Reinfection with influenza A (H3N2) virus in young children and their families.

The frequency and consequences of reinfection with influenza A virus were studied by longitudinal observation of families for a three-year period in which two epidemics of influenza A (H3N2) occurred. Seven children followed from birth were reinfected 10-25 months after their first infection. Two children were reinfected by the same H3N2 virus while the others were reinfected with a closely related variant. At least five of these reinfections were accompanied by respiratory illness. Reinfection illness was similar to that accompanying primary infection. For children in the second and third year of life during the 1978 epidemic, the rate of infection was the same for those who had been previously infected (seven of 12) as for those who had not been previously infected (22 of 40). Reinfection was detected in 26% of older siblings and 6% of parents. The occurrence of reinfection may have important implications for elucidation of the protective immune response and for development of prophylaxis for influenzal infections.