The burden of influenza is substantial in all children, including those without any underlying medical conditions. Both influenza A and B viruses cause a high rate of morbidity and a large demand for hospitalization in the paediatric population. Although most children with influenza are treated as outpatients, especially among young children, influenza-attributable illnesses often cause hospitalization. Limited data exist on the clinical effectiveness of oseltamivir treatment of influenza in the youngest age group of children.

We performed a prospective cohort outpatient study to investigate the burden and clinical presentation of influenza and the impact of oseltamivir among infants during their first year of life. A substantial proportion of infants, 13.5%, were infected with influenza viruses during their first influenza season. In this special age group, oseltamivir treatment rapidly decreased the viral load in nasopharyngeal secretions as well as the duration and severity of symptoms.

We also performed a retrospective study to investigate the differences between influenza A and B infections in children <16 years of age who were hospitalized with virologically confirmed influenza at the Department of Paediatrics and Adolescent Medicine, Turku University Hospital. In the same age group, we also searched for changes in the demographic, management and clinical features of children hospitalized with influenza during the 25-year period of 1993-2018. Our results revealed that there were no significant differences between influenza A and B infections in hospitalized children when the outcomes were adjusted for age. During the 25-year period, the relative proportion of hospitalized children <2 years of age almost halved, while the proportion of children aged 6-15 years almost tripled. The median duration of hospitalization in all children shortened from 2 days to 1 day.

According to our results, the clinical severity of influenza A and B is similar in hospitalized children. Our findings strengthen the importance of influenza B in contributing to the total morbidity of influenza. The results of our outpatient studies provide new information about the incidence and clinical features of influenza among infants and demonstrate the benefit of antiviral treatment in children during their first year of life.