Emergence and epidemiology of dominant variants of human metapneumovirus in the United States between 2016 and 2021.

Human metapneumovirus (HMPV) causes acute respiratory disease worldwide and is the second leading cause of lower respiratory infection and hospitalization in young children in the USA. There is no licensed vaccine or therapeutic. HMPV mutates rapidly; however, the specific genomic features that explain strain dominance remain undefined because there is limited routine genomic surveillance of HMPV. We analyzed prospectively collected nasal specimens and medical data from 8,000 pediatric acute respiratory infection cases and sequenced 219 HMPV whole genomes from Pittsburgh, PA, between 2016 and 2021. Only A2, B1, and B2 subgroups were detected. The dominant subgroup varied between seasons. Variants with an in-frame 111- or 180-nucleotide (nt) insertion that nearly duplicates the preceding flanking region in the 660-nt G gene (encodes the attachment protein) were the predominant A2 viruses detected by 2016-2017. Among B2 viruses, variants with smaller in-frame insertions in the same location of the G gene became dominant by 2017-2018. Each insertion length formed a distinct phylogenetic clade. The insertions are in the ectodomain and contain positively charged residues or predicted O-glycosylation sites. Epidemiological analysis revealed that HMPV infection was independently associated with age, insurance type, and comorbidities. Elevated disease severity was independently associated with age and comorbidities, although not with HMPV subgroup. To our knowledge, in the USA, this is the earliest detection of the A2 insertion variants and the first report of the B2 insertion variants. It is the largest population-based genomic HMPV study that provides a detailed phylodynamics and epidemiological analysis of prospectively collected clinical specimens.IMPORTANCEHuman metapneumovirus (HMPV) is a leading cause of lung infection and pediatric hospitalizations worldwide for which there is no licensed vaccine or therapeutic. Because HMPV mutates rapidly, understanding which mutations enhance its ability to multiply and spread is important for the development of interventions and treatments. We prospectively collected patient data and nasal specimens from children with symptoms of acute respiratory illness. The predominant A2 and B2 HMPV variants circulating in the population contained insertions in the attachment protein, which suggests that these insertions may be advantageous to the virus. Furthermore, our analysis suggests that age, insurance type, and underlying health conditions were associated with HMPV infection. Age and underlying health conditions were associated with elevated HMPV disease severity, whereas HMPV subgroup was not. This large HMPV genomic epidemiological study provides insight into patient factors associated with disease and the emergence of the dominant variants in the USA.