Objective:To investigate the epidemiological characteristics, genotypes and genetic evolution of respiratory syncytial virus (RSV) isolated in Shanghai from April 2020 to December 2021, which was a period from the COVID-19 outbreak to the phase of regular epidemic prevention and control.Methods:This retrospective study collected the nasopharyngeal secretions or nasopharyngeal aspirates of children with acute respiratory tract infection (ARTI) admitted to the Shanghai Children′s Hospital from April 2020 to December 2021. PCR-capillary electrophoresis and RT-PCR were used for virus identification and the amplification of the gene fragment of the second hypervariable region of RSV G protein. Homology analysis and phylogenetic analysis were conducted using bioinformatics software. Chi-square test was used to compare the detection rates of RSV. Results:A total of 6 211 samples were collected and 13.62% (846/6 211) of them were positive for RSV. The positive rates of RSV in male and female patients were 14.07% (503/3 574) and 13.01% (343/2 637), respectively, with no significant gender difference (χ 2=1.467, P=0.226). The highest detection rate of RSV was found in children ≤6 months of age, and the rate of RSV infection decreased gradually with age (χ 2=352.942, P<0.001). No RSV-positive specimens were detected from April 2020 to August 2020, after which the detection rate of RSV gradually increased with two epidemic peaks occurring from December 2020 to February 2021 and from August to October 2021. The predominant epidemic subtype was RSV subtype B in 2020 and the first 9 months of 2021, which was gradually replaced by RSV subtype A in the last 3 months of 2021. The 176 strains of RSV subtype A obtained in this study were all ON1 genotype, and the nucleotide homology of the Shanghai epidemic strains was 90.20%-99.50%. All of the 250 strains of RSV subtype B were BA9 genotype, and the nucleotide homology of the Shanghai epidemic strains was 90.10%-100.00%. Conclusions:From April 2020 to December 2021, with the regular prevention and control of COVID-19, there is a change in the epidemic season of RSV. The prevalent genotypes of RSV subtypes A and B are ON1 and BA9, respectively, and the subtype A gradually replaces subtype B as the most prevalent subtype.