The phase 3b, double-blind, randomised, placebo-controlled trial enrolled 1,457 influenza-positive index patients and 2,681 household contacts across 15 countries from 2019 to 2024. The index patients, aged 5 to 64, were assigned to receive either baloxavir or a placebo within 48 hours of symptom onset. The primary endpoint was laboratory-confirmed influenza transmission to household contacts by day 5.

Key Findings:

• Treatment with baloxavir reduced the odds of untreated household members contracting the virus by 32%.
• Transmission resulting in symptomatic influenza was lower with baloxavir (5.8% vs. 7.6%), though the difference was not statistically significant (P=0.16).
• Baloxavir led to a faster reduction in viral titers, with a mean reduction of 2.22 log₁₀ TCID₅₀/mL by day 3 compared to 1.85 log₁₀ TCID₅₀/mL for placebo.
• Drug-resistant viruses emerged in 7.2% of baloxavir-treated index patients but were not detected in household contacts, suggesting limited transmission risk.
• No new safety concerns were identified, with adverse events reported in 4.6% of baloxavir-treated patients compared to 7.0% in the placebo group.

'These results highlight baloxavir's potential not only to treat influenza but also to reduce its spread within communities,' said Professor Benjamin Cowling, co-author of the study and Helen and Francis Zimmern Professor in Population Health, Chair Professor of Epidemiology, and Head of the Division of Epidemiology and Biostatistics, School of Public Health, HKUMed. 'This dual effect could transform how we manage seasonal influenza and prepare for future pandemics.'

The study underscores the complementary role of antiviral drugs alongside vaccination, particularly in unvaccinated populations or during pandemics when vaccines may not be immediately available.