MONTREAL -- Quebec could change its vaccine strategy based on new data out of Israel about the efficacy of the first dose, on its own, of the Pfizer COVID-19 vaccine, says a top adviser in the province.

Israel just provided the world with its first large-scale, real-world hint of how effective the first dose of the Pfizer vaccine is before the booster, and it doesn’t seem reassuring for places that have delayed the second shot, including Quebec and the United Kingdom.

“We not only monitor the data that comes from Quebec but also what is observed around the world,” Dr. Gaston De Serres, a chief adviser on Quebec’s vaccine strategy, told CTV News.

“Yes, we are looking at the data from Israel and the [Quebec immunization committee] could make recommendations based on this data if necessary," he said.

Data on 200,000 elderly Israelis suggests that the first shot alone only lowered infections by 33 per cent—about a third of the roughly 90-per-cent rate that many experts around the world have predicted.

It’s “concerning in terms of the single-dose policy decision,” said a U.K. scientist, John Robertson, who had previously written about his concerns about the U.K.’s decision, like Quebec’s, to delay booster shots.

Importantly, in terms of understanding the new data, Israel is not delaying boosters. It’s following the timeline set out by Pfizer and giving the second, or “booster,” shot 21 days after the first. 

That means the Israeli numbers pertain only to the short time when the elderly people in question had gotten their first shots but hadn't yet gotten boosters. The numbers suggest that even when giving the shots on schedule, the vaccinees didn’t have nearly the protection that was predicted in that short pre-booster interval.

The data doesn’t call into question how well the two doses together work. Pfizer's trial data showed that together, the doses are 95 per cent effective.

But the Pfizer trial wasn’t meant to prove the efficacy of the first dose alone, so the estimates on how well it works without the booster have been just that—estimates—with scientists looking back at the data and trying to gauge whether delaying the second shots will work.

Delaying the boosters, as Quebec is doing for up to 90 days, is meant to give more people a first shot and some heightened, if imperfect, immunity.

The Israel data doesn't help with a bigger uncertainty in places like Quebec: whether, and how much, that first-dose protection could last after the 21-day mark if the booster isn't given. Pfizer says its trial provided no data on this, and the Israel numbers can't fill that gap either.

On Thursday, Quebec's chief public health officer, Dr. Horacio Arruda, said he’s also following the news from Israel but is waiting to read the written results, when they're available, before acting on them. He said he'll pay special attention to the vaccine's effects on people of different ages.

"We will follow it and if there is some data showing that the efficacy is too low... we're going to change [the approach]," Arruda said.

So far all the reports out of Israel are from experts there speaking to media, rather than published findings. A Montreal infectious diseases expert, Dr. Matthew Oughton, also said Thursday he's waiting for written data.

"There’s no manuscript yet to actually analyze," Oughton said.

ISRAEL'S FINDINGS SO FAR

Israel has moved very quickly on vaccination, inoculating 2.2 million Israelis over the last month. It made an agreement to get rapid delivery of the Pfizer-BioNTech vaccine in return for tracking the effects and sending the manufacturer detailed data.

Two Israeli experts have spoken about the results in recent days.

According to Israeli news channel i24 News, the leader of the country’s vaccine drive, Nachman Ash, told Israeli Hebrew-language outlet Army Radio that "many people have been infected between the first and second injections of the vaccine," and that it was "less effective than we thought.”

Ran Balicer, an Israeli doctor and epidemiologist, and an adviser to the World Health Organization, spoke to the UK outlet Sky News, explaining more about what was found.

"We compared 200,000 people above the age of 60 that were vaccinated,” the outlet quoted Balicer as saying.

“We took a comparison group of 200,000 people, same age, not vaccinated, that were matched to this group on various variables,” he said. 

Scientists then compared the daily rate of positive COVID-19 cases between the two groups. They found at first, unsurprisingly, there was no difference in the first two weeks after the shot—the vaccine takes about two weeks to kick in.

After that, starting at 14 days post-vaccination, “a drop of 33 per cent in [positive cases] was witnessed in the vaccinated group and not in the unvaccinated,” Balicer told Sky News.

He called it “really good news,” considering the group did have much more protection than their unvaccinated peers. 

SHORT OF ESTIMATES, THOUGH MANY QUESTIONS

However, that number fell far short of the estimate in recent weeks: Dr. De Serres in Quebec, as well as the UK vaccine advisory committee and many other experts, had all said they believed the first shot would be about 90 per cent effective, at least for several weeks, allowing them to delay the booster.

Pfizer has maintained that its trial data only showed a rate of 52.4 per cent efficacity before the second shot and that it knows nothing about what would happen past 21 days.

One question remains around how well the single dose worked to help people fight off serious infections, even if they tested positive for the virus—a key measure. On Wednesday afternoon, Israel's Minister of Health said Ash's comments had been taken "out of context" on this.

The minister clarified that Ash had been discussing how Israel "[has] yet to see a decrease in the number of severely ill patients," not infections, according to the BBC.

And Balicer suggested the surprise in Israel's data may have come partly from the fact that those studied so far have all been elderly, whereas Pfizer's trial subjects were a mix of ages. The immune systems of the elderly aren’t as strong as those of younger people.

Balicer said he expects the Israeli numbers to rise once more young people are included in the group studied.  

He also said that real-world data is not the same as trial data—and on the upside, Israel’s data proves beyond a doubt that the vaccine does work, and on the same kind of timeline the Pfizer trial showed.

"This is not the ideal setting of a randomized controlled trial where everything from coaching maintenance to selection of the population of interest is done in a very meticulous way,” he said.

“This is the real world. And so by seeing the real-world impact so early on in the same direction and in the same timing as we've seen in the clinical trials is something that makes us very hopeful.”

According to the BBC, Balicer also said that after the first 33-per-cent drop in infections, the rate of cases continued to drop—meaning immunity appeared to keep growing stronger, in those vaccinated with the first dose—but it was too soon to know more.

QUEBEC URGED TO TAKE A SECOND LOOK

Robertson, a professor of surgery at the University of Nottingham, said Wednesday that he thinks the Israeli results provide strong evidence for Quebec and similar jurisdictions to change course if they’ve delayed second doses.

Earlier this month, Robertson co-published an opinion piece for the BMJ British medical journal arguing that delaying the second dose wasn’t based in firm science.

“The personal and population risks have even greater relevance and urgency for Quebec given the real-life data reported from Israel,” he said Wednesday.

“The second dose should be given on Day 22 as in the Phase 3 trials and approved by regulatory agencies worldwide.”

Pfizer said it has no comment yet on the new data and can only speak about the results of its Phase 3 trial.

--With files from CTV's Emily Campbell