How Iowa Got the Mumps

The largest outbreak of mumps to hit the U.S. in nearly 20 years has epidemiologists scratching their heads. After all, the illness, with its characteristic inflamed salivary glands and swollen throat, is relatively rare in North America, thanks to the widespread availability of a childhood vaccine against measles, mumps and rubella (the so-called MMR shot).

Nobody was too alarmed in December when the first few cases were reported to Iowa's health officials; the state has been averaging about five cases of mumps a year, so the numbers weren't out of line. By last week, however, the total of known and suspected cases had jumped to 365, and the disease seemed to be spreading to nearby states. Illinois and Kansas have reported spikes in mumps infections. And although the outbreak appears to have started in college-age adults, the latest information shows that mumps is now striking people ages 2 to 80.

What's really puzzling about the Iowa outbreak is that most of the victims seem to have been fully vaccinated. Among those whose medical history has been confirmed, two-thirds got both of the required doses of MMR.

So what gives? There are several possibilities. Doctors know that about 5% of the people who get vaccinated against mumps don't get protected because their immune system, for some reason, doesn't respond. Or perhaps their protection has weakened over the years, or maybe the vaccine is not as effective as it should have been. To sort all that out, investigators from Iowa and the Centers for Disease Control and Prevention are studying residents who became ill as well as those who were exposed to the virus but didn't get sick.

Another possibility the investigators are sure to explore is whether there is any link between the Iowa outbreak and an ongoing epidemic of mumps that sickened at least 43,322 people in England and Wales last year. (The British outbreak, in turn, may be tied to a 1994 campaign to vaccinate older children that focused on measles and rubella but was unable, for complicated reasons, to include a mumps shot.) The viruses responsible for the British and American outbreaks have similar genetic makeup, but further testing will be required to determine whether they are related.

Fortunately, mumps doesn't usually cause problems beyond swelling and misery that last about five days. Even so, doctors watch for signs of meningitis or, in young men, a testicular inflammation that could theoretically lead to infertility.

College students may be at particular risk because they live in such close quarters and are more likely to exchange saliva. Nancy Anderson, director of the Longbrake Student Wellness Center at the College of Wooster in Ohio, advises students to wash their hands regularly and avoid going out if they get sick. Wooster also requires incoming students to have received both doses of MMR vaccine.

But as the Iowa outbreak reminds us, the mumps vaccine is not 100% effective. That's why you have to stay alert in an outbreak, even if you've been inoculated.