Epidemiological characterization and phylogenetic analysis of human metapneumovirus isolated from children in Ningbo, China, 2020-2024.

Human metapneumovirus (hMPV) is an important cause of acute respiratory infections in children, but genomic surveillance data from Ningbo, a coastal port city of China, remain limited. We investigated the epidemiology, genetic diversity and phylogeographic patterns of hMPV among children in Ningbo from 2020 to 2024 using 6,632 respiratory specimens from paediatric outpatients and 26 hMPV-positive samples. The overall hMPV-positive rate was 3.62% (240/6,632), with a peak in 2022 (6.86%). An atypical summer peak in 2022 and a prolonged 2023-2024 season suggested altered seasonality in the post-COVID-19 period. Preschool-aged children (1-6 years) were the most affected age group and the proportion of viral co-infections among hMPV-positive cases increased significantly over time. Phylogenetic analysis of the 26 genomes showed co-circulation of four lineages, with B2 and A2.2.2 predominating across multiple years. We identified one A2.2.2 strain (20240959) carrying a 111-nt (37-aa) insertion in the G gene, whereas other A2.2.2 strains retained the classical non-duplicated G sequence. Several lineage-associated substitutions in F protein were observed between lineages A, B1 and B2, and mapping suggested that a substantial proportion of these sites fell within predicted linear B-cell epitope-prone regions. Phylogeographic reconstruction indicated multiple introductions of hMPV into Ningbo from other parts of China and from overseas. These findings demonstrate the genetic and epidemiological complexity of hMPV circulation in a major port city and underscore the need for continued, genome-informed surveillance to monitor hMPV evolution.