Phylodynamics of human metapneumovirus and evidence for a duplication-deletion model in G gene variant evolution

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BACKGROUND: In December 2024, human metapneumovirus (hMPV) gained global attention amid rising cases in Chinese hospitals, prompting a World Health, Organization (WHO) statement indicating case numbers remained within expected ranges. To assess whether a variant of public health concern was emerging and to examine global hMPV phylodynamics, we conducted a genomic surveillance study in western Washington State. STUDY DESIGN: We sequenced hMPV genomes from inpatient and outpatient samples collected between 2021 and 2025 in western Washington State and constructed phylogenomic and phylodynamic trees. RESULTS: We obtained 60 hMPV-A and 39 hMPV-B genomes, including 13 from November 2024 to January 2025. Following COVID-19 pandemic disruptions, hMPV seasonality returned to typical patterns after 2023. Genomic analysis showed hMPV-A predominance since 2022-23, with co-circulation of A2b1, A2b2, B1, and B2 sublineages. The A2b2 sublineage was most prevalent and all genomes carried a 111-nt G gene duplication. Phylogenetic evidence suggests the 111-nt variant evolved from a prior 180-nt duplication via a 69-nt deletion rather than through independent duplication events. Most sublineages showed multiple co-circulating clades, except A2b1. Phylodynamics showed recovery of global diversity after pandemic-related declines and a higher evolutionary rate in hMPV-A compared to hMPV-B. No distinct evolutionary features of heightened concern were detected. CONCLUSIONS: Despite recent concerns, our findings indicate that hMPV circulation in, the USA follows expected seasonal patterns, with ongoing introductions of diverse viral variants from preexisting sublineages rather than emergence of a novel variant. Continued genomic surveillance is essential, particularly as vaccine development progresses.

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