The epidemiologic features of respiratory syncytial virus (RSV) infection were investigated by detecting the virus in throat swab specimens from patients with acute respiratory symptoms attending the sentinel surveillance clinics in Yokohama City in 5 seasons from July 1998 to June 2003. Throughout the 5 seasons, RSV was found from 181 in 2683 specimens tested (6.7%) by virus isolation in cell culture or genome detection using nested RT-PCR, and this detection rate followed that of influenza virus (infl.v.) (441/2683; 16.4%), while the proportion of RSV isolates in a season fluctuated from 12 to 22% of all causative viruses identified. Analysis of monthly detected number of strains revealed that the peak of RSV isolation was present in December which preceded that of infl.v. by 2 months. Moreover, RSV strains were isolated sporadically during late spring to early autumn (from May to September) when infl.v. was scarcely detected. Among 181 RSV strains, 172 could be subgrouped; 104 were identified as subgroup A, while 68 were B. Subgroup A were detected more frequently throughout the 5 seasons (57%), though the proportion varied seasonally and subgroup B exceeded both in 2000/2001 and 2002/2003 seasons (61% and 70%, respectively). Clinical characteristics of RSV-infected patients were compared with those infected with infl.v. Age distribution of cases revealed that RSV detected predominantly (79%) from lower age groups (less than 5 years) compared to infl. v (41%). As for the proportion of cases showing clinical symptoms of lower respiratory inflammation predominated in RSV-infected irrespective of age groups.
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