Evidence of homosubtypic but not heterosubtypic immunity in young children following wild-type influenza illness.

The effect of subtype-specific influenza illness in young children on the probability of influenza illness in the following year was analyzed for placebo recipients in four 2-year vaccine efficacy studies. There was evidence of homosubtypic protection but no evidence of heterosubtypic protection; however, a moderate-to-low level of heterosubtypic protection could not be ruled out. This analysis supports previous observations that children do not develop lasting, clinically significant heterosubtypic protection after influenza illness.