The efficacy of the Pfizer BioNTech vaccine in preventing Covid-19 infections has declined by nearly a third in Israel, largely due to the spread of the Delta variant, data from the country's health ministry suggests.
The vaccine had a 64 per cent efficacy rate from early June until early July, the latest figures show, down from 94 per cent a month earlier. The decrease coincides with a period in which the government reversed coronavirus restrictions and the Delta variant spread through the country.
The vaccine is still conferring strong protection against severe symptoms of the virus, with hospital admission rates remaining low. The ministry's data shows the vaccine's efficacy in preventing hospital admission was 93 per cent from June 6 to July 3, compared to a 98 per cent rate in the previous period.
But the rising cases offer a "preliminary signal" that the vaccine may be less effective at preventing mild symptoms from the Delta variant, the chairman of Israel's national expert panel on Covid-19, Ran Balicer, said.
While stressing it was "too early to precisely assess vaccine effectiveness against the variant", Balicer told AFP that "some decrease in vaccine effectiveness against mild illness - but not severe illness - is likely".
The Delta variant of the coronavirus was first identified in India in October and has since spread to nearly 100 countries worldwide. It is more than twice as contagious as the original Covid-19 virus and has forced a number of governments, including in the UK, to delay or rethink lifting pandemic restrictions.
The Delta variant's emergence as the "dominant strain" in Israel has led to a "massive shift in the transmission dynamic", said Balicer.
On Monday, Israel reported the highest rate of new infections in three months, with the ministry recording 343 new cases over the past 24 hours. After a peak of more than 10,000 new cases in one day in January, new daily cases had fallen to the single digits in June.
In the past fortnight, 90 per cent of new cases have been caused by the delta variant. About half of new cases have been detected in fully vaccinated patients, and about half in children, with a handful of returning travellers testing positive.
Experts "remain hopeful that the vaccine effectiveness against serious illness will remain as high as it was for the Alpha strain" – first identified in the UK in December, Balicer said.
The number of fully vaccinated Israelis experiencing severe symptoms after contracting the virus had increased from roughly one every other day up to five per day, Balicer estimated.
"It is encouraging that we still maintain zero deaths for the last 12 days," he added.
After recently reimposing an indoor mask rule for public spaces, Prime Minister Naftali Bennett was scheduled to meet Health Minister Nitzan Horowitz on Monday to discuss the latest outbreak and whether to advise a third booster shot for certain demographics.
Israel may also consider limiting the size of permitted gatherings and reintroducing the "Green Pass" system that limited access to certain spaces to vaccinated people, Bennett said on Sunday.
Pfizer CEO Albert Bourla has said people will "likely" need a third dose of a Covid-19 vaccine within 12 months of getting fully vaccinated.
A Pfizer spokesperson declined to comment on the data from Israel but told Bloomberg that other research suggested the vaccine provided ongoing protection against new mutations. Available evidence suggests the vaccine "will continue to protect against these variants", she said.
Israel's world-leading coronavirus response has fully vaccinated 57 per cent of the population and 88 per cent of those over 50, who are considered most at risk of serious illness.