Although it has one of the world’s fastest vaccine rollouts, Israel has been experiencing a new surge in cases stemming from the spread of the delta variant, the dominant strain in many countries.

On Monday Israel said it would begin offering a third vaccine dose to patients with weakened immune systems. This came after Pfizer cited early data showing a third shot can sharply increase immune response. The company plans to request U.S. emergency authorization in August for a third booster dose for the general population there.

At this point in Israel, there is no evidence to prove it is “appropriate” to administer a third shot to the entire population, Ran Balicer, chairman of the national expert advisory team on Covid-19 response, said in an interview with Bloomberg Television on Tuesday. There hasn’t been “clear evidence on waning immunity,” he said.

New cases in the country have risen from less than ten to as much as 500 a day in the past month and critical cases have more than doubled to 47. That is still a fraction of the peak hospitals saw earlier this year now that about 56% of Israelis are fully vaccinated, all with the Pfizer Inc.-BioNTech SE vaccine.

“There are other potential explanations to some of the rising current cases,” said Balicer, who is also the chief innovation officer at Clalit Health Services, the country’s largest health maintenance organization. “Therefore, until we have such evidence, I don’t feel that we can take this decision,” he said, when asked whether Israel was considering rolling out a third shot for the entire population.

For now, Israelis who have undergone heart, lung, kidney or liver transplants and cancer patients receiving chemotherapy are among those eligible for a third dose.

Balicer said that along with the spread of the delta strain, cases are rising because people are less concerned with the virus and that there is a general return to normal life.

In an early bid to halt the virus’s spread, the government reinstated indoor mask-wearing last month and pushed back the date to allow vaccinated tourists by a month. Prime Minister Naftali Bennett’s cabinet is considering steps to prevent mass infections originating from large-scale gatherings.

“We do feel we have a high level of uncertainty of where this wave might stop if we take a laissez-faire approach,” Balicer said. “But we’re trying not to hurt economy in the process.”