Background: To determine the epidemiological trends in pediatric influenza and changes in healthcare resource use from 2005 to 2021 using nationally representative outpatient database in Japan.

Methods: We conducted a retrospective cohort study consisting of 3.5 million children with 177 million person-months during 2005-2021 using Japan Medical Data Center claims database in Japan. We investigated trends in incidence rates of influenza and changes in healthcare resource use (eg, use of antivirals) over 17 years. Generalized estimation equations were used to investigate the impact of the 2009 influenza pandemic and the coronaviral disease 2019 (COVID-19) pandemic on incidence rates of influenza and related healthcare utilization.

Results: Annual incidence rates of influenza were estimated at 55 cases per 1000 person-years with a 93% relative increase [95% confidence interval (CI): 80%-107%] during the 2009 influenza pandemic and a 99.4% relative reduction (95% CI: 99.3%-99.4%) during the COVID-19 pandemic. Similar patterns were observed for health resource use, total healthcare costs, admission rates and antiviral agent use. Approximately 80% of children with influenza received prescriptions for antivirals. The most prescribed antivirals were oseltamivir, but we observed a temporal increase in zanamivir use during 2007-2009, an increasing trend in laminamivir use during 2010-2017, a temporal increase in baloxavir use in 2018. Symptomatic medications with serious side effects (codeine, salicylate, sedative antihistamine) showed decreasing trends over the study period.

Conclusions: Influenza incidence and healthcare resource use were largely affected by the 2009 influenza pandemic and the COVID-19 pandemic. Our study indicates improvement of quality in healthcare delivered to children.