Increased susceptibility to measles in infants in the United States

Background. Women born in the United States after measles vaccine licensure in 1963 transfer less measles antibody to their infants than do older women. This may result in increased susceptibility to measles among infants. Objective. To determine the effect of maternal year of birth on the risk for measles in infants. Methods. We enrolled 128 unvaccinated infants less than or equal to 15 months of age who had documented exposure to measles from 1990 through 1992 in a retrospective cohort study. We interviewed their mothers by telephone to obtain demographic data, medical and vaccination history, and details of measles exposure and outcome. We used logistic regression analysis to estimate the effect of maternal year of birth. Results. Infants whose mothers were born after 1963 had a measles attack rate of 33%, compared with 12% for infants of older mothers. In logistic regression analysis, the adjusted odds ratio for maternal year of birth (born after 1963) was 7.5 (95% confidence interval 1.8, 30.6). Other significant risk factors were older infant age, mothers who developed measles after delivery, and exposure within 2 days of the rash onset of the exposing case. Conclusions. Infants whose mothers were born after 1963 are more susceptible to measles than are infants of older mothers. An increasing proportion of infants born in the United States may be susceptible to measles. Infants at high risk of exposure to measles should be vaccinated at 12 months of age. Vaccination programs that reduce transmission of the measles virus in the general population reduce the risk of infant exposure to measles.