Vaccination-induced HI antibody response to intraepidemic influenza A(H3N2) virus variants of the 1996-1997 epidemic season

Intraepidemic antigenic and genetic variation was indicated when H3N2-subty pe influenza A virus strains isolated during the 1996-1997 epidemic season in Finland were studied for reactivity in the haemagglutination inhibition (HI) assay and for nucleotide sequences coding for the variable HA1 domain of viral haemagglutinin. Thirty prevaccination- and postvaccination paired sera taken from subjects who had been vaccinated against influenza during t he previous autumn were studied for the presence of HI antibody to the homo logous vaccine virus A/Nanchang/933/95, and five field strains representing the genetic and antigenic variability of the 1996-1997 epidemic season. Th e lowest vaccination-induced HI titres in each of the three age groups were detected in the two field strains that had been isolated from vaccinated p atients and belonged to two different genetic sublineages. The intraepidemi c variability of the 1996-1997 field strains in HI reactivity may be indica tive of circulation of virus strains that may be capable of breaking throug h vaccination-induced immunity better than the other strains. (C) 2001Wiley -Liss,Inc.