A rubella epidemic in an unvaccinated pregnant population. 2. Seroepidemiology

The study is based upon primary rubella infections detected in a collection of 7,781 serum pairs from as many pregnant women out of a total number of about 12,500 in the Oslo area of Norway in 1974. In the spring of that year, a rubella outbreak occurred. The results obtained on the serum pairs were compared and supplemented with acute serodiagnostic data obtained from the files of the virus laboratories, informations obtained from the mothers when interviewed in 1976 and from the files of application for legal abortions. From October 1973 through December 1975 a total of 118 serologically confirmed pregnancy infections were detected in the area, 94 of which took place between February and July 1974. The year following the outbreak showed scattered cases, whereas the last half of 1975 was free of cases. The pairs of the collection covered about one third of the pregnancy months occurring between February 1974 through January 1975, and the rubella infections diagnosed by seroconversion during this period indicated an attack rate for the epidemic period of 2.8% pregnancy months, and 0.35% for the post-epidemic period. 50% of the infections went unrecognised when they occurred, whereas only 17% seemed to have been subclinical. It is estimated that at least 9 children may have been born with rubella sequelae following infections during this period, when the legal abortions because of rubella in taken into consideration.