Potential limitations may result from the differences in criteria for inpatient treatment and use of diagnostic methods depending on the individual decision by the admitting physician. The number of hospitalizations corresponded to a conservative incidence estimate of at least 118 PIAH per 100,000 children <18 years of age. However, this may considerably underestimate the true pediatric PIAH incidence as only patients with laboratory-confirmed PIA were included; children hospitalized with respiratory symptoms or influenza-like illness without being tested for influenza were not captured in this study.