Antigenic drift and variability of influenza viruses.

Annual influenza epidemics are caused by rapid evolution of the viral genome. Continuous and extensive antigenic variation has been shown for hemagglutinin (HA), the principal immunizing antigen of the virus. Monitoring of the antigenicity of circulating influenza viruses is necessary for selection of the most suitable vaccine strains. In this study, characterization of influenza A/H3N2 and influenza B viruses recently circulating in Germany was performed by molecular and antigenic analysis. Sequencing and phylogenetic analysis of the HA1 gene revealed that two distinct groups of H3N2 viruses co-circulated during 1997/1998. The majority of isolates clustered with the new drift variant A/Sydney/5/97, as was also shown by antigenic characterization. A noteworthy genetic drift of H3N2 viruses was evident during the winter 1998/1999. However, serological characterization using hemagglutinin inhibition tests did not result in detection of viruses belonging to different groups as confirmed by molecular analysis. Influenza B viruses isolated during 1996/1997 were antigenically closely related to the prototype vaccine strains B/Beijing/184/93 or B/Harbin/7/94. Molecular analysis demonstrated that our German 1996/1997 isolates differed by nine amino acids from B/Harbin/7/94 and represented a group of viruses that was completely different from the Harbin strain. Retrospective studies revealed the circulation of B/Yamanshi/166/98-like viruses in Germany already during the 1996/1997 season. Our results suggest that molecular analysis of the HA gene is important to complement the antigenic characterization for a better selection of appropriate vaccine strains.