Respiratory syncytial virus under 2 years of age: hospitalization trends and risk factors for severe disease – preliminary data from the Portuguese sentinel network

Introduction and Objectives: Respiratory Syncytial Virus (RSV) infection is an important cause of hospitalization in children under five years. A national RSV sentinel network was set up in Portugal in April 2021. We describe the trends in RSV hospitalizations until September 2022 and identify risk factors for severe disease. Methods: Acute respiratory infections in hospitalized children under two years were reported and tested for RSV. RSV disease severity was defined by the need for ventilation or admission to an intensive care unit. Risk ratios were used to assess the association between gender, age group, gestational age, birthweight, chronic conditions, RSV subtype and severity of disease. Results: We detected two RSV off-season epidemics in June 2021 to February 2022 and May to September 2022. 63.3% of RSV-related hospitalizations occurred in children under six months old and 8.0% had chronic conditions. 11.0% had severe disease. Children under six months and with chronic conditions had, respectively, an 18-fold risk and a 2-fold risk of developing severe illness. Discussion: The off-season RSV epidemics were probably triggered by the relaxation of COVID-19 physical distancing measures and immunity debt. In the first epidemic, the proportion of children with severe disease was higher than reported by previous studies, however, this result is probably overestimated due to the high proportion of cases notified by central hospitals. Age < 6 months and chronic conditions predispose to severe disease. As several factors may change the pattern of RSV activity, causing more severe outbreaks at different times, countries should implement year-round RSV surveillance systems.