[Serum immunity against influenza viruses A/Hongkong, A/Zürich, A/Victoria and A/Swine in the Swiss population].

The HI antibody against the influenza virus A/Zürich/18/75 was titrated in 2351 sera from 17- to 75-year-old blood donors living in 24 different parts of Switzerland. 518 sera selected from these donors with the same distribution of age and location were tested against the viruses A/Hongkong/1/68, A/Victoria/3/75 and A/Swine/15/31. The percentage frequency of 91.7% and 89.4% positive sera reveal almost complete exposure of the population to A/Hongkong and A/Zürich. But only half of the donors possess antibody against the A/Victoria/3/75 virus. It is therefore reasonable to expect that the A/Victoria virus or a closely, antigenetically related virus will cause influenza cases in the winter of 1976/77. Despite small regional differences the distribution of influenza in Switzerland appears to be uniform, in view of the fact that the geometric mean titre of the donors from different parts of the country showed no significant deviation from the total geometric mean. The largest number of positive sera were in the youngest and oldest age groups and the smallest in the 30- to 50-year age group. Antibody to A/Swine/15/31 virus was found almost exclusively in sera from persons aged 50 or over.