The Food and Drug Administration on Wednesday offered the first public look at Pfizer’s application for a booster coronavirus shot, two days before an outside advisory committee of experts is scheduled to meet to recommend whether or not the agency should approve the company’s request.

It also comes amid significant disagreement about the need for boosters between career scientists at the agency and top Biden health officials, who have already started planning a broad booster campaign for this fall.

In a 23-page document reviewing the company’s application, regulators examined safety and immune response data on roughly 300 adults who received a booster shot of Pfizer-BioNTech’s vaccine six months after their second dose, finding an increased immune response in study participants, even as they said that coronavirus vaccines were holding up powerfully against severe forms of Covid-19. There were no serious safety concerns associated with the booster injection, the regulators reported.

Pfizer said in a separate filing that one month after a third injection, levels of neutralizing antibodies against the Delta variant in a subgroup of trial volunteers were between five and seven times higher, roughly, than they were a month after the second dose. The company also reiterated its findings that the effectiveness of its vaccine against symptomatic disease fell from about 96 percent to about 84 percent by six months after the second shot, although it held steady against severe disease.

Pfizer argued in its filing that ebbing of the vaccine’s potency was the dominant reason for breakthrough infections among vaccinated people in Israel, which has relied almost exclusively on the Pfizer vaccine and has vaccinated its population faster than the United States.

But the F.D.A. regulators wrote that while waning immunity is one potential factor in breakthrough infections, other variables, including the Delta variant, may also have contributed to the cases.

In an interview, Pfizer officials acknowledged that the company’s booster study was quite small. But they said that the data they have delivered meets the F.D.A.’s criteria for justifying third shots for people 16 and up. Pfizer has another, much bigger booster study underway, with results expected this fall.

The F.D.A.’s analysis noted that Pfizer provided data on immune response against the Delta variant, by far the dominant variant in the U.S., in only two dozen people. Understanding the effectiveness of boosters against variants would likely be critical to the F.D.A.’s review, the document suggested. “Available data should support the effectiveness of the booster dose, particularly against currently circulating” variants, regulators wrote.

The analysis also suggested that regulators are cautiously weighing studies from Israel, which top Biden administration officials have said were key to their decision to recommend starting a booster campaign this month. Israel is already providing booster shots to most of its population.

“While observational studies can enable understanding of real-world effectiveness, there are known and unknown biases that can affect their reliability,” the regulators wrote. Studies in the United States “may most accurately represent vaccine effectiveness in the U.S. population,” they added.