Citing new data, Pfizer outlined a case for booster shots, but there’s a debate over whether they’re needed.

Pfizer offered new evidence on Wednesday in support of booster shots of its potent vaccine, reporting that the vaccine’s power wanes slightly over time but may be improved with a third dose.

The new data, which have not yet been peer-reviewed nor published in a scientific journal, nonetheless confirm that the two-dose vaccine offers potent protection against the coronavirus. Whether booster shots will ever be widely needed is far from settled, the subject of heated debated among scientists.

So far, federal health officials have said boosters for the general population are unnecessary. And experts questioned whether vaccinated people should get more doses when so many people have yet to be immunized at all.

“There’s not enough evidence right now to support that that is somehow the best use of resources,” said Natalie Dean, a biostatician at Emory University in Atlanta.

Still, the findings raise questions about how much protection two doses will provide in the months to come. And with coronavirus cases surging again in many states, the data may influence the Biden administration’s deliberations about delivering boosters.

If third shots are cleared for the general population, the boosters would likely represent a multi-billion-dollar business for Pfizer.

In a study posted online, Pfizer and BioNTech scientists reported that the vaccine’s effectiveness against symptomatic infection wanes slightly over time, but remains strong in preventing severe disease.

The vaccine had a sky-high efficacy rate of about 96 percent against symptomatic Covid-19 for the first two months following the second dose, the study showed. But the figure declined by about 6 percent every two months after that, falling to 83.7 percent after six months.

Against severe disease, however, the vaccine’s efficacy held steady at about 97 percent.

“It’s not a big drop, but it is noteworthy,” Dr. Dean said. “Overall, they find that the vaccine is still performing very well, at very high efficacy.”

The study period ended before the rise of the Delta variant, the highly contagious version of the virus that now dominates in the United States and makes vaccines somewhat less effective against infection.

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 received 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 with 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.

In the new study, the researchers followed the volunteers for six months after vaccination, up to March 13. Over the entire period, the researchers estimated, the vaccine’s efficacy was 91.5 percent against symptomatic Covid-19. (The study did not measure the rate of asymptomatic virus infections.)

But within that period, efficacy did gradually drop. Between one week and two months after the second dose, the figure was 96.2 percent. In the period from two to four months following vaccination, efficacy fell to 90.1 percent. From four months after vaccination to the March cutoff, the figure was 83.7 percent.

Those figures still describe a remarkably effective vaccine, however, and may not convince critics that booster shots are widely needed.

Earlier on Wednesday, Pfizer reported that a third dose of its vaccine significantly increases blood levels of antibodies against several versions of the virus, including the Delta variant.

Results were similar for antibodies produced against the original virus and the Beta variant, which was first identified in South Africa. Pfizer and BioNTech expect to publish more definitive research in the coming weeks.

The announcement was a preliminary snapshot of data contained in an earnings statement. The finding has not been peer-reviewed nor published in a scientific journal. And although antibody levels are an important measure of immunity, they are not the only metric. The body has other defenses that turn back infection.

Pfizer also said in its statement that vaccines for children ages 5 through 11 years could be available as early as the end of September. The vaccine is already authorized in the United States for everyone ages 12 and up.

Pfizer’s vaccine brought in $7.8 billion in revenue in the last three months, the company said, and is on track to generate more than $33.5 billion this year.

The vaccine is poised to generate more sales in a single year than any previous medical product, and by a wide margin. The sales figures are poised to translate into billions of dollars in profit for the drug maker.