Clinical Factors Associated with Documented Radiographic Pneumonia in Adults with Human Metapneumovirus Infection: A Hospital-Wide Cohort Study in Japan

Background/Objectives: Human metapneumovirus (hMPV) is increasingly recognized as an important cause of adult respiratory infection, but data on pneumonia remain limited. We evaluated clinical characteristics, temporal detection patterns, and clinical factors associated with radiographic pneumonia among adults with laboratory-confirmed hMPV infection. Methods: We conducted a single-center retrospective observational study at Kyorin University Hospital, Tokyo, Japan, from December 2020 to February 2026. Adults aged ≥18 years with hMPV detected by multiplex polymerase chain reaction were included. The primary outcome, documented radiographic pneumonia, was defined by newly identified pneumonia-compatible parenchymal abnormalities on chest radiography and/or chest computed tomography (CT). Logistic regression was used to identify clinical factors associated with this outcome. Results: Among 590 hMPV-positive patients, 192 adults were included; 121 were managed as outpatients and 71 were hospitalized. Pneumonia was identified in 68 adults, including 43 of 71 hospitalized patients. Adult hMPV detection shifted from summer predominance in 2022–2023 toward increased spring and winter detections in 2024–2025 after the onset of the coronavirus disease 2019 (COVID-19) pandemic. By 2025, pneumonia was identified in 24.0% of outpatients and 65.5% of hospitalized patients. In the primary multivariable model, higher C-reactive protein (CRP) concentrations (adjusted odds ratio per 1 mg/dL, 1.12; 95% confidence interval (CI), 1.03–1.22; p = 0.008) and abnormal auscultation (adjusted odds ratio, 3.45; 95% CI, 1.21–9.83; p = 0.021) were independently associated with pneumonia. Conclusions: Adult hMPV infection was frequently associated with radiographic pneumonia, particularly among hospitalized patients. Elevated C-reactive protein concentrations and abnormal auscultation may help identify adults with hMPV infection who are more likely to have pneumonia.