The Clinical Effectiveness and Tolerability of Oseltamivir in Unvaccinated against Influenza Pediatric Patients in Two In-fluenza 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 goal was to assess the epidemiological features of two consecutive seasonal in-fluenza cases in children following the COVID-19 pandemic and also to observe the clinical ef-fectiveness and tolerability of oseltamivir in hospitalized children who were not vaccinated against influenza and who had different influenza subtypes, including A (H1N1), A (H3N2), and influenza B, and to identify specific comorbidities associated with influenza in children. We performed an observational study on 1300 children, enrolled between 1st October 2022 and 30th May 2023 and 1st October 2023-4th of 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 sea-son, 791 pediatric patients tested positive for influenza and received Oseltamivir. Of these, 89% (704/791) had influenza A, with 86.4% subtypes A(H1N1) and 13.6% of cases had A(H3N2), and influ-enza B 11%(87/791) 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 influen-za, 56.9% were male gender and were treated with Oseltamivir. 81.6 % had Influenza A, and 18.4% had influenza B. Treatment with neuraminidase inhibitors like Oseltamivir, 2mg/kg/dose twice daily for 5 days, was well tolerated by the children, and we recorded no deaths. The duration of fever hospitaliza-tion 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 disorders, 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 (p=0.001) as well as post-infection complications (p=0.005) in both groups. Oseltamivir was less effective in Influenza A (H1N1) than A(H3N2), (p=0.0035). In this study, we evaluated the clinical effectiveness of oseltamivir therapy for all influenza types/subtypes in children, and the length of hospitalization. We identified comorbidities associated with prolonged dura-tion of hospitalisation. Influenza vaccination should be the main tool in the prevention of influenza and its complications in children, especially with comorbidities