Risk of Measles Is Higher Among Infants of Younger Mothers
Nov. 4, 1999 (Atlanta) -- Infants of mothers born after 1963 -- the date of the measles vaccine licensure -- are more likely to contract measles than are infants of older mothers, according to a study in the November issue of Pediatrics. The study is the first to document this increased susceptibility and provides further evidence for measles vaccination at 12-15 months.
"The main finding is that the susceptibility to measles is increasing in infants in the U.S. because more and more infants are born to mothers who have vaccine-induced measles immunity," lead author Mark Papania, MD, MPH, tells WebMD. Papania is acting chief of the Measles Elimination Activity branch of the CDC's National Immunization Program.
The study involved about 125 unvaccinated infants younger than 15 months who were exposed to measles in the early 1990s. Mothers were interviewed by telephone for details of measles exposure and outcome, demographic data, and medical and vaccination history.
The researchers found that the attack rate for measles was 33% among infants whose mothers were born after 1963; the rate was 12% for infants of older mothers.
Papania explains that infants rely on maternal antibodies for protection against measles. Antibodies are molecules in the immune system that help the body fight infection. During pregnancy, antibodies are transported from the mother to the developing fetus. Antibodies against measles form following a measles infection or after receiving a measles vaccine.
When a woman is immune to measles due to having a vaccine, lower antibody levels develop than if she developed immunity to the disease after having had measles herself, according to Papania. Therefore, in women with a vaccine-induced measles immunity, lower antibody levels give less antibody to their children," he says. "So, the infants become susceptible [to measles] at an earlier age."
According to Papania, "Women who have had measles disease have higher [antibody levels] than women who have been vaccinated," he says. "Women born before the vaccine was introduced almost all had measles disease and have higher [antibody levels]. Women born after the vaccine was introduced have lower [antibody levels], so you would expect that their children might become susceptible at an earlier age. That is exactly what we found."
Currently, the CDC recommends that children be vaccinated against measles at 12-15 months of age. "We must vaccinate on time," says Papania. "If we wait longer [than 15 months] we are opening up a window of susceptibility and we could start to have cases [of measles] again."
"In the U.S., this is an important phenomenon -- that children of vaccinated mothers are more susceptible," says Papania. "But it's under control because we have such high [vaccine] coverage among school children and preschool children that these kids aren't being exposed to measles. The important thing is, if the same ... [increased susceptibility] were to happen in countries where measles is still endemic [contagious], is that going to have a big impact on infant mortality?" He says that he hopes future research will address this issue.
Close to one million people worldwide died of measles in 1998, according to the World Health Organization.