Human metapneumovirus infections--biannual epidemics and clinical findings in children in the region of Basel, Switzerland.

Human metapneumovirus (hMPV) epidemics vary in time and severity. We report findings for PCR for hMPV and respiratory syncytial virus (RSV) performed on nasopharyngeal aspirates (NPA) of hospitalized and outpatient children with respiratory tract infections between October 2004 and April 2008. A total of 3,934 NPAs were tested for hMPV and 3,859 for RSV. Of these, 198 (5%) were hMPV positive and 869 (23%) were RSV-positive. Median age was 17 months and 9 months for hMPV and RSV, respectively. Fifty-nine percent of hMPV and 58% of RSV patients were hospitalized. Proportions of hMPV positive samples for the four winter seasons were 0.4%, 11%, 0.2%, and 14%. For RSV, they were 28%, 15%, 28%, and 28%. HMPV epidemics follow a biannual variation in our area. Major epidemics were observed in winter seasons starting in odd years (2005/06 and 2007/08), minor epidemics in those starting in even years (2004/05 and 2006/07). RSV epidemics usually follow a reciprocal biannual pattern, leading to annually alternating major RSV and hMPV epidemics.