Respiratory syncytial virus in pediatric influenza-like illness cases in Lombardy, Northern Italy, during seven consecutive winter seasons (from 2014–2015 to 2020–2021)

Introduction: Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract illness in young children and can also cause influenza-like illness (ILI). Here we investigated the epidemiological features of RSV infection in pediatric ILI cases in Lombardy (a region in Northern Italy accounting nearly 10 million inhabitants) from 2014–2015 to 2020–2021 winter seasons. Material and Methods: Data for this study were retrieved and statistically analyzed from the database of virological influenza surveillance of the regional reference laboratory for Lombardy within the Italian influenza surveillance network (InfluNet). Results: RSV accounted for nearly 19% of pediatric ILI with a risk of infection nearly two-fold greater than that of individuals ≥15 years. RSV positivity rate increased to 28% considering 0–5 years old children. Although in children ≤5 years the risk of infection from influenza viruses resulted nearly two-fold higher than the risk of RSV infection, the age group 4–6 months and 7–12 months showed a five-fold greater risk of infection from RSV than from influenza. Children ≤5 years of age with pre-existing underlying health conditions had a nearly five-fold greater risk of getting RSV infection than otherwise healthy 0–5 years old children. RSV was identified in ILI cases <15 years of age in all considered winter seasons except in the 2020–2021 season. Discussion: Sentinel surveillance of ILI allowed us to identify groups at higher risk of RSV and influenza infection and to define the start, duration, timing, and intensity of the RSV and influenza community circulation. This surveillance approach can be implemented to assess the RSV circulation and impact in a real-time manner.