A single oral dose of baloxavir marboxil, a cap-dependent endonuclease inhibitor, is approved for patients with influenza A or B infection; however, real-world evidence is limited. We evaluated the effectiveness of baloxavir vs neuraminidase inhibitors in reducing the incidence of severe illness in influenza outpatients aged 5-11 years. In this retrospective cohort study, we analyzed individual-level data from patients treated with these antivirals, using a large, Japanese health insurance claims database (JMDC). Patients were included at the first date of diagnosis of the influenza virus infection (Day 1) in two influenza seasons. The primary outcome was the incidence of hospitalization from Day 2-14. Of 196,749 included patients (Season 2018/2019, n = 103,709; Season 2019/2020, n = 93,040), 20.9 % received baloxavir, 38.8 % received oseltamivir, 28.7 % received laninamivir, and 11.6 % received zanamivir. In each treatment group, 61-70 % patients had influenza A. The incidence of hospitalization from Day 2-14 was significantly lower for baloxavir than for oseltamivir, laninamivir, and zanamivir. The adjusted risk ratio (95 % CI) for oseltamivir, laninamivir, and zanamivir were 1.86 (1.30-2.68), 2.11 (1.37-3.25), and 1.90 (1.31-2.77), respectively, compared with baloxavir. Comparative incidence of hospitalizations between agents were unaffected by season or virus type. Using a large, Japanese health insurance claims database, a lower rate of hospitalization was demonstrated in children aged 5-11 years with an influenza virus infection when treated with baloxavir vs neuraminidase inhibitors. Thus, single dose, oral baloxavir may reduce the incidence of severe illness in these patients.

Recurrent outbreaks of coronavirus disease 2019 (COVID-19) in Japan highlight the continued need for effective prevention, particularly household transmission. This study clarifies the epidemiological profile of household transmission of COVID-19 and influenza by quantifying transmission rates according to demographic and relational characteristics. An anonymized health insurance claims database was used to identify index patients diagnosed with COVID-19 or influenza between November 2021 and August 2023. Transmission events were defined as cases in which household members were diagnosed with the same infection in the week following the index patient’s diagnosis (Day 1). Analyses were stratified by the age of the index patients and age-based transmission patterns between index patients and household members. A total of 1,001,509 index patients with COVID-19 and 207,090 with influenza met the inclusion criteria. Transmission rates were highest among index patients aged <12 years (COVID-19, 40.17%; influenza, 33.51%). Transmission from children was disproportionately high, while age-combination analysis demonstrated elevated transmission both from and to children for both diseases. COVID-19 exhibited increased transmission among adults. As the primary drivers of household transmission for both COVID-19 and influenza, preventive strategies should be tailored to children. COVID-19 interventions must also address transmission among adults to strengthen household-level infection control.