Hypoxemic Human Metapneumovirus Pneumonia in a Young Immunocompetent Man in the First Autumn After the Reclassification of Coronavirus Disease

Although human metapneumovirus<i> </i>(hMPV) infection can induce severe symptoms in older adults or immunocompromised patients, it usually causes mild symptoms in young immunocompetent adults. The prevalence of hMPV infectious disease is highest during the late winter and early summer. We report a hypoxemic case of hMPV infection in a young immunocompetent man that occurred in the first autumn after the reclassification of coronavirus disease (COVID-19) from Class 2 to Class 5. A multiplex-nested polymerase chain reaction (MN-PCR) test using a sputum sample was useful for rapid detection of co-infection with hMPV and <i>Haemophilus influenzae (H. influenzae)</i> as causes of pneumonia and acute respiratory failure. In early October 2023, a 24-year-old healthy man without comorbidities visited our hospital with a fever, sore throat, and nasal congestion. The antigen test results for influenza and COVID-19 were negative. After 6 days, he presented with a productive cough and exertional dyspnea. He was immediately admitted to our hospital due to hypoxemia with a percutaneous arterial oxygen saturation of 90% breathing room air. Although bacterial pneumonia was suspected based on bilateral ground-glass opacities and infiltrative shadows on chest computed tomography (CT), the MN-PCR test using a nasopharyngeal swab indicated hMPV infection. On the 2^nd day of admission, the MN-PCR test using a sputum sample (Group 5 in the Geckler classification) indicated that hMPV and<i> H. influenzae </i>were the causative pathogens for pneumonia. We initiated oxygen supplementation and administered intravenous ceftriaxone sodium hydrate and oral azithromycin hydrate. The patient was discharged on the 7^th day of admission with improved pneumonic shadows and hypoxemia. hMPV infection can occur in any season in the post-COVID-19 era. Even in young immunocompetent adults, hMPV infection can cause pneumonia which may progress to acute respiratory failure with other bacterial co-infection. Conversely, it is clinically important to pay attention to the involvement of hMPV and other respiratory viruses in pneumonia cases that initially appear to be bacterial based on CT findings. MN-PCR test using a good-quality sputum sample is valuable not only for rapid detection of causative bacteria and viruses for pneumonia but also for identifying these co-infections.