Variation in frequency of natural reinfection with influenza A viruses.

To investigate the frequency of natural reinfection with influenza A viruses, 55 children followed from birth along with the 44 other children and 85 adults in their families were observed for varying periods between 1975 and 1981 in the Houston Family Study. Persons infected in 1976 or early 1977 with influenza A subtype H3N2 experienced a high rate of reinfection during the 1977-78 season (63% in one group of young children) and additional reinfections in 1980-81. Persons in whom H3N2 infection was first observed in 1977-78 had little or no reinfection in 1980-81; one reinfection was observed in 1980-81 among 19 young children at risk. Only one reinfection with influenza A subtype H1N1 was observed following introduction of the virus in early 1978 despite exposure in 1978-80 and a substantial outbreak in 1980-81. Statistically significant protection from reinfection in young children followed from birth was not noted for the 1977-78 H3N2 outbreak but was present in 1979-81 for both H3N2 and H1N1 viruses. Reasons for variations in reinfection suggested by this data include 1) socioeconomic and other factors influencing intensity of exposure, 2) age at primary infection, 3) varying degrees of antigenic difference between sequential variants of a major subtype, and 4) unrecognized immunogenicity or virulence differences between subtypes and variants.