Post-outbreak study: Mumps immunity waning

Washington -- A postmortem of a mumps outbreak in 2006 -- the nation's largest in 20 years -- is raising questions about the effectiveness of the vaccine.

Investigators who took a close look at the reasons behind the 6,584 cases of mumps that hit Midwestern states in March through May of 2006 found that a surprisingly large percentage of those who contracted the illness had received the recommended two doses of the measles, mumps and rubella vaccine.

In contrast, investigations of past outbreaks of vaccine-preventable diseases often revealed that low immunization rates were responsible.

The study found that among those infected in the eight hardest-hit states, 63% overall and 84% of those 18 to 24 years old had received two MMR injections. The first dose of the vaccine usually is administered when a child is between 12 and 15 months of age with a second dose between 4 years and 6 years.

In 2006, the national, two-dose coverage rate among adolescents was 87%, the highest in U.S. history, said the authors.

The study, published in the April 10 New England Journal of Medicine, was conducted by the Centers for Disease Control and Prevention and several state health departments.

"This is a really important study from a couple of perspectives," said Martin Myers, MD, executive director of the National Network for Immunization Information, an educational group based in Galveston, Texas, to which many medical organizations belong, including the AMA.

The study raises the possibility that the vaccine's effectiveness wanes about 10 years after a second dose, said Dr. Myers, a professor of pediatrics at the University of Texas Medical Branch at Galveston. It also points up that mumps can be a serious disease, although it is thought of as the least dangerous of the measles, mumps and rubella trio. "There were still 5% of these [infected] people who had complications," he said.

Nonetheless, it is important to accent the positive, said a number of vaccine experts, including Dr. Myers. Without the high level of vaccination in the U.S., the tally of mumps cases could have soared into the tens and hundreds of thousands, said Amy Parker, MPH, co-author of the study and an epidemiologist at the CDC.

A mumps outbreak that swept through the United Kingdom at about the same time affected more than 70,000 people. "Our high vaccine coverage was protective of having it spread much further," she said.

"My take on it," said Dr. Myers, "is that the outbreak probably came from the United Kingdom, where immunization rates have been declining because of misleading and false information about the safety of the measles vaccine."

The mumps vaccine in the U.K., as in the U.S., is administered in combination with the measles and rubella vaccines.

Although no scientific evidence has been found to link the measles vaccine and adverse health effects, fears persist that the vaccine or the preservative once used in it cause autism. These worries have led many parents in this country and Great Britain to forgo the shots for their children.

"If more people in the U.K. had been vaccinated, we probably never would have had this outbreak in the United States," said Dr. Myers. "Misinformation about vaccine safety has global implications."

The threat of exposure from abroad is also likely to be an ongoing problem, said the study's lead author Gustavo H. Dayan, MD, formerly with the CDC but now director of clinical development at vaccine manufacturer Sanofi Pasteur in Swiftwater, Pa. He noted that few countries vaccinate their children against mumps.

William Schaffner, MD, chair of the preventive medicine department at Vanderbilt University School of Medicine in Nashville, Tenn., and a member of the Infectious Diseases Society of America's board of directors, believes that all developed countries should provide their children with two doses of mumps vaccine.

The disease carries rare but dangerous complications, he said. "Having taken care of young men with orchitis and even a patient with encephalitis, those are not nice things." Mumps can also cause hearing loss, pancreatitis, meningitis and inflammation of the ovaries.

Since the vaccine's effectiveness has come into question, a new vaccine would be welcome, he said. "The lights have to stay on in the laboratory at night."

The mumps vaccine, which was approved in 1967, was developed by Maurice Hilleman, PhD, from the mumps virus that infected his 5-year-old daughter, Jeryl Lynn. The Jeryl Lynn strain is still in use.

Without a more effective vaccine on the horizon, the public health world will likely begin debating whether and how to boost immunity using the currently available vaccine. Groups such as the federal Advisory Committee on Immunization Practices will probably discuss whether there should be a third dose of vaccine and when that dose should be administered, experts said.

"There would be questions about cost effectiveness and whether to use a monovalent mumps vaccine or MMR," said Dr. Schaffner. "Would this be administered in adulthood or pre-college, and is it necessary at all? After all, this only occurred once in 20 years," he added. "But I think the ACIP is very likely to examine these data and discuss them."