Prevalence of rubella antibodies in Massachusetts schoolchildren.

In 1982, 1,871 (79%) of 2,368 eligible 6th, 10th and 12th grade students in Massachusetts participated in a statewide serosurvey for rubella antibodies. Sera were screened at the Centers for Disease Control (CDC) by a reference hemagglutination inhibition assay at 1:8, equivalent to approximately 15 International Units (IU)/ml. Sera negative by the CDC hemagglutination inhibition assay were retested using an enzyme immunoassay, a passive hemagglutination assay, and a commercial hemagglutination inhibition test. The approximate screening levels were 10 IU/ml, 7.5 IU/ml, and 5 IU/ml, respectively. Overall seroprevalence levels varied from 76.4% screening at 15 IU to 93.1% including seropositives from any of the tests. Persons with a school record of vaccination had significantly higher seroprevalence levels than persons without records. However, only 78.3% of persons with a record had antibody greater than or equal to 15 IU compared with 60.0% without records; considering any detectable antibody, the comparison is 95.6% versus 71.4%. The low titers in vaccinees appeared to be due to a falloff of antibody with time since vaccination. Of students with a single vaccination noted in the record with exact dates, 92.3% who were vaccinated 0-4 years prior to the study had antibody at 15 IU compared with less than 78% of students with antibody who were vaccinated five or more years prior to the study. In contrast, using more sensitive assays, there was no significant decline in seroprevalence with time since vaccination. Revaccination studies and epidemiologic data suggest that almost all persons with detectable antibody whether above or below 15 IU/ml are immune to rubella. Thus, immunity levels in Massachusetts schoolchildren in the 6th, 10th, and 12th grades are probably in excess of 90%.