MEASLES ANTIBODIES AND HERD-IMMUNITY IN 20-YEAR-OLD AND 40-YEAR-OLD NORWEGIANS

The introduction of a measles vaccination programme in Norway in 1969 using one dose of vaccine, and since 1983 two doses, was followed by a substantial decrease in the incidence of the disease. Since 1992, the annual incidence has been less than 20 cases. Small clusters and outb reaks have occasionally been observed among military personnel and unv accinated children. This paper describes a seroepidemiological investi gation of the level of immunity among 1,188 military conscripts, aged 18-28 years (mean 20.7) compared with 695 healthy 40-year-olds. The co nscripts had been offered measles vaccine in infancy, in some cases al so at 12-13 years of age, but they had also been exposed to wild measl es virus, since the virus continued to circulate many years after the vaccination had started. The measles immunity in this group is conside red to indicate the immunity level among the first 5 cohorts offered m easles vaccine in Norway. The 40-year-olds had grown up in a community with no measles vaccination. Their level of immunity gives an indicat ion of the level finally obtained when there are no vaccinations, and thus of the level that would induce herd immunity against measles in t he Norwegian population. The aims of the vaccination programme must be to obtain a corresponding immunity. The results of the investigation show that the percentages with measles antibodies in the respective gr oups were 92.3 and 98.1. The observation of measles outbreaks among yo ung Norwegian conscripts, as well as reports from several countries on outbreaks in university and college settings with levels of seroposit ivity of even more than 95%, indicate that the seropositivity in the 2 0-year-old group may be too low to afford protection, especially when this group is living under close conditions. Consideration should be g iven to the need for an intensification of the existing vaccination pr ogramme to ensure that the protection level needed for herd immunity i s reached.