Pfizer Inc.’s COVID-19 vaccine declined in protection against hospitalization after four months, while Moderna Inc.’s remained stable, U.S. researchers found in an analysis of data from 21 U.S. hospitals across 18 states.

Two doses of either vaccine provided more protection against hospitalization than the one-dose Johnson & Johnson vaccine, the study found, though Pfizer’s advantage over J&J narrowed over time, according to the study published Friday by the Centers for Disease Control and Prevention with collaborators across the country. All three vaccines provided substantial protection after four months — Moderna’s was 92% effective against hospitalization by then, with Pfizer’s at 77% and J&J at 68%.

The data, published Friday, may influence the debate over whether Americans should receive a third dose of vaccine to ward off the virus. Advisers to the Food and Drug Administration are expected to vote Friday on whether to recommend a booster shot, and they’ve mostly had to rely on data from Israel and the U.K. on whether the shots’ effectiveness wanes over time.

The U.S. is facing a surge of COVID-19 infections fueled by the highly transmissible delta variant, particularly among unvaccinated parts of the country, and breakthrough infections among vaccinated people have become more common.

The CDC study looked at 3,689 nonimmunocompromised adults from March to August.

The researchers noted that the vaccine effectiveness differences between Moderna and Pfizer’s shots, which both use a mechanism called messenger RNA, could be due to differences in timings between doses. The second dose of the Pfizer vaccine is typically delivered after three weeks, while Moderna patients wait four weeks. They also noted several limitations to the study, including the fact that a relatively small number of patients had received the J&J vaccine compared with the mRNA vaccines.

Previous studies have found that Moderna’s vaccine appears to generate more antibodies than Pfizer’s, though it’s not clear if antibodies are even the most important component in immunity over the long term.

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