Ensitrelvir Effectively Prevents Spread of COVID-19 in Households

THURSDAY, March 20, 2025 (HealthDay News) — For household contacts of individuals with COVID-19, taking ensitrelvir within 72 hours of symptoms is effective at preventing infection and generally well tolerated, according to a study presented at the annual Conference on Retroviruses and Opportunistic Infections, held from March 9 to 12 in San Francisco.

Akimasa Fukushi, M.D., of Shionogi & Co., Ltd, in Osaka, Japan, and colleagues randomly assigned household contacts (HHC) of index patients (IP) with COVID-19 to receive either ensitrelvir (1,030 participants; day 1: 375 mg; day 2 to 5: 125 mg) or placebo (1,011 participants) within 72 hours of symptom onset in the IP.

The researchers found that the overall proportion of HHC developing confirmed COVID-19 was significantly lower in the ensitrelvir group (2.9 percent) versus the placebo group (9.0 percent; risk ratio [RR], 0.33) for HHC with central laboratory-confirmed SARS-CoV-2 negativity by reverse transcription polymerase chain reaction (RT-PCR). For HHC regardless of central laboratory RT-PCR results at baseline, similar results were seen (RR, 0.43). Across both groups, the proportions developing treatment-emergent adverse events (TEAEs; 15.1 percent for ensitrelvir and 15.5 percent for placebo) or serious TEAEs (0.2 versus 0.2 percent) were similar, as were the TEAEs seen in >1.5 percent of patients in either group: headache (2.9 versus 2.5 percent), diarrhea (1.8 versus 1.3 percent), and influenza (1.1 versus 1.6 percent). No COVID-19-related hospitalizations or fatalities were seen.

“In addition to vaccination, postexposure prophylaxis with timely use of an oral antiviral would be a valuable way to help prevent COVID-19 illness in people who have been exposed, especially people at high risk for severe disease,” presenting author Frederick G. Hayden, M.D., from the University of Virginia in Charlottesville, said in a statement.

The study was funded by Shionogi & Co., Ltd, the manufacturer of ensitrelvir.

Abstract

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