The Pfizer-BioNTech vaccine’s effectiveness wanes slightly over time, according to newly released data from the companies, but remains strong in preventing severe disease. With coronavirus cases surging again in many states, the findings may influence the Biden administration’s deliberations about delivering a booster shot.

The vaccine had a sky-high efficacy rate of about 96 percent against symptomatic Covid-19 for the first two months, the study showed, but then declined about 6 percent every two months after that, falling to 83.7 percent after six months. Against severe disease, its efficacy held steady at about 97 percent. The data was posted online on Wednesday and has not been published in a scientific journal.

Despite the decline, the data confirm that the vaccine gives potent protection against Covid-19. Still, the study raises questions about how much protection two doses will provide in the months to come. Adding to these concerns is the rise of the Delta variant, which makes vaccines somewhat less effective against infection. The variant became dominant only after the study ended. But recent studies have also shown that vaccines remain strongly protective against the worst outcomes of Covid-19 caused by the Delta variant.

The findings come from 42,000 volunteers in six countries who participated in a clinical trial that Pfizer and BioNTech began last July. Half of the volunteers got the vaccine while the other half got a placebo. Both groups received two shots spaced three weeks apart. The researchers compared the number of people in each group who developed symptoms of Covid-19, which was then confirmed by a P.C.R. virus test.

When the companies announced their first batch of results, the vaccine showed an efficacy against symptomatic Covid-19 of 95 percent. In other words, the risk of getting sick was reduced by 95 percent in the group that got the vaccine compared to the group that got the placebo.

That result — the first for any Covid-19 vaccine — brought an exhilarating dose of hope to the world in December when it was riding what had been the biggest wave of the pandemic. Since then, the Pfizer-BioNTech vaccine has made up the majority of shots that Americans have received, with more than 191 million doses given so far, according to the Centers for Disease Control.

After the first analysis, the Pfizer and BioNTech researchers continued to follow the volunteers. The research became more challenging as time passed, because volunteers who got the placebo could ask to get the vaccine once it was authorized in their country.

Understand the State of Vaccine Mandates in the U.S.

For the new study, the researchers followed the volunteers for six months after vaccination, up to a cutoff date of March 13. Looking over that entire period, the researchers estimated the vaccine’s efficacy at 91.5 percent against symptomatic Covid-19. (The study did not measure the rate of asymptomatic virus infections.)

But within that period, the efficacy did gradually drop. Between one week and two months after the second dose, the efficacy was 96.2 percent. In the period between two and four months, the efficacy fell to 90.1 percent. And between four months and six months, the efficacy hit 83.7 percent.

Each estimate came with a margin of uncertainty. But over the six months of the trial, there was a clear decline in efficacy.

The new study comes on the heels of data from Israel suggesting that the Pfizer-BioNTech’s protection may be waning there. But experts have pushed back against a rush to approving a booster there. The data have too many sources of uncertainty, they say, to make a precise estimate of how much effectiveness has waned. For example, the Delta-driven outbreak hit parts of the country with high vaccination rates first and has been hitting other regions later. “Such an analysis is still highly uncertain,” said Doron Gazit, a physicist at Hebrew University who analyzes Covid-19 trends for the Israeli government.