TEL AVIV—Data from Israel suggests Pfizer Inc.’s vaccine has become less effective in preventing infection of Covid-19 in recent weeks, although it remains a robust bulwark against serious illness as governments around the world scramble to respond to the rampant spread of a new Delta variant of the coronavirus

The findings, which are preliminary and based on a small sample, suggest that after two shots the vaccine was 39% effective at reducing the risk of infection and 40% effective at reducing the risk of symptomatic disease during a period when the Delta variant dominated cases in Israel, according to the country’s Health Ministry. The vaccine was 91% effective at preventing severe illness in the same period between June 20 and July 17, the ministry said.

“It’s important to say that in terms of protecting against severe illness, hospitalization and death, the Pfizer vaccine is still very very efficient,” said Nadav Davidovitch, a member of the expert advisory panel to the Israeli government on the coronavirus. “We are still not sure about whether the reduction in effectiveness is due to time passed or [a] question of the variant.”

Mr. Davidovitch said the data was based on hundreds of cases and should only be considered preliminary.

Pfizer said the company and its partner BioNTech SE are confident in the protection and safety of the two-dose vaccine, which it said prevents severe disease and hospitalizations. Two doses continue “to be highly efficacious in preventing Covid-19, including variants and to date, no variant, including Delta, appears to have escaped the protection of the vaccine,” Pfizer said.

Natalie Dean, an assistant professor in the department of biostatistics and bioinformatics at Emory University, said not enough is known to draw firm conclusions from the study. The time from vaccinations in January isn’t very long, and such a major drop would need to be replicated, she said. “It’s just such a striking difference and whenever it’s like that you always want to see some other verification,” she said.

Dr. Dean also said the study may be limited in its ability to ensure the vaccinated groups are compared with similar unvaccinated people. While the study’s investigators consider people’s ages, sex and time, factors such as occupation, location and whether they live in congregate settings aren’t accounted for.

Israel used the Pfizer vaccine to conduct a world-leading campaign that offered glimpses of how governments might combat the Covid-19 pandemic.

Israel has restored indoor face-mask requirements to check the spread of the Delta variant. A visitor inside the Church of the Holy Sepulchre in the Old City of Jerusalem on July 15.

Israel has restored indoor face-mask requirements to check the spread of the Delta variant. A visitor inside the Church of the Holy Sepulchre in the Old City of Jerusalem on July 15.

Photo: abir sultan/Shutterstock

The new Israeli data is likely to fuel debate about how soon vaccines lose potency and whether governments should begin administering booster shots. Earlier in July, Israel began offering a third dose to immunocompromised individuals, and health officials are studying whether one is necessary for the wider population.

The Israeli Health Ministry previously said it calculates the effectiveness of a vaccine by analyzing the number of infections among the vaccinated compared with those who were unvaccinated in the given period. It also accounts for factors such as the week of infection and age and whether the person was infected in the past.

The ministry said the latest data might be biased because of where coronavirus tests were conducted. Many were carried out in areas of outbreaks and among the elderly, and only a small number of tests were conducted among younger people who were vaccinated. Older Israelis were the first to be vaccinated; many were fully inoculated by the end of January and currently make up the most of cases of severe illness.

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Israel’s new data suggests a lower potency for the Pfizer shot than was found by another major U.K. study published this week. Published in the New England Journal of Medicine and sponsored by Public Health England, the study involved nearly 20,000 people and said the vaccine was 88% effective at protecting against symptomatic disease. A previous Israeli study, based on data collected from June 6 through early July during the start of the Delta outbreak, found Pfizer protected 64% of inoculated people from infection and was 94% effective at preventing severe illness.

Before the Delta outbreak, Israel’s Health Ministry said Pfizer was 94% effective at preventing infection and ensured 97% of people inoculated avoided severe illness.

Israel’s latest figures come as the country tackles a worsening outbreak of the Delta variant.

The seven-day average for new cases of the virus in Israel rose to over 1,100 on Friday, up from around 10 for the first half of June. The total number of severely ill cases stands at 81. More than 80% of the adult population in Israel has been fully inoculated, largely with the Pfizer vaccine.

To prevent further infections, Israel has returned to some restrictions it had largely abandoned after rolling out a rapid vaccination campaign. The government recently started requiring proof of vaccination, a note of recovery from Covid-19 or a recent negative test to be able to attend indoor events with more than 100 participants. It is expected to soon extend those requirements for access to sports venues, gyms, restaurants, tourist attractions and houses of worship.

On Thursday, the government also approved a plan to require all unvaccinated individuals coming into the country to enter quarantine for a week, no matter the results of a test on arrival.

Corrections & Amplifications
The Israel data suggests the Pfizer vaccine after two shots was 39% effective at reducing the risk of infection and 40% effective at reducing the risk of symptomatic disease. An earlier version of this article incorrectly said that after two shots the Pfizer vaccine protected 39% of people from infection and 40% against symptomatic disease. (Corrected on July 23)

Write to Rory Jones at rory.jones@wsj.com