WASHINGTON (BLOOMBERG) - Pfizer's Covid-19 vaccine declined in protection against hospitalisation after four months, while Moderna's remained stable, United States researchers found in an analysis of data from 21 American hospitals across 18 states.
Two doses of either vaccine provided more protection against hospitalisation than the one-dose Johnson & Johnson (J&J) vaccine, the study found, though Pfizer's advantage over J&J narrowed over time, according to the study published on Friday (Sept 17) by the Centres for Disease Control and Prevention (CDC) with collaborators across the country.
All three vaccines provided substantial protection after four months - Moderna's was 92 per cent effective against hospitalisation by then, with Pfizer's at 77 per cent and J&J at 68 per cent.
The data, published on 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 were expected to vote on Friday on whether to recommend a booster shot, and they have mostly had to rely on data from Israel and Britain on whether the shots' effectiveness wanes over time.
The US is facing a surge of Covid-19 infections fuelled 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 non-immunocompromised adults from March to last month.
The researchers noted that the differences in vaccine effectiveness between Moderna and Pfizer's shots, which both use a mechanism called messenger ribonucleic acid (mRNA), 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 is not clear if antibodies are even the most important component in immunity over the long term.