Concurrent antigenic analysis of recent epidemic influenza A and B viruses and quantitation of antibodies in population serosurveys in Italy.

Laboratory investigations of virus isolation and serum antibodies in a Mediterranean country (Italy) demonstrated that influenza A and B viruses, and often both, circulated every winter in Italy. The winter 1987/88 was characterized by a low level of influenza activity, as shown by the limited number (47) of influenza virus isolates, the majority of which (61%) belonged to the influenza B type. In contrast, the 1988/89 influenza season was exclusively associated with the circulation of influenza type A viruses. The A(H1H1) subtype was largely predominant (97%), as compared to the low incidence of the A(H3N2) subtype (3%). During the 1989/90 winter a co-circulation of A and B influenza viruses was observed, A(H3N2) strains being responsible for 96% of the virologically confirmed cases. Antigenic analysis of the virus isolates showed some antigenic variation in influenza A viruses of both H1N1 and H3N2 subtypes, whilst antigenic stability was found among the influenza B virus isolates. Overall, the above virological findings correlate with the data concerning the pattern of influenza virus circulation in Northern Europe and the UK during the three years surveyed. The results of serum antibody surveys conducted in each post-epidemic period are also reported.