Antigenic variation of neuraminidase of human type A influenza (H3N2) viruses isolated in Berlin (West).

After the emergence of the A/Hong Kong/1/68 (H3N2) strain of influenza virus antigenic variation of the hemagglutinin and neuraminidase antigens have been demonstrated leading to the identification of the variants A/England/42/72,a/Port Chalmers/1/73 and A/Scotland/840/74. This study describes antigenic changes of neuraminidases in influenza viruses isolated since 1968 in Berlin (West). A collection of 13 isolates of human type A influenza was studied in neuraminidase inhibition tests. The results are in line with data from other laboratories indicating a major change in the neuraminidase antigen as early as December 1969. The strains isolated at that time cross-reacted with an antiserum against the N2-Hong Kong enzyme to less than 50%. During the following years (1970 to 1972) the neuraminidase remained fairly stable. Serological cross-reactions showing 47 to 38% inhibition as compared to the homologous N2-antigen. The neuraminidase of the A/Berlin/3/72 strain revealed a close antigenic relationship to the later appearing A/Port Chalmers 1/73 variant. Two strains isolated in 1975 (January) showed an even further drift away from the then representative A/Port Chalmers/1/73 strain. The fact that the neuraminidase antigens of the Berlin viruses had changed in December 1969 may account for the severe 1969/70 influenza epidemic in Berlin affecting a large proportion of the population. Prevalent anti-neuraminidase antibodies may play a role in restricting a variant carrying a new hemagglutinin to primarily infected individuals by cross-reacting with a closely related enzyme.