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As Israel battles a new surge in coronavirus cases, its health ministry said on Thursday that although the efficacy of the Pfizer-BioNTech vaccine is high against serious illness, its protection against coronavirus infection may have decreased significantly compared to this winter and early spring. .

Researchers who analyzed the government’s national health statistics estimated that the Pfizer shot was only 39 percent effective at preventing infection in the country in late June and early July, compared to 95 percent from January to early April. However, in both time periods, the vaccine was more than 90 percent effective at preventing serious illness.

Israeli scientists cautioned that the new study is much smaller than the first and measures cases over a narrower timeframe. As a result, a much wider range of uncertainty surrounds its estimates, which can also be skewed by a variety of other factors.

Head of Israel’s Covid-19 National Expert Advisory Panel, Dr. Ran Balicer said the challenges in making accurate estimates of vaccine efficacy are “huge”. He said more careful analysis of the raw data is needed to understand what’s going on.

“I think the data should be taken very carefully because of the scarcity of numbers,” said Eran Segal, a biologist at the Weizmann Institute of Science, who advises the Israeli government on vaccines.

However, the new estimates raise concerns that the vaccine may be losing some of its effectiveness, both in Israel and elsewhere, including the United States. Possible causes include the rise of the highly contagious Delta variant or decreased protection from vaccines over time.

Israel launched an aggressive campaign with the Pfizer vaccine in January, and the country has achieved one of the highest vaccination rates in the world, with 58 percent of the population fully vaccinated. At the start of the campaign, government researchers began estimating how much the shot reduced people’s risk of contracting Covid-19.

Them published Based on records from January 24 to April 3, they estimated their results for May to be 95 percent effective in preventing coronavirus infection in the country. In other words, the risk of contracting Covid-19 in vaccinated people decreased by nearly 100 percent compared to unvaccinated people. Researchers also estimate that the vaccine is 97.5% effective against serious diseases.

Cases have fallen, from a peak of more than 8,600 cases per day in January to just a few dozen people testing positive daily across Israel in the months that followed. Vaccination likely played a role in this decline, along with strict government restrictions on travel and meetings.

Israel began to relax its restrictions in the spring. At the end of June, cases increased again. Now, with more than a thousand people testing positive every day, Israel is returning some restrictions this week.

In the new surge, some people who tested positive for coronavirus were fully vaccinated. Epidemiologists expected such breakthrough infections, as with all vaccines.

Researchers at the Department of Health took another look at the vaccine’s effectiveness, limiting their analysis to the increase from June 6 to July 3. 64 percent.

More recently, they did another analysis. This time, they looked at cases between June 20 and July 17. During this period, they estimated the effectiveness of the vaccine was even lower: only 39 percent against infection.

Still, they estimated the vaccine’s efficacy against serious diseases remained high, at 91.4 percent.

If a vaccine is 39 percent effective, it does not mean that 61 percent of people who get the vaccine are infected with the coronavirus. Instead, it means that vaccinated people are 39 percent less likely to become infected than unvaccinated people. So even at this lower percentage, the data show that vaccinated people are significantly less likely to become infected than unvaccinated people.

The small number of people in the last study means the actual efficacy may be lower or higher. What makes the numbers even more uncertain is that the new surge has not yet spread evenly across the country. Travelers who found the highly contagious Delta variant brought it back to neighborhoods where vaccination rates are relatively high.

The new outbreaks have yet to suffocate Orthodox Jewish or Arab Israeli communities where vaccination rates are lower. This imbalance can make the vaccine appear less effective than it actually is.

In addition, the ages of vaccinated persons varied considerably over the different time periods studied. For example, those who got their vaccinations in January differed in one important respect from those who got their vaccinations in April: they were over 60 years old. the vaccine itself, but their advanced age – or another factor that researchers haven’t yet taken into account.

Still, the new estimates have some researchers thinking about what might happen to vaccines. The delta variant became more common in Israel in June, raising the possibility that it might be good to avoid the vaccine.

In Britain, where Delta started increasing earlier in the year, researchers estimated the effectiveness of the Pfizer-BioNTech vaccine against the variant, based on a review of everyone vaccinated in the UK by 16 May. reported It was stated in the New England Journal of Medicine that it was 88 percent effective against symptomatic Covid-19.

Another possibility is that the Pfizer-BioNTech vaccine is becoming less and less effective. Department of Health researchers found that people vaccinated in January had a higher rate of new infections than those vaccinated in April.

If after six months the vaccine is really running low, the consequences could be huge. It could affect the Israeli government’s current negotiations on whether to give people a third chance. Dr. If vaccines are indeed losing some of their effectiveness, Segal says it may be wise to use boosters to fight the Delta-borne epidemic.

“If a third booster is safe and seems like it would really benefit, I think it’s definitely something we should do as quickly as possible,” he said.

Also head of innovation at Clalit Health Services, Dr. Balicer said he and his colleagues are working on their own study of the vaccine’s effectiveness in Israel, using Clalit’s health records to account for such confounding factors.

Dr. “I definitely think there’s been a reduction, but not as much as assumed based on raw data, and that’s just not to blame,” Balicer said. “Now we’re trying to figure it out cleanly.”