Seroprotection against Measles Among British Columbia Women of Childbearing Age

Background: An outbreak of measles occurred in British Columbia in the spring of 2010. Unimmunized children and under-immunized adults of childbearing age were the two prominent groups affected. Population-based seroprotection in adults was examined using banked prenatal specimens. Purpose: To assess seroprotection to measles among women of childbearing age in British Columbia to help guide immunization policy. Methods: Banked prenatal sera submitted for routine testing to the Provincial Health Services Authority Laboratory from January 2009 through December 2010 for years of birth from 1960 to 1979 were chronologically selected for testing. Specimens were tested for anti-measles IgG by the Behring assay. A required sample size of 662 samples in each of two birth cohorts was estimated based on assumed measles seropositivity of 95% in the 1960-1969 cohort and 92% in the 1970-1979, 5% type I error, and 60% power. Results: 1326 samples were included in the analysis. Ninety-two percent (1219/1326) of samples demonstrated immunity to measles. Ninety-five percent (95%CI 94,97) of women with years of birth 1960-9 had seroprotective levels, whereas 88% (95%CI 86,91) of women born 1970-79 had seroprotective levels with a trend to decreasing immunity with younger age observed when plotted by 5-year age groups. Reduction in seroimmunity at younger ages was associated with a higher proportion of equivocal results. Conclusion: Measles is a highly transmissible viral infection and population protection required to maintain herd immunity is estimated at 95%. Immunity as assessed by anti-measles IgG in younger adults is below the levels required to interrupt transmission.