Clinical Report: Impact of Timing on Baloxavir Treatment Regarding Serial Interval

Overview

This study used household survey data from Japan to model how the timing of baloxavir treatment relates to the serial interval between influenza cases and estimated secondary transmission within households. Earlier treatment was associated with a greater reduction in the modeled serial interval distribution. The observed difference in household secondary attack rates (SARs) between treated and untreated index cases was not statistically significant.

Background

Influenza spreads rapidly and creates a substantial healthcare burden. Baloxavir inhibits an early stage of viral replication through a mechanism distinct from neuraminidase inhibitors. Previous studies have associated baloxavir treatment with lower household transmission, but how treatment timing, the serial interval, and SAR relate to one another has been unclear.

Objective

To estimate the model-based association between baloxavir treatment timing, the serial interval, and household secondary transmission of influenza.

Approach

Researchers analyzed an internet-based survey of Japanese households reporting symptomatic influenza between October 2018 and February 2019. They compared household transmission by treatment status and timing, then developed a likelihood-based model linking the serial interval distribution to SAR. The serial interval analysis included 95 index cases: 73 treated with baloxavir and 22 untreated.

Data Highlights

• The observed SAR was 0.23 among baloxavir-treated index cases and 0.29 among untreated index cases; this difference was not statistically significant.

• The model estimated a 31.54% reduction in serial interval density after baloxavir treatment.

• Treatment within 24 hours of symptom onset was associated with a 25.34% reduction in the area under the modeled serial interval distribution. This was not a measured 25.34% reduction in household infections.

Key Findings

• Modeled reductions in the serial interval distribution became smaller as the time from symptom onset to baloxavir treatment increased.

• Earlier treatment was associated with lower estimated household transmission risk under the model’s assumptions.

• This study compared baloxavir-treated index cases with untreated cases; it did not directly compare their infectious periods with those of oseltamivir-treated cases.

Interpretation

The results suggest a relationship between earlier baloxavir treatment and lower modeled household transmission risk. The serial interval measures time between symptom onsets in an index case and a secondary case; it does not directly measure how long the index case remained infectious.

Clinical Implications

The model may inform further research on treatment timing and influenza transmission. The nonsignificant observed SAR difference and the assumptions used to link serial intervals with transmission risk do not establish an optimal prescribing time or prove that earlier baloxavir treatment prevented household infections.

Limitations

The survey relied on reported illness and lacked information about household contacts’ treatment or isolation practices. Transmission chains beyond the index case were unavailable, and the model assumed that secondary infections originated from that case. Few index cases received baloxavir at least 72 hours after symptom onset.

Conclusion

Earlier baloxavir treatment was associated with greater reductions in the modeled serial interval distribution and lower estimated household transmission risk. The observed SAR comparison was not statistically significant, so the findings do not establish a causal reduction in household transmission.

Related Resources & Content

The publication details below appear in the supplied paper or its reference list:

• Kim J, Ryu M, Nishiura H, Lee H. “Association of baloxavir treatment timing with serial interval and household transmission of influenza through a likelihood-based analysis.” International Journal of Infectious Diseases. 2026;172:109094.

• Umemura T, Mutoh Y, Kawamura T, et al. “Efficacy of baloxavir marboxil on household transmission of influenza infection.” Journal of Pharmaceutical Health Care and Sciences. 2020;6:21.

• Komeda T, Takazono T, Hosogaya N, et al. “Comparison of household transmission of influenza virus from index patients treated with baloxavir marboxil or neuraminidase inhibitors: a health insurance claims database study.” Clinical Infectious Diseases. 2021;72:e859–e867.

• Ikematsu H, Baba T, Saito MM, et al. “Comparative effectiveness of baloxavir marboxil and oseltamivir treatment in reducing household transmission of influenza: a post hoc analysis of the BLOCKSTONE trial.” Influenza and Other Respiratory Viruses. 2024;18:e13302.

• Monto AS, Kuhlbusch K, Bernasconi C, et al. “Efficacy of baloxavir treatment in preventing transmission of influenza.” New England Journal of Medicine. 2025;392:1582–1593.