Age‐Specific Effectiveness of Antiviral Agents on the Secondary Household Transmission of Influenza

OBJECTIVES: We investigate the effects of age, vaccination history, treatment timing, and type of antiviral agent in index cases and household members on the risk of influenza transmission. METHODS: We conducted a retrospective household survey in Japan between October 2018 and February 2019, enrolling 915 households (3248 members) with influenza cases. Secondary attack rates (SARs) and odds ratios (ORs) were calculated for subgroups stratified by treatment timing, antiviral agent, and age group. RESULTS: When treatment was initiated within 48 hours of symptom onset, both index cases and household contacts aged 0-14 and 15-50 years exhibited significantly higher odds of household transmission compared with those aged 51 years and older. Early treatment was associated with lower odds of transmission, with Baloxavir (OR = 0.74, 95% CI: 0.57-0.96) and Laninamivir (OR = 0.57, 95% CI: 0.41-0.79) showing lower ORs than Oseltamivir. CONCLUSIONS: Our findings highlight that both clinical and demographic factors substantially influence household influenza transmission. Early initiation of antiviral treatment, combined with the use of Baloxavir or Laninamivir, significantly reduces the risk of secondary transmission, particularly in younger populations who play a central role in household spread.