Prevalence and Molecular Characteristics of Human Metapneumovirus Among Children in Shanghai, China, 2021-2025

At the end of 2024, human metapneumovirus (HMPV) infections surged significantly in China, attracting widespread attention and concerns at home and abroad. This study aims to investigate the prevalent and molecular characteristics of the HMPV infection in children during 2021-2025 in Shanghai, China, including the latest epidemic prevailing in December 2024. The demographic information and the results of laboratory pathogen detection from July 2021 to May 2025 were collected and analyzed. The G gene of HMPV was amplified and sequenced using Sanger sequencing, followed by phylogenetic analysis to determine the genotypes. A total of 26 952 pediatric patients with acute respiratory tract infection (ARTI) were enrolled in this study. The overall positive rate of HMPV was 4.65% (1252/26952). The popularity mainly peaked in Winter and early Spring, with the highest detection rate in 1~3 years age group and with no statistically significant gender difference. Among 206 successfully sequenced G gene samples, four genotypes were identified: A2c (66.99%, 138/206, all of which were the A2c_111nt-dup variant), B2 (24.76%, 51/206), A2b (7.77%, 16/206), and B1 (0.49%, 1/206). From 2021 to 2024, the A2c_111nt-dup variant was dominant. However, a genotype shift to B2 occurred in December 2024, leading to a significant surge in HMPV infections that persisted into 2025. The dominant genotype of HMPV has changed from A2c_111nt-dup to B2, causing the outbreak in Shanghai from December 2024 to April 2025. Sustained surveillance is essential for predicting and monitoring its prevalence.