December 1999
ELK GROVE VILLAGE, Ill. - Infants born to mothers who received measles vaccine (mothers born after 1963) may be at increased risk of developing measles themselves prior to vaccination, according to a report in the November issue of Pediatrics.
A retrospective study of 128 unvaccinated infants who were exposed to measles between 1990 and 1992 showed that those whose mothers were born after 1963 had a measles attack rate of 33%, compared with 12% for infants of older mothers.
Most infants receive maternal antibody that offers them initial protection until it is depleted, meaning that how long an infant is protected depends on the mother's antibody titers. Women who were effectively vaccinated have lower antibody titers than those who contracted measles.
Children of mothers vaccinated against measles have lower measles antibody titers at birth, higher rates of seronegativity at 6, 9 and 12 months and higher seroconversion rates to measles vaccine at 6, 9 and 12 months.
Studies conducted before 1963 showed that 85% of people in the United States developed measles by age 10. Surveys conducted from 1965 to 1978 showed that the percentage of 10-year-olds with a history of measles decreased from 70% of children born in 1955 to 14% of children born in 1968.
Measles resurged in the United States from 1989 to 1991 and affected infants under 16 months the most. Infants under 16 months accounted for 24% of the 55,622 reported cases during that timeframe, despite comprising only 2% of the general population.
Although the risk of measles exposure is low in the United States, measles is still endemic in many other countries. As a result, infants leaving the United States should receive measles vaccine or immune globulin. Infants who are at least 6 months old should also be vaccinated if they are known to be exposed to measles or at high risk of exposure in outbreak settings. Infants younger than 6 months whose mothers have developed measles after delivery may receive immune globulin.
Mothers of infants exposed to measles were interviewed in Texas and New Jersey. Of the 128 infants exposed, 34 (27%) developed measles. The interval between initial exposure and rash onset ranged from 10 to 21 days. Infants of mothers born after 1963 had an attack rate of 33%, compared with 12% of infants of mothers born earlier. In cases where mothers had measles after delivery, 80% of infants also got measles. Infants younger than 6 months were less likely to get measles than infants 6-11 months or 12-15 months. Infants exposed at home were more likely to contract measles than those exposed at day care centers. Duration of exposure correlated with risk. In addition, breast-fed infants were less likely to develop measles than those who were not breast-fed.
Another study of U.S. infants showed that measles antibody seroprevalence decreased from 79% at 6 months to 48% at 9 months and 16% at 12 months for infants whose mothers were born before 1963. For infants of mothers born after 1963, measles antibody seroprevalence decreased from 44% at 6 months, 23% at 9 months and 5% at 12 months. This indicates that children of mothers born after 1963 may be more susceptible to measles at a younger age.
For more information:
- Papania M, Baughman AL, Lee S, et al. Increased susceptibility to measles in infants in the United States. Pediatrics 1999;104(5):e59.
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