The Clinical Effectiveness and Tolerability of Oseltamivir in Unvaccinated Pediatric Influenza Patients during Two Influenza Seasons after the COVID-19 Pandemic: The Impact of Comorbidities on Hospitalization for Influenza in Children.

Antiviral therapy such as oseltamivir has been recommended for hospitalized children with suspected and confirmed influenza for almost 20 years. The therapy is officially authorized for newborns two weeks of age or older, however, questions about its safety and effectiveness still surround it. Our goals were to assess the epidemiological features of two consecutive seasonal influenza cases in children following the COVID-19 pandemic; to observe the clinical effectiveness and tolerability of oseltamivir in hospitalized children who were not vaccinated against influenza and had different influenza subtypes, including A(H1N1), A(H3N2), and B; and to identify specific comorbidities associated with influenza in children. We performed an observational study on 1300 children, enrolled between 1 October 2022 and 30 May 2023 and between 1 October 2023 and 4 May 2024, to the IX Pediatric Infectious Diseases Clinical Section of the National Institute of Infectious Diseases "Prof. Dr. Matei Balș". During the 2022-2023 influenza season, 791 pediatric patients tested positive for influenza and received oseltamivir. Of these, 89% (704/791) had influenza A, with 86.4% having subtype A(H1N1) and 13.6% of cases having A(H3N2), and for influenza B, 11% (87/791) of the pediatric patients. Of the total group, 59% were male, and the median age was 2.4 years (1.02-9.28). For the 2023-2024 influenza season, 509 pediatric patients tested positive for influenza, with 56.9% being of the male gender and who were treated with oseltamivir. Of these patients, 81.6% had influenza A and 18.4% had influenza B. Treatment with neuraminidase inhibitors, specifically oseltamivir, 2 mg/kg/dose administered twice daily for 5 days, was well tolerated by the children, and we recorded no deaths. The duration of hospitalization for patients with a fever after the oseltamivir administration was significantly longer for patients with A(H1N1) infection than A(H3N2), during both seasons. We identified more complications in the 2022-2023 season and a decreasing number of influenza B for the 2023-2024 season. Among children with comorbidities, the most common were asthma, gastrointestinal diseases, and metabolic and endocrine diseases. In terms of effectiveness, oseltamivir significantly reduced the intensity of influenza symptoms, thus reducing the number of days of hospitalization (