Epidemiologic characteristics and seasonal distribution of human metapneumovirus infections in five epidemic seasons in Stockholm, Sweden, 2002-2006.

The presence of human metapneumovirus (hMPV) was analyzed retrospectively by reverse transcriptase-polymerase chain reaction (RT-PCR) in five epidemic seasons, in Stockholm, 2002-2006. The occurrence of hMPV was compared with five common respiratory viruses; respiratory syncytial virus, influenza A virus, influenza B virus, parainfluenza virus and adenovirus. With a detection rate of 2.9% (n = 143/4,989) in nasopharyngeal samples over the whole period, hMPV was the fourth most common respiratory virus after RSV, influenza A and parainfluenza virus. hMPV genotype A dominated over genotype B, out of 91 genotyped virus samples 87 belonged to genotype A and four belonged to genotype B. Approximately 50.3% (n = 72/143) of the hMPV positive patients were <3 years, 49.7% (71/143) were > or =3 years and 38,5% (n = 55/143) were <1 year. The relative frequencies of hMPV infections in the three age groups were 2.8% (72/2,579), 2.9% (71/2,410) and 2.6% (55/2,122), respectively. This age distribution differed from RSV, influenza A, B and parainfluenza virus. hMPV epidemics peaked in March, not coincident with RSV or parainfluenza virus. In successive epidemic seasons, large outbreaks of hMPV alternated with small outbreaks in a regular, biannual pattern. Large hMPV virus epidemics were anticyclical to large RSV epidemics. It is concluded that the epidemiology of hMPV differs markedly from other common respiratory viruses.