Hours before the F.D.A. vote, the C.D.C. released a study showing waning protection of the Pfizer vaccine.

The Centers for Disease Control and Prevention released data on Friday indicating that the level of protection against Covid hospitalizations afforded by the Pfizer-BioNTech vaccine dropped significantly in the four months after full inoculation.

The data was released hours before a scientific advisory committee to the Food and Drug Administration recommended authorizing booster shots for recipients of the Pfizer coronavirus vaccine who are 65 or older or are at high risk of severe Covid-19, at least six months after the second shot.

The new study found that from two weeks after recipients got their second dose — a point at which they are normally considered fully vaccinated — to four months later, the Pfizer vaccine was 91 percent effective in preventing hospitalization. Beyond 120 days, though, its effectiveness fell to 77 percent.

The Moderna vaccine showed no comparable decrease in protection over the same time frame: It was 92 percent effective against hospitalizations four months after recipients’ vaccination, a level virtually identical to its 93 percent effectiveness before then.

The study said that not enough participants had received the one-shot Johnson & Johnson vaccine to compare its performance. Overall, though, the Johnson & Johnson shot has been 71 percent effective in preventing hospitalizations.

The C.D.C. study released on Friday supported some others that suggested the Pfizer vaccine may offer less protection from hospitalization over time. But the available data is far from unanimous.

Other studies have shown that Pfizer’s effectiveness against hospitalization has remained above 90 percent, despite the spread of the Delta variant and the lengthening time since people received their second shots. Pfizer has said that data from Israel suggest a falling effectiveness against severe disease, though it appears that Israel and the United States define “severe disease” differently.

The latest C.D.C. study was based on an analysis of roughly 3,700 adults hospitalized across the United States from March to August.

People with compromised immune systems, who typically don’t respond as well to vaccines, were excluded from the study. Nevertheless, the vaccinated patients tended to be older people — the Pfizer cohort had a median age of 68 — and it was unclear whether vaccine effectiveness had changed much in younger age groups. Previous studies have shown lower levels of protection in older people.

The authors of the study said that the gap in the performance of the Moderna and Pfizer vaccines could stem from higher doses of mRNA in the Moderna shots or the four-week space between doses of the Moderna vaccine. (Pfizer vaccines were given three weeks apart.) It’s also possible, they said, that other, unnoticed differences in the study participants receiving either shot could have also influenced the results.