A mumps outbreak at The Ohio State University has grown to 28 cases and officials are concerned the number could rise with students returning from spring break, a city health department spokesman said on Tuesday.
He said those infected include 23 students and a staff member at the university in Columbus, Ohio, and four other people with connections to students or links to the community.
The spokesman, Jose Rodriguez, said all but one had received at least one dose of the mumps vaccine. The university does not require students to be vaccinated, he said.
Students began returning from spring break on Monday and five cases have been reported since then.
"The possibility is always there for more cases," Rodriguez said. "With the long incubation period mumps has, it is a recipe for a prolonged outbreak."
Mumps is a contagious disease that causes painful swelling of the salivary glands. The number of cases reported annually in the United States has dropped 98 percent since the mumps vaccine was introduced in the 1960s, according to the U.S. Centers for Disease Control and Prevention.
Dr. Gregory Wallace, a CDC expert on mumps and other communicable diseases, said people who are vaccinated may still get the mumps, but typically have fewer and less severe complications than those who are not.
Mumps cases are uncommon in the United States, but not unheard of on college campuses. A multi-state outbreak in 2006 led to nearly 6,600 reported cases with more than 80 percent of the people saying they were attending college.
The Ohio State University said in early March that the school's health center had diagnosed students with mumps. Rodriguez and university spokeswoman Liz Cook said the source of the outbreak had not been confirmed.