JERUSALEM — The Israeli government says its analysis has shown the Pfizer-BioNTech coronavirus vaccine appears to be less effective against infections caused by the Delta variant compared to other strains of COVID-19.

In a brief statement issued on Monday, the government said that as of June 6, the vaccine provided 64 percent protection against infection. In May when the Alpha variant dominated in Israel and the Delta strain had not yet spread widely, Israel found that the shot was 95.3 percent effective against all infections.

The government added that the vaccine was now 93 percent effective in preventing severe disease and hospitalizations, compared to 97 percent reported in the medical journal The Lancet in May.

The statement cited top-line figures, but did not release underlying data or other details about its analysis. A team at Hebrew University said in a separate statement that it was too soon to tell how much the Delta variant was affecting vaccine efficacy.

Dr. Ashish Jha, dean of Brown University's School of Public Health, was also cautious about drawing conclusions. "Best data still suggest mRNA vaccines offer a high degree of protection against infection and superb protection against severe illness. Let's await more data but as of now ... If you're vaccinated, I wouldn't worry," he tweeted.

In another statement on Tuesday, Israel's Ministry of Health released some data about illness caused by COVID-19 and offered an expanded explanation of the vaccine's protectiveness. Despite an apparent decline in the vaccine's ability to prevent all infections during the spread of the Delta variant, the statement emphasized its continuing benefit in preventing severe cases.

Israel has deployed the Pfizer vaccine to everyone over the age of 12, and its early and quick rollout gave scientists one of the first real-world snapshots of its efficacy.

The government said the drop in efficacy is likely due to the spread of the Delta variant in Israel. This more infectious strain of the virus was first identified in India earlier this year and is also known as B.1.617.2.

Pfizer said it could not comment on unpublished data, but a recently published lab study it conducted with the University of Texas Medical Branch that found its vaccine was effective against lab-made versions of the Delta strain and others. The study found that full vaccination elicits an immune response that should be expected to protect people well against infection with the new variants.

Israel is one of the most vaccinated countries in the world, with more than 60 percent of the population fully inoculated and two-thirds having received at least one dose.

The statement highlights a big risk going forward: the emergence of new variants that might evade some of the protection provided by vaccines.

Public health officials are stressing that the current shots offer good protection against the Delta variant.

A study by Public Health England found this month that the Pfizer-BioNTech and Oxford-AstraZeneca vaccines were each highly effective — at 96 percent and 92 percent respectively — against hospitalizations from the Delta variant after two doses.

Separately, preliminary findings of a Scottish study published in The Lancet last month found that the Pfizer vaccine provided 79 percent protection against all infections from the Delta variant, compared with 92 percent against the Alpha variant.

The same study, which analyzed data from 5.4 million people in Scotland found the Oxford-AstraZeneca vaccine offered 60 percent protection against infection with the Delta variant compared with 73 percent for the Alpha variant.

Moderna's vaccine was found in lab experiments to work against new variants such as the Delta strain, the company said.

Johnson & Johnson said lab tests of its one-shot coronavirus vaccine suggest it provides protection against the Delta variant.

However, this can change if the virus mutates further. That's why doctors and public health officials want more people to get vaccinated. "The more we allow the virus to spread, the more opportunity the virus has to change," the World Health Organization (WHO) advised last month.

Israel lifted most of its coronavirus restrictions in early June. However, the government then reinstated an indoors mask mandate after a spike in cases caused by the Delta variant. It also appointed a "special manager" to prevent the entry of the coronavirus and its variants into Israel and approved a plan to build a permanent testing facility at Ben Gurion Airport.

The country has reported a slight increase in daily cases in the past few weeks, but only a handful of people have died of the disease in Israel in the past month. — CNN