A rubella epidemic in an unvaccinated pregnant population. I. Screening methods and serological results.

During the 1974 rubella epidemic in Oslo, paired sera from 7781 pregnant women were examined for rubella-specific IgG antibodies using the single radial haemolysis (SRH) test. 12.4% of the women with seronegative in early pregnancy, with no significant variations between the different age groups. In the seropositive samples, there was a mean decrease in the rubella IgG concentration of about 30% during 22 gestational weeks. 53 seroconversions were found. In addition, 22 primary infections were detected by the demonstration of rubella specific IgM antibodies using an indirect immunofluorescence test. The persistence of the rubella IgM antibodies was examined and a large individual variation was found, from less than 1 month up to 4 months after onset of the disease. The serological techniques, particularly the SRH test, were found to be well suited for mass screening.