Human metapneumovirus was first isolated from humans with respiratory tract infections in 2001 and is closely related to respiratory syncytial virus (RSV). This paper examines its prevalence in a large cohort of otherwise healthy children followed from birth to 5 years of age. Children attending a primary care clinic in Tennessee between 1976 and 2001 with acute respiratory tract infections had nasal washings collected and cultured for common respiratory viruses.

There were 1127 episodes of acute lower respiratory tract infection in the 2009 children attending the clinic. In 687 cases nasal washings were obtained and 408 (59%) were culture negative for viruses. 248 of these culture negative specimens remained available for subsequent polymerase chain reaction and 49 (20%) were positive for human metapneumovirus. Extrapolation of these data suggests that human metapneumovirus can be isolated in 12% of all acute lower respiratory tract infections in this cohort. The spectrum of clinical diagnoses was comparable to that caused by RSV: 59% had bronchiolitis, 18% croup, 8% pneumonia, and 14% exacerbations of asthma. The virus was also detected in 15% of samples from children with upper respiratory tract infection, but in only one of 86 asymptomatic children.

Causality cannot be assumed from this study. The use of different viral detection methods makes frequency comparisons problematic, and there is potential for selection bias as 39% of respiratory tract infection episodes did not provide samples for analysis. However, it is likely that human metapneumovirus is a new pathogenic virus in children. This paper should lead to further work to examine its prevalence outside the US and in other age groups.