Baloxavir safety and clinical and virologic outcomes in influenza virus-infected pediatric patients by age group: age-based pooled analysis of two pediatric studies conducted in Japan

BACKGROUND: Anti-influenza treatment is important for children and is recommended in many countries. This study assessed safety, clinical, and virologic outcomes of baloxavir marboxil (baloxavir) treatment in children based on age and influenza virus type/subtype. METHODS: This was a post hoc pooled analysis of two open-label non-controlled studies of a single weight-based oral dose of baloxavir (day 1) in influenza virus-infected Japanese patients aged < 6 years (n = 56) and ≥ 6 to < 12 years (n = 81). Safety, time to illness alleviation (TTIA), time to resolution of fever (TTRF), recurrence of influenza illness symptoms and fever (after day 4), virus titer, and outcomes by polymerase acidic protein variants at position I38 (PA/I38X) were evaluated. RESULTS: Adverse events were reported in 39.0 and 39.5% of patients < 6 years and ≥ 6 to < 12 years, respectively. Median (95% confidence interval) TTIA was 43.2 (36.3–68.4) and 45.4 (38.9–61.0) hours, and TTRF was 32.2 (26.8–37.8) and 20.7 (19.2–23.8) hours, for patients < 6 years and ≥ 6 to < 12 years, respectively. Symptom and fever recurrence was more common in patients < 6 years with influenza B (54.5 and 50.0%, respectively) compared with older patients (0 and 25.0%, respectively). Virus titers declined (day 2) for both age groups. Transient virus titer increase and PA/I38X-variants were more common for patients < 6 years. CONCLUSIONS: The safety and effectiveness of single-dose baloxavir were observed in children across all age groups and influenza virus types. Higher rates of fever recurrence and transient virus titer increase were observed in children < 6 years. TRIAL REGISTRATION: Japan Pharmaceutical Information Center Clinical Trials Information JapicCTI-163,417 (registered 02 November 2016) and JapicCTI-173,811 (registered 15 December 2017). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12887-023-03841-5.