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Background: This phase 2b/3, randomized, placebo‐controlled trial explored the efficacy and evaluated the safety of ensitrelvir. This trial involved individuals with asymptomatic infection with severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) and patients with mild symptoms of coronavirus disease 2019 (COVID‐19). Methods: The trial was conducted at 57 medical institutions in Japan, South Korea, and Vietnam (study period: January 6–August 14, 2022). Eligible participants were randomized (1:1:1) to the ensitrelvir 125‐mg, ensitrelvir 250‐mg, or placebo group, received the allocated intervention orally, and were followed up until Day 28. Participants self‐rated the severity of 14 typical COVID‐19 symptoms and recorded the data in an electronic diary. Results: In total, 572 participants (194, 189, and 189 in the ensitrelvir 125‐mg, ensitrelvir 250‐mg, and placebo groups, respectively) were included in the intention‐to‐treat population. Ensitrelvir 125‐mg group observed a 77% reduction in the risk of developing any of the 14 COVID‐19 symptoms or fever and a 29% reduction in the risk of worsening of such symptoms or fever versus placebo (statistically nonsignificant). The viral RNA, viral titer, and time to infectious viral clearance observed a statistically significant decrease versus placebo. Most treatment‐related adverse events (TEAEs) were mild to moderate in severity, and the most common TEAE observed across groups was a decrease in high‐density lipoprotein. Conclusions: Our exploratory results suggest a potential reduction in the risk of development or worsening of COVID‐19 symptoms with ensitrelvir. Ensitrelvir showed antiviral efficacy and was well tolerated. Trial Registration: Japan Registry of Clinical Trials identifier: jRCT2031210350. [ABSTRACT FROM AUTHOR]

In a post‐hoc analysis of the phase 3 BLOCKSTONE study (JapicCTI‐184180), we investigated household transmission of baloxavir‐resistant (PA/I38X) influenza viruses. Using baloxavir resistance rates from prior clinical trials and the rate of influenza transmission observed in the study, the predicted number of PA/I38X transmission events was 4.8, assuming wild type and PA/I38X viruses were equally transmissible. However, no PA/I38X viruses were observed. These results suggest a low potential for baloxavir‐resistant influenza virus transmission from treated to untreated individuals, potentially due to reduced viral/transmission fitness for PA/I38X viruses and/or low viral titres at the time when resistant viruses arise. [ABSTRACT FROM AUTHOR]

Baloxavir marboxil (BXM) demonstrated a rapid and profound decline in infectious viral titer 1 day after BXM administration. Rapid reduction in virus titer is a characteristic of BXM. There may be a possibility that drug carryover effects have impacts on the observed antiviral effects due to the poor correlation that was observed between viral titer reduction and alleviation of influenza symptoms. Here, we report possible carryover effects of baloxavir acid (BXA), an active form of BXM, on infectious titer testing. Our findings indicate that there is little impact of BXA carryover on the infectious titer testing. [ABSTRACT FROM AUTHOR]