Robustness of measles immunity in parturient women in Gipuzkoa, Basque Country, Spain, in the post-vaccination era

The infants of mothers with vaccine-induced immunity lose passive acquired measles antibodies earlier than infants of naturally infected mothers. This study included two cohorts of parturient women: one composed of women who gave birth in 1990 (end of the epidemic period), and another comprising women who gave birth in 2006 (after eight years without virus circulation). Immunoglobulin G (IgG) antibodies against measles (IgG-AM) were investigated by enzyme immunoassay in stored serum samples (-40°C). Measles-IgG titres of >400 mIU/mL were found in all 185 parturient women who gave birth in 1990, all with natural immunity. Of 185 women who gave birth in 2006, most of whom had vaccine-induced protection, measles-IgG were undetectable in 4.9% (<150mIU/mL), values were borderline in 7% (150-299 mIU/mL), and the geometric mean titre was lower (p<0.001), being 3.4 to 3.8 times lower in women aged <28 years. The changing levels of maternal measles antibodies suggest that in Spain, the window of susceptibility to measles in infants is increasing. To protect susceptible infants against measles in countries with long-established vaccination programs where measles immunity in parturient women was artificially acquired, it is essential to ensure that both doses of the routine measles vaccine achieve a coverage of >95%, and that infants receive the first vaccination dose before 15 months of age (e.g. at 12 months).