Age-specific neutralizing antibody levels against human metapneumovirus during the post-COVID-19 pandemic period: a cross-sectional seroepidemiological study in Taizhou city, Eastern China

The COVID-19 pandemic has altered the epidemic patterns of human metapneumovirus (hMPV) globally. As no licensed prophylactic products currently exist for hMPV, understanding population immunity is essential for identifying susceptible groups and informing prevention strategies. This study aimed to quantify age-specific hMPV neutralizing antibody (NAb) levels and seroprevalence in the postpandemic period. We analyzed serum samples collected in a previously published community-based cross-sectional sero-epidemiological survey conducted in Taizhou city, eastern China, from June 2 to June 30, 2024, using a multistage, age-stratified sampling design with additional oversampling of children aged < 5 years. Serum samples were tested for hMPV NAb levels via a fluorescence-based microneutralization assay, with seropositivity defined as a titer of 1:30. We estimated geometric mean titers (GMTs) and seroprevalence rates across age groups. Factors associated with the NAb titer were examined using generalized linear models. We also examined the relationship between RSV- and hMPV-specific antibody responses, given their potential cross-reactivity. A total of 1303 participants aged 16 months to 93 years were included. Antibody levels were lowest among children aged 16 to < 36 months (GMT: 217, 95% CI: 159–295). The GMTs increased to 331 among children aged 3–4 years and exceeded 1000 among those aged 5–11 years, remaining elevated throughout life. The highest GMTs were observed in adults aged 65 years and older (1829, 95% CI: 1620–2064). Among children aged < 5 years, NAb titers increased by 74.7% (95% CI: 14.4%–167.0%) for each additional year of age. Approximately 67.8% (137/202) of the participants were seropositive in the age groups of 16 to < 36 months. From 6 years onward, the seroprevalence was close to or reached 100%. Higher RSV prefusion F IgG antibody titers were significantly associated with higher hMPV NAb titers (Spearman’s ρ: 0.302, P < 0.001), which may reflect shared epidemiological exposures, individual differences in immune responses, or possible cross-reactive immunity between the two viruses. This study provides population-based evidence of age-specific hMPV NAb patterns in the post-COVID-19 pandemic period. The relatively low seroprevalence in children aged 16 to < 36 months highlights the importance of continued hMPV surveillance in this potentially susceptible population.