As Pennsylvania State University copes with an ongoing outbreak of mumps, infectious-disease experts are investigating why vaccinated young people are getting sick and whether a booster shot would help.
"We didn't see outbreaks like this 10 years ago," said Kelly L. Moore, director of the Tennessee Immunization Project and chair of the mumps work group for the Advisory Committee on Immunization Practices (ACIP). Its vaccination recommendations become official policy for the U.S. Centers for Disease Control and Prevention when approved by the CDC director.
The government already recommends two shots, given at 12 to 15 months and just before entering school. The second shot was added in 1989.
In recent outbreaks, public health authorities have suggested a third dose for people in affected communities. Penn State recently began suggesting shots for several hundred students it deemed at high risk for mumps exposure.
Penn State had its first case Jan. 29 and had 36 probable or confirmed cases on March 3, just before students headed out for spring break.
The total is now 45, said Shelley Haffner, infectious-disease manager at Penn State's student health center. She said the school would have a vaccination clinic on Thursday. Students will be expected to submit the cost of the vaccine, which the school is still working out, to their insurance companies. The MMR (measles, mumps, rubella) vaccine usually costs about $150, she said.
All but one of the students with mumps symptoms has had at least one dose of the vaccine, Haffner said. The first student to become ill was fully vaccinated. Students are supposed to have two doses before entering the university, but some may not have had them at the correct age, she said.
The Pennsylvania Department of Health said Wednesday that it was not aware of mumps cases at any other Pennsylvania colleges.
Experts in 2012 decided that the evidence for a third shot was "inconclusive," Moore said. Now, because of multiple outbreaks of mumps over the last two years, the CDC and ACIP are studying the issue again. A recommendation is expected next year, she said.
Her group had its first meeting last month, so it doesn't have answers yet, but it will investigate a lot of questions. Those include whether and how much the vaccine's effectiveness wanes over time and whether a third dose could help prevent or shorten outbreaks. Another question is whether the vaccine's formula, which has been based on the same strain of virus since it was introduced in 1967, needs tweaking. Most of the current cases involve a different strain of mumps.
Outbreaks since 2006 have occurred in well-vaccinated groups, including college and high school students.
Before the mumps vaccine was developed, there were about 186,000 cases a year in the United States. Not all were reported then, so the CDC said the actual numbers were likely much higher. Since then, there have been many years when only a couple of hundred cases were reported.
But there was a spike in cases in 2006, mainly among college students in the Midwest. That year, 6,500 people got sick.
There was another spike last year, with more than 5,000 cases reported. As of the end of February, 1,077 cases had been reported this year.
Trish Perl, an infectious-disease doctor at the University of Texas who is on the board of the Infectious Diseases Society of America, said there was some evidence that the vaccine becomes less effective over time.
"That is probably contributing to the outbreaks we've been seeing," said Kristen Feemster, director of research for the Vaccine Education Center at Children's Hospital of Philadelphia.
Giving third doses during outbreaks is becoming more common, but it's not clear how effective that is. "Mumps is really a pretty infectious disease" and it takes two weeks after a shot for increased immunity to kick in, Perl said.
Effectiveness of the vaccine is estimated at 77 percent after one dose and 88 percent after two.
People infected with the virus can spread it for two to three days before they have symptoms and five days after. The incubation period is long — 16 to 18 days — and some infected patients never have symptoms.
Outbreaks in the last decade have mainly been among vaccinated people who belong to groups with frequent, close contact. "The other thing that plays a role," Moore said, "is how intense and prolonged is your exposure to the virus." College students, who live and go to class in groups, and may exchange saliva through kissing and sharing of drinks, are prime candidates for exposure. The disease largely has not spread to wider communities.
In most people, mumps causes relatively mild symptoms such as fever, headache, muscle aches, fatigue, and lack of appetite. The classic symptom is swelling and tenderness of the glands just below or in front of the ear or jaw.
Mumps can occasionally cause more severe complications, including inflammation of the testicles, brain, tissues covering the brain and ovaries as well as deafness.
Perl and Moore both pointed out that vaccinated mumps victims have far fewer complications than those who have not gotten the shots. Haffner said that only one Penn State student has had a mumps complication and that did not require hospitalization.
Before vaccination, 12 percent to 50 percent of male mumps victims got testicular inflammation, Perl said. Three percent to 10 percent of boys and men who have been vaccinated get it. The percentage who suffered hearing loss has fallen from 4 percent to 1 percent or less, according to the Morbidity and Mortality Weekly Report, a CDC publication. The percentage needing hospitalization fell from 5.5 percent to 1 percent to 2 percent.