14-05-2025
Bronchiolitis in Late Spring? Think Human Metapneumovirus
It is mid-April in a pediatrician's office in the southeastern United States. A worried mother holds a 3-month-old on her lap. The baby has audible wheezes, clear nasal discharge, and mild nasal flaring. The symptoms began shortly after the child started group childcare.
Kristina A. Bryant, MD
'This can't be RSV,' the mother insisted. 'She had the RSV shot before she left the newborn nursery, and you shared with me how effective that is.' The pediatrician consulted the electronic health record and confirmed that the baby had received nirsevimab on the second day of life. During a discussion with the family during a prenatal consultation, he had shared the results of a study conducted by the Centers for Disease Control and Prevention's (CDC's) New Vaccine Surveillance Network during the 2023-2024 respiratory virus season. Among infants entering their first respiratory syncytial virus (RSV) season, nirsevimab was 90% effective at preventing RSV-associated hospitalization.
The pediatrician affirmed that nirsevimab protects most babies from getting sick enough to be hospitalized with RSV but gently reminded the mom that it might not prevent every infection.
'Surely this isn't flu,' the mother said. 'I had the flu shot during my pregnancy. That's supposed to protect my baby during the first 6 months of life.' The pediatrician recalled a paper published in JAMA Pediatrics that found that a flu shot during pregnancy reduced the risk for flu in babies younger than 6 months of age by one third. Flu hospitalizations were reduced by approximately 40%.
While praising the mom for doing everything she could to protect her baby against viral infections, he noted that vaccination reduced but did not completely eliminate the risk for infection, and influenza, especially influenza B, was still circulating in the community. He recommended testing for RSV, flu, and COVID-19.
The mother was relieved when the test was negative. The baby was discharged home with supportive care and the diagnosis of viral bronchiolitis. The following day, the baby worsened and presented to a local emergency department. An extended viral panel performed on a nasal swab revealed the diagnosis: human metapneumovirus (hMPV).
Like RSV, hMPV is a member of the Pneumoviridae family, and clinical manifestations are often similar. Just like RSV, hMPV is a common cause of bronchiolitis in children. It can also cause upper respiratory tract infections, croup, pneumonia, otitis media, and asthma exacerbations.
hMPV infections are common, if less well known to parents than RSV and influenza. Nearly all children are infected at least once by age 5. According to the American Academy of Pediatrics, the incidence of hospitalizations attributable to hMPV in young children is lower than RSV but similar to influenza and human parainfluenza type 3 (hPIV3). Children hospitalized with hMPV tend to be older than those hospitalized with RSV.
Prior to the COVID-19 pandemic in the US, hMPV typically circulated from early January to early June, peaking in late March. By comparison, RSV season started earlier (typically late October), peaked in late December, and trailed off in April. There was considerable overlap with RSV circulation. Co-infections with these two viruses are well-described and may be associated with more severe disease. Seasonality for both hMPV and RSV was disrupted during the pandemic, and according to a recent CDC report that analyzed data from the National Respiratory and Enteric Virus Surveillance System (NREVSS), less than 5 weeks of co-circulation of RSV and hMPV occurred in most regions of the US during the 2022-2023 and 2023-2024 seasons. Notably, typical hMPV patterns have returned. Clinicians curious about hMPV circulation in their region can check out the NRVESS interactive dashboard at
Ultimately, the baby with hMPV returned to the pediatrician for follow-up and the mother wanted to talk about what might have been done to prevent this infection.
No vaccines are currently available to prevent hMPV, but candidate vaccines are being studied in adults and children. The safety and immunogenicity of an investigational combination mRNA vaccine targeting both hMPV and hPIV3 was recently evaluated in a phase 1b study. In children 12-59 months who were seropositive for both viruses at baseline, the investigational vaccine was well tolerated and boosted antibody levels. A study comparing two investigational live attenuated hPIV3/hMPV vaccines in children aged ≥ 24 months to < 60 months of age is ongoing.
For now, the best ways to prevent hMPV are the commonsense approaches recommended for the prevention of all viral respiratory tract infections. Wash hands frequently with soap and water. Avoid touching your eyes and mouth (or the eyes and mouth of your baby) with unwashed hands. Clean surfaces regularly that might be contaminated with hMPV, such as shared toys. Finally, stay away from sick people if you can. That recommendation, the mom and the pediatrician agreed, is the one that often seems impossible.
