Parainfluenza virus type 3: seasonality and risk of infection and reinfection in young children.

In Houston the temporal occurrence of infections with parainfluenza virus type 3 has evolved from an endemic to an epidemic pattern. Continuous virological surveillance for six years demonstrated that most infections occurred the late winter or spring after influenza virus activity. At least two-thirds of children observed in the Houston Family Study were infected with this virus in each of the first two years of life, and the risk of illness was about 30/100 children per year. After two years of age, the infection and illness rates dropped to 32 and 8 per 100 child-years, respectively. Most lower-respiratory-tract disease was associated with primary infection, and the risk for infection was greater during the second year for the smaller proportion of children who escaped infection during the first year. The risk during the first year may have been modified by passively acquired maternal antibody.