Respiratory syncytial virus activity--United States, 1999-2000 season.

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness (LRTI) among infants and children worldwide (1) and is an important cause of LRTI among older children and adults (2). Despite the presence of maternal antibodies, most hospitalizations occur among infants aged <6 months, and nearly all children are infected by age 2 years (3). Although primary infection is usually most severe, reinfection throughout life is common (4). In temperate climates, RSV infections occur primarily during annual outbreaks, which peak during winter months (5). In the United States, RSV activity is monitored by the National Respiratory and Enteric Virus Surveillance System (NREVSS), a voluntary, laboratory-based system. This report summarizes trends in RSV activity reported to NREVSS from July 1999 through June 2000 and presents preliminary surveillance data from July 8 through November 21, 2000, which indicate that RSV community outbreaks are becoming widespread.