Two-year periodicity of respiratory syncytial virus epidemics in Switzerland.

The annual respiratory syncytial virus (RSV) epidemics vary in time and severity. The aims of this study were (1) to describe the time-related pattern of RSV epidemics in Switzerland and (2) to deduce the most effective time period for administration of prophylactic measures to high-risk patients. Descriptive study of (1) RSV hospitalizations between 1997 and 2001 at a pediatric hospital serving a population of 1 million and (2) of national RSV detection rates reported by diagnostic laboratories between 1988 and 1999. 497 RSV hospitalizations and 8,574 reported RSV detections occurring during four and 12 epidemics, respectively, were analyzed. There was fixed alternation of minor and major epidemics differing in the number of RSV infections (two to fourfold), evolution (median interval from onset to peak 13 weeks, range 4-13 weeks vs 8 weeks, range 7-10 weeks; p = 0.065) and median duration (26 weeks, range 24-29 weeks vs 19.5 weeks, range 18-21 weeks; p = 0.005). For minor epidemics it was estimated that a maximum of 85.6% (range, 79.4-86.6%) of annual RSV infections could be covered by a standard five-dose regimen of the monoclonal anti-RSV antibody palivizumab, if initiated in week 50. During major epidemics the most effective time of initiation would be week 43 (88.7%; range 81.9-94.6%). RSV epidemiology in Switzerland is characterized by fixed biannual variation. In the absence of active RSV surveillance, such periodicity is useful for scheduling RSV prophylaxis and for hospital resources management.