Chronological analysis on isolation of antigenic variants of A(H3) influenza virus in 2002–2003 influenza season in Sendai and Fukuoka, Japan

Antigenicity of all the A(H3) viruses which we isolated from specimens collected in Sendai and Fukuoka, Japan in 2002–2003 influenza season was examined by hemagglutination inhibition test (HI) using the ferret antiserum against A/Panama/2007/99 (H3N2), the vaccine strain for the past three influenza seasons, provided by WHO Influenza Collaborating Center, Japan. All the viruses were isolated in MDCK cell system. From the beginning of the epidemic, we found that many isolated virus strains reacted with the standard antiserum more than 4- to 16-fold less than its homologous titer, in both cities. The number of isolation of such antigenic drifted viruses increased and dominated the isolates in this season. We show here the chronological appearance of such variants in this season. Furthermore, we performed genetic analyses on the HA genes of the isolates along with those of previous two consecutive seasons. Thus it was revealed that the isolates of this season make a clear cluster different from the cluster in the previous seasons, and the variants of 2002–2003 season make a different cluster from those of previous two seasons. Moreover, no difference was found in clusters which could explain the difference between these variants and nonvariants, and among variants isolated in four cities. [Copyright &y& Elsevier]