the doses we have available."

He also echoed Russell's acknowledgement of "unknowns" — chiefly, slippery evidence related to the length of time a first dose would provide protection.

That could be a problem if, for example, a second dose is not available for more than 90 days after the first dose was given.

In that case, Deonandan said, immunity might fade and the person "would once again be susceptible and wouldn't know it."

It could also mean the entire portion of the population that didn't receive their second dose might need to start the two-dose regimen all over again. 

All things considered, that still doesn't tip the balance against the 90-day delay.

"I don't know of any real harm in receiving a third or fourth booster," Deonandan said.

"So one can imagine a scenario where we get all the shots into many arms now and then later in the year, when the disease is not raging across the country, people can casually go to their drugstore for a quick booster."

First dose's power might have been underestimated

In Newfoundland and Labrador, which is currently in the thick of a variant-fuelled outbreak, Rodney Russell noted New Brunswick's 90-day-delay strategy with interest.

Russell, a COVID-19 antibodies researcher and professor of immunology and infectious diseases at Memorial University of Newfoundland, said there are increasing suggestions that the first vaccine dose might be a more powerful weapon than initially suspected.

"The reason you give the booster is literally to boost the immunity, it gives your immune system another taste of the bug and so basically your body says, 'OK, this is not a one-off,'" Russell said. "But the first shot will induce some immunity."

That might be really good immunity, or it might be partial immunity, he said.

"But what we are seeing now … and these are very early reports … is that even the first dose is enough to keep most people from getting very sick."

Although the vaccines' own manufacturers, Pfizer and Moderna, advise a booster shot within 28 days, Russell noted the fast-moving nature of the pandemic, and the science related to it, have already changed the landscape since the original trials.

"The original trials were based on two doses, and original data indicated you'd get a much better response to vaccines after a booster," he said. "And that's all the time they had to try, they didn't have an opportunity to try every possible timeframe or distance between doses."

But now that the real-time data is coming out, "and that's what matters," Russell said, if it increasingly proves that high percentages of people are protected with a single dose, then that changes the situation — especially if vaccines are limited.

"We're all kind of winging it and new data is coming on a global, almost daily basis," Russell said. "I've never experienced science at that pace."

One might be almost as good as two: study

Earlier this week, two Canadian researchers said in a letter published in the New England Journal of Medicine that a single dose of the Pfizer-BioNTech vaccine might be almost as good as two.

They found that the Pfizer-BioNTech vaccine has 92.6 per cent efficacy after just one dose. Another letter to the same journal said Moderna's first dose has 92.1 per cent efficacy.

A second dose only increases Pfizer's efficacy to 94.8 per cent, according to the researchers, Dr. Danuta Skowronski of the British Columbia Centre for Disease Control and Dr. Gaston De Serres of the Institut National de Santé Publique du Québec.

"The benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose," they wrote.

The letter doesn't mention any ideal time period for a second dose.

On Friday, Canada's top doctor said she was "very optimistic" about the findings, particularly in light of the fact that  Canada's supply of vaccines will be comparatively small over the coming months.

"For the next few months, we're not going to have a lot of people vaccinated," Chief Public Health Officer Dr. Theresa Tam said. "That's just a fact."

However, she said, "there are studies now beginning to emerge both abroad and in Canada that just that initial dose packs quite a punch."