[The influenza season 1997/'98 and the vaccine composition for 1998/'99]

The 1997/'98 influenza season in the Netherlands was marked by influenza A/H3N2 activity which never reached a true epidemic level. There was no real peak activity either but a prolonged period of increased activity of approximately eight weeks with a maximum in week 13, when sentinel physicians reported 16.6 cases of influenza-like illness per 10,000 inhabitants. It was not until week 18 of 1998 that the influenza activity declined to baseline levels. During the season, almost exclusively influenza A/H3N2 viruses were isolated, of which the majority resembled the new strain influenza A/Sydney/5/97 (H3N2). Further analysis of these variant viruses revealed that, although there was some cross-reactivity with the vaccine strain (A/Nanchang/933/95), no optimal protection could be expected to be induced by the vaccine. Antigenic characterisation of the sporadic influenza A/H1N1 and influenza B viruses showed that these were related to the vaccine strains. As a result of these findings, the World Health Organization (WHO) recommended to change the H3N2 strain in the influenza vaccine for the season 1998/'99 to an influenza A/Sydney/5/97(H3N2)-like strain. Based on epidemiological data from other countries, it was also decided to change the influenza A/H1N1 component to an influenza A/Beijing/262/95 (H1N1)-like strain.