A Comparative Study of Hospitalized Versus Community Cases in Portugal (2021–2023)

Background: Respiratory syncytial virus (RSV) is the leading cause of acute respiratory infection (ARI) in young children, but its genetic diversity requires ongoing surveillance. Methods: From 2021 to 2023, a total of 619 and 94 RSV-positive samples from the National Respiratory Syncytial Virus Surveillance Network (VigiRSV) and the Sentinel Influenza and other respiratory viruses surveillance network (Sentinel ISN), respectively, were analysed. The RSV A and RSV B typing was assessed by a multiplex real-time RT-PCR. Sanger sequencing was performed on a subset of samples (n = 495). Phylogenetic analysis was carried out on partial glycoprotein G sequences. Clinical and epidemiological data were compared through Pearson Chi-Square tests. Results: RSV Subgroup A was more prevalent (53.5%, 85/159) in the 2021/2022 season, whereas in the 2022/2023 season, it was RSV Subgroup B (82.1%, 435/530) in both networks. RSV A strains in VigiRSV clustered mainly to A.D.1 (39.0%, 39/100), whereas in Sentinel ISN, they clustered in A.D.5 (30.0%, 3/10). RSV Type B clustered mainly to B.D.E.1 (96.6%, 372/385) in both networks. All lineages cocirculated during the study period and in both surveillance networks. Regional clusters were identified for both subgroups. Conclusions: This study provides new insights into RSV genetic variability in Portugal, namely, the cocirculation of lineages and intravariability among lineages within both subgroups during the study period and in all Portuguese regions. However, our study is based on partial sequencing of the G gene, and because of this limitation, our results should be considered with great caution.