Four more cases of mumps have been confirmed since last Wednesday in the outbreak connected to Harvard University, meaning a total of 13 people have been diagnosed in that cluster in recent weeks, according to the Massachusetts Department of Public Health.
An additional 10 laboratory-confirmed cases have occurred across the state since Jan. 1, and health officials are still investigating whether any of the illnesses are connected.
Tufts University officials said one mumps case had been confirmed Monday. A second Tufts student has a suspected case of mumps, and officials are awaiting test results.
Both students, who attend the Medford campus, have been isolated and are doing well, university spokesman Patrick Collins said Tuesday. He said the university does not know if the two cases are linked.
Last week, Bentley University in Waltham and the University of Massachusetts Boston each reported one confirmed case of mumps among students. Separately, an athlete visiting UMass Boston from another school was found to have the disease.
Massachusetts law requires undergraduates to be vaccinated against mumps before admission, and most of the Harvard students had been vaccinated, according to state health officials. They theorize the illnesses occurred because the vaccine, which is effective about 88 percent of the time, failed to work, or because the virus mutated in ways that rendered the vaccine less effective.
Mumps outbreaks occur from time to time even in highly vaccinated populations. Isolated cases are also reported every year. In Massachusetts, there were six cases in 2015, five cases in 2014, and 71 in 2013, when an outbreak at Boston College resulted in 39 confirmed and probable cases.
This year, in addition to the Harvard outbreak, clusters of mumps cases have been reported at colleges in Indiana, California, and New Hampshire. Similar college outbreaks occurred in California in 2011 and in 11 states in 2006.
Mumps symptoms include puffy cheeks or jaws from swollen salivary glands, as well as fever, headache, muscle aches, and fatigue. The virus spreads through coughing, sneezing, sharing utensils, and touching surfaces where infected mucus or saliva droplets have landed.
Most people recover fully, but sometimes mumps infects men’s testes, causing pain and affecting fertility. Less commonly, the virus attacks women’s ovaries and, very rarely, it can infect the brain or the lining of the brain and spinal cord.