Association of Baloxavir Treatment Timing with Serial Interval and Household Transmission of Influenza through a Likelihood-Based Analysis

BACKGROUND: Baloxavir treatment is associated with reduced influenza transmission within households, and the serial interval varies by treatment status. However, it remains unclear how baloxavir-induced changes in the serial interval relate to household transmission. We aimed to quantify the model-based association between baloxavir treatment timing and the serial interval and household transmission risk. METHODS: We conducted a household survey of influenza cases in Japan between October 2018 and February 2019. We defined the likelihood-based model integrating the serial interval distribution by treatment status and the secondary attack rate (SAR) using individual-level data from index cases. Using this model, we estimated the reduction in the serial interval associated with baloxavir treatment. RESULTS: Compared with untreated index cases, baloxavir-treated cases were estimated to have a serial interval density reduced by 21.42% following treatment. Treatment within 24 hours was associated with a 0.1685 reduction in the area under the curve, with smaller reductions as treatment was delayed. Earlier treatment was associated with a shorter, more concentrated distribution, whereas treatment 72 hours after onset resembled untreated cases. CONCLUSIONS: Our findings highlight that baloxavir treatment is associated with a shorter serial interval and lower estimated secondary household transmission risk. We provide model-based estimates suggesting that earlier administration is associated with a greater reduction in serial interval density and estimated transmission risk, which may inform public health strategies for infection control.