FDA recommended Pfizer boosters for persons over 65 who are at high risk. The Centers for Disease Control and Prevention reported on Friday that the level of protection against Covid hospitalizations provided by the Pfizer-BioNTech vaccine declined dramatically four months after full vaccination.

The data was disclosed just hours before a scientific advisory council for the Food and Drug Administration suggested that recipients of the Pfizer coronavirus vaccine who are 65 or older or at high risk of severe Covid-19 have booster injections at least six months after the second shot.

The latest study discovered that the Pfizer boosters vaccine was 91 percent effective in preventing hospitalization from two weeks after patients received their second dosage — a time at which they are generally considered completely immunized — to four months later. However, after 120 days, its efficiency dropped to 77%.

The Moderna vaccine showed no corresponding loss in protection over the same time period: it was 92 percent effective against hospitalizations four months after recipients were vaccinated, a level nearly identical to its previous 93 percent efficiency.

According to the study, there were not enough participants who had received the one-shot Johnson & Johnson vaccine to compare its effectiveness.

The C.D.C. study, which was announced on Friday, corroborated previous research that suggested the Pfizer boosters vaccination may provide reduced protection against hospitalization over time.

Other trials have shown that despite the distribution of the Delta version and the extended period since participants received their second doses, Pfizer’s effectiveness against hospitalization has remained above 90%. According to Pfizer, data from Israel indicate a decline in effectiveness against severe disease, albeit it appears that Israel and the United States define “severe disease” differently.

The most recent C.D.C. study was based on an examination of around 3,700 persons hospitalized in the United States from March to August.

People with weakened immune systems, who normally do not respond well to immunizations, were barred from participating in the trial. Nonetheless, the vaccinated patients were mostly older persons — the Pfizer cohort had a median age of 68 — and it was unclear whether vaccination effectiveness had altered much in younger age groups.

According to the study’s authors, the difference in performance between the Moderna and Pfizer boostersvaccinations could be due to larger dosages of mRNA in the Moderna shots or the four-week delay between doses of the Moderna vaccine. It’s also possible that additional, undiscovered variations in the study individuals who received either shot could have influenced the results, according to the researchers.

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