(Bloomberg) -- In a real-world study of people with Covid-19, Pfizer Inc.’s antiviral Paxlovid was less effective at preventing the infection’s most serious cases than it was in initial trials. 

The pill was just 37% effective at preventing hospitalization or death in high-risk patients when compared to no Covid treatment at all, according to an observational study published Thursday in JAMA Network Open. In earlier studies, it cut the risk of hospitalization or death by as much as 89%. 

Paxlovid is still an important drug for Covid’s worst outcome: The new study showed it was 84% effective against death alone.

The study published Thursday measures how the drug is performing in the real world, rather than carefully designed and monitored clinical trials. Both the earlier trial and the newer study measured people at high risk of developing Covid complications.

Pfizer’s initial Paxlovid studies were conducted in unvaccinated people, which could explain some of the differences in outcomes. The coronavirus has also evolved since the studies that provided more positive results.

Despite the lower efficacy against hospitalization and death in the new study, the antiviral is still “definitely recommended for high-risk patients,” says Danyu Lin, a biostatistics professor at the University of North Carolina at Chapel Hill and lead author of the study.

The study also found that another therapy — Merck & Co.’s Lagevrio — had similar efficacy to Paxlovid. Merck’s drug works through a genetic mechanism and isn’t recommended for use in pregnant women. Sill, it should be considered when patients can’t take Pfizer’s, which sometimes interacts with other medications, Lin said.

High-risk patients who took Lagevrio saw a 41% reduction in hospitalizations and deaths in the observational study compared to those who took no treatment, while earlier studies showed reductions of only about 30%. The Merck drug was also 77% effective against death in the JAMA study.

The study looked at electronic health records of 68,867 patients at several hospitals in Cleveland and Florida.   

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