Background.

Respiratory syncytial virus (RSV) is the major cause of bronchiolitis and pneumonia in young children. Prevention of RSV disease in children in certain high risk groups through use of immunoglobulin preparations has been recommended by the American Academy of Pediatrics since 1998. A more precise understanding of the timing of annual RSV epidemics should assist providers in maximizing the benefit of these preventive therapies. The objective of this study was to determine whether current national RSV surveillance data could be used to define the timing of seasonal outbreaks

Methods.

Weekly RSV testing data from the National Respiratory and Enteric Viruses Surveillance System for the period July 1990 through June 2000 were analyzed. RSV season onset week, peak week and duration were calculated for the entire United States, Census regions and select local laboratories. Season variability was estimated by comparing calculations for individual RSV seasons to median measurements for the entire surveillance period

Results.

RSV seasons in the South region began significantly earlier (P< 0.05) and lasted longer (P< 0.05) than seasons for the rest of the nation. RSV seasons in the Midwest region began significantly later (P< 0.01) and were shorter (P< 0.05) than those for the rest of the nation. Local RSV seasons varied substantially by year and by laboratory. The variability between laboratories generally increased with distance between laboratories

Conclusions.

Onset weeks and durations of RSV seasons vary substantially by year and location. Local RSV data are needed to accurately define the onset and offset of RSV seasons and to refine timing of passive immune prophylaxis therapy recommendations.