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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‐subtype 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 the previous autumn were studied for the presence of HI antibody to the homologous vaccine virus A/Nanchang/933/95, and five field strains representing the genetic and antigenic variability of the 1996–1997 epidemic season. The 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 patients and belonged to two different genetic sublineages. The intraepidemic variability of the 1996–1997 field strains in HI reactivity may be indicative of circulation of virus strains that may be capable of breaking through vaccination‐induced immunity better than the other strains. J. Med. Virol. 65:584–589, 2001. © 2001 Wiley‐Liss, Inc.