After the worst mumps outbreak in 20 years disproportionately struck college students last spring, many feared the disease would pose a threat to campuses when students returned this fall. So far in September, a cluster of 33 infected college students in Illinois and another suspected case on a Virginia campus have medical professionals on alert, even though they say it’s too early to tell if the new cases are linked with the spring’s outbreak or if they herald another record year for infections of students.

In DuPage County, Illinois, two institutions -- Wheaton College and Benedictine University -- have experienced confirmed mumps cases so far this fall: 32 at Wheaton and 1 at Benedictine, David Hass, spokesman for the DuPage County Health Department, said Wednesday. More cases of the highly contagious disease are expected to occur at Benedictine, where the only confirmed case was diagnosed Monday, Hass said. At Wheaton, new cases have arisen throughout the month since the first diagnosis September 7, said Tiffany Self, a spokeswoman. Meanwhile, several states away, the University of Virginia reported a “highly probable” case of mumps after a student visited its health center Friday.

All those affected in Illinois are college students who received the recommended two-course vaccination series protecting against measles, mumps and rubella, Hass said. Experts say a double dose of shots is effective 90-95 percent of the time.

Mumps, commonly associated with swelling of the salivary glands, generally is not considered a serious disease, although it can, in rare circumstances, lead to side effects including meningitis, inflammation of the testicles -- which can lead to infertility -- spontaneous abortion, and permanent deafness. The viral disease has generally been well contained by a vaccination program begun in 1967, but the number of cases mysteriously increased nearly ninefold last spring. Between January 1 and May 2, 2,597 cases were reported in 11 states, mainly in the Midwest, the Centers for Disease Control and Prevention reports. In comparison, between 2001 and 2003, fewer than 300 cases were reported each year nationally.

Of those affected by the spring outbreak, 38 percent were in the 18-24 age range, many of whom were in college, where close quarters can accelerate spread of the virus. Hass said it’s too early to comment on whether the cases at Wheaton and Benedictine are linked to the spring outbreak, but an expert on infectious diseases said incomplete efficacy of the two-course vaccine series, combined potentially with waning immunity, could leave young adults susceptible to periodic mumps outbreaks.

“We are really defining right now how long the immunity from the mumps vaccine lasts, how durable it is, how good it is,” said Leigh Grossman, head of the division of pediatric and infectious diseases at UVa. “I think what we’re learning, much like what we’ve learned with pertussis, measles, and now mumps -- this is a group of kids who never had this disease, were properly vaccinated, and are again susceptible in their late teens/early adult years.”

“Whether it’s vaccine failures coupled with waning immunity, there’s a large group of (susceptible) students that are out there. So at some level we’re going to have to accept that these illnesses are not gone and we will see them sporadically in groups that are susceptible and living close together.”

In an April briefing, the CDC's director, Julie Gerberding, said waning immunity from the vaccines did not appear to be playing a role in the outbreak, explaining that if waning immunity were the primary problem, older populations would be at greater risk.

“So we are looking into this as one of several possibilities, but I think right now what we know about this vaccine's efficacy, what we know about the under-vaccinated people in this age cohort, and what we know about the sociology of life in some of these community settings, we have ample explanation for why the virus is spreading the way it is,” Gerberding said. The CDC did not respond Wednesday to a request for updated information on the topic, but John Dorman, clinical professor of medicine at Stanford University, said he has not heard any concerns about waning immunity of the mumps vaccine.

But if immunity from the vaccine isn’t decreasing over time, and the efficacy of the vaccine has remained constant, then why was there an increase in cases this year? It’s possible, said Dorman, speaking on behalf of the American College Health Association, that the spring outbreak was caused by a more virulent strain, leading to the increase in infections. But while the vaccine’s efficacy rate of 90 to 95 percent is considered pretty high, Dorman suggested the outbreak would focus greater attention on further improving the current vaccine -- right now, the MMR vaccine offers less protection against mumps than it does against measles or rubella, he said.

The three institutions affected by the latest mumps cases require students to be inoculated against mumps, but state laws grant students the opportunity to seek waivers for medical, religious, or philosophical reasons. At UVa, Wood said the university contacted 1,149 students who had turned in incomplete health forms, lacked a second dose of the vaccine, or had turned in waivers to alert them of a possible mumps case on campus and the opportunity to get inoculated at the student health center.

At Benedictine, college officials developed a plan last week in the wake of mumps cases at nearby Wheaton, and, upon having a confirmed case Monday, had reason to use the plan. They contacted every student -- in addition to posting the news online and on fliers, said Mercy Robb, the university’s spokeswoman. The affected students at UVa. and Benedictine are home recovering, while those at Wheaton either returned home or were placed in isolation apartments for the duration of the illness -- 24 students there have since returned to class, said Self, Wheaton’s spokeswoman.

The college health association stresses that the MMR vaccine is the most effective way to combat the spread of mumps, and urged colleges to consider deferring registration until students provide evidence of two doses in a June letter co-written by the CDC chief and the association's president.