Human Metapneumovirus Seasonality and Co-Circulation with Respiratory Syncytial Virus - United States, 2014-2024.

Human metapneumovirus (hMPV) infections cause acute respiratory illness and lower respiratory tract disease. Respiratory syncytial virus (RSV) is a closely related virus within the Pneumoviridae family, and hMPV and RSV infections are associated with similar clinical manifestations. Although no specific antiviral therapies or vaccines exist for hMPV, vaccines and monoclonal antibody products are available to protect against severe RSV disease. This report summarizes hMPV circulation relative to the timing of RSV epidemics before, during, and after the COVID-19 pandemic. Polymerase chain reaction testing results reported to the National Respiratory and Enteric Virus Surveillance System during July 2014-June 2024, were analyzed. Before the COVID-19 pandemic, the median hMPV season onset, peak, and offset occurred in early January, late March, and early June, respectively (median duration = 21 weeks). The 2021-22 season was atypically long (35 weeks); seasonality reverted to more typical patterns during the 2022-23 and 2023-24 seasons. In the two COVID-19 pandemic seasons (2021-22 and 2022-23) and one postpandemic season (2023-24), RSV offsets occurred earlier in January (2021-22 and 2022-23) or March (2023-24) than before the pandemic, when the median offsets occurred in April. The annual interval from peak RSV to peak hMPV circulation increased from a prepandemic median of 11.5 weeks (range = 2-17 weeks) to 19 weeks (range = 19-20 weeks) during and after the pandemic. Fewer than 5 weeks of cocirculation of RSV and hMPV occurred in most regions during the 2022-23 and 2023-24 seasons. Real-time surveillance of RSV and hMPV co-circulation patterns can help guide clinician-directed testing and supportive care, optimize the use of prevention products, prompt detection of and response to outbreaks, and help ensure health care system preparedness for seasonal increases in illnesses.