Antigenic and molecular surveillance of influenza virus strains in the period 1990-1992

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Influenza virus is subject to frequent antigenic changes. As a consequence, the World Health Organization (WHO) operates a world-wide laboratory network to monitor these changes. RIVM participates in this system. It isolates influenza virus strains from clinical specimens obtained from a sentinel station network of general practitioners. It also compares Dutch virus strains with foreign virus strains and with the virus strains used in the current influenza vaccine. The results are reported to the WHO and to the Ministry of Health of the Netherlands. The results of such comparisons enable the Dutch health authorities to consider adaptations of the WHO-proposal about the vaccine for the next season to the epidemiological situation in the Netherlands. A total of 256 influenza A virus strains isolated in 1990-1992 in the Netherlands and other countries were analysed by haemagglutination-inhibition (HI) tests using ferret antisera and monoclonal antibodies and, partly, by determination of the nucleotide sequences of the haemagglutinin gene (HA1). In the season 1991/92, most strains belonged to the subtype A(H3N2), a minority to the subtype A(H1-N1). The overall antigenic structure was similar to that of the corresponding vaccine strain for both subtypes. This means that the influenza vaccine will have rendered optimal protection against the circulating virus strains in 1991/92. A markedly heterogeneous geographic distribution was noted with the two most frequently isolated HI-variants of subtype A(H3N2). One prevailed in the Netherlands and in Spain, the other in Sweden, France, and the United States. Even within the Netherlands the proportions of the two variants showed regional variation. This heterogeneity did not have implications for the vaccine efficacy.