Comparison of influenza B/Hong Kong virus infections among infants, children, and young adults.

An influenza B/Hong Kong viral epidemic was monitored by surveillance of respiratory illness in three different age groups. Prospective viral monitoring of febrile respiratory illness was a useful mirror of the epidemiologic behavior of influenza in the community. Influenza B virus infection in infants and young children was distinguished by high fever and respiratory symptoms and was occasionally associated with otitis media. In older children and young adults, systemic and gastrointestinal complaints were more prominent. Of young children experiencing their first infection, a hemagglutination-inhibiting antibody response was seen in only one-third. Eleven children who received inactivated B/Hong Kong virus vaccine three years earlier were not protected from clinical infection but exhibited an anamnestic serum antibody response.