Special populations.

Bronchiolitis is a leading cause of hospitalisation in infancy, with respiratory syncytial virus (RSV) being the most common pathogen. Younger age, especially infants younger than 3 months of age, environmental factors and genetic susceptibility, are associated with increased risk of hospitalisation. Most importantly, conditions such as prematurity, in particular if associated with chronic lung disease, congenital heart disease, lung disease such as cystic fibrosis, neuromuscular disease or impairment, or congenital or acquired immune deficiencies, are associated with increased risk of RSV hospitalisation and severe RSV lung disease. In these high risk populations, a 3- to 10-fold increase in the rate of RSV hospitalisation has been observed, justifying RSV-specific prophylaxis with palivizumab during the first, and in the populations at highest risk, the second RSV season. Studies have demonstrated a significant reduction (approximately 50%) in the rate of RSV hospitalisation in high-risk infants treated with palivizumab during the RSV season.