The findings underscore the importance of having children receive the recommended two doses of the measles-mumps-rubella (MMR) vaccine, according to researchers at the Centers for Disease Control and Prevention (CDC) in Atlanta.
Mumps is a viral infection that causes fever, headache, muscle aches and swollen glands around the jaw. Most people recover in a few weeks, but in a small number of cases the infection can have complications such as inflammation of the brain (encephalitis) that can be life-threatening; hearing loss (usually temporary); or painful inflammation of the testicles or ovaries.
A mumps vaccine became available in the U.S. in 1968, and was recommended for all children in 1977, as part of the combined MMR vaccine (which had been licensed in 1971). After a resurgence of mumps outbreaks in the mid-1980s -- mainly among high school and college students -- public health officials began recommending that children receive a second MMR dose when they enter school.
By 2005, there were only 314 cases of mumps reported in the U.S. for the year.
In 2006, however, an outbreak of mumps, seen largely among college students in eight Midwestern states, brought that year's number up to nearly 6,600 cases.
Another outbreak that began at a summer camp in June of last year eventually sickened more than 1,500 people in New York and New Jersey, largely children and teenagers between the ages of 7 and 18.
In both of those outbreaks, many of the young people who became sick had gotten two doses of the MMR vaccine.
The 2006 outbreak prompted the current study, lead researcher Dr. Preeta K. Kutty told Reuters Health. She and her colleagues wanted to gauge the prevalence of mumps antibodies -- which signal immunity against the virus -- among Americans of different age groups in the years before the 2006 outbreak.
Using data from a U.S. national health study conducted between 1999 and 2004, they found that 90 percent of 6- to 49-year-olds had antibodies to mumps in their blood.
The lowest prevalence was among people born between 1967 and 1976, at 86 percent.
Among Americans born between 1977 and 1986 -- the age group hardest hit by the 2006 outbreak -- about 90 percent had mumps antibodies. And among non-Hispanic whites in that age group -- who were more affected by the outbreak than other racial groups -- only 87 percent had mumps antibodies.
People born between 1967 and 1976 were probably less affected by the 2006 outbreak because they were old enough to be out of school and not living in close quarters, Kutty said. So the combination of less-than-optimal mumps immunity levels and communal living appears key.
It's estimated that between 90 percent and 92 percent of the population must be immunized against mumps to provide so-called herd immunity -- where the level of immunity in the whole population is high enough to prevent large outbreaks of an infection.
So we need to achieve high levels of vaccination, Kutty said. The bottom line for parents, she said, is that children should get the recommended two doses of the MMR vaccine; the first is given between the ages of 12 and 15 months, and the second between the ages of 4 and 6 years.
Some parents balk at having their children receive the MMR out of concern that is has been linked to the risk of autism. However, the research that suggested an MMR-autism link a decade ago has since been widely discredited, and numerous international studies have failed to find an association between the vaccine and autism.
In an editorial published with the study, Dr. M. Patricia Quinlisk of the Iowa Department of Public Health raises the question of whether two doses of MMR are enough to prevent mumps outbreaks -- since, in both of the recent large outbreaks, many of those who fell ill had gotten two shots.
If two doses are not enough, she writes, one option might be to give a third dose. Another would be to delay the second dose until adolescence, since mumps immunity may wane over time.
We believe that two doses of MMR are good for conferring mumps protection, Kutty said.
The double dose is estimated to be 80 to 90 percent effective against mumps, according to the CDC. Whether any changes to the vaccination schedule could make it more effective, Kutty said, is an open question.