Smith College in Northampton, Massachusetts, held a meningitis vaccination clinic Thursday as a student remained hospitalized for the bacterial infection.
The Centers for Disease Control estimates 20 cases a year of strain-B meningitis across the country. That's the strain found in two UMass students last semester, and suspected in a Smith student last week.
Even two or three cases is considered an outbreak.
Most students already get vaccinated against four other strains of meningitis.
CDC medical officer Sarah Meyer said the B-strain vaccine, licensed in 2014, was harder to develop.
"Based on the clinical trials, the vaccines seem to be pretty effective in providing short term protection against most strains of serogroup B disease," said Meyer. "It's only recently been available in this country, so we're continuing to learn how effective these vaccines are."
Most meningitis vaccines work by injecting a tiny part of the protein capsule that surrounds the bacteria -- so the body then creates an immune response without actually causing the disease.
Meyer said one reason it was harder to develop the B-strain vaccine is because the strain itself is similar to human neural tissue, and scientists wanted to make sure the body would not start to reject its own tissue.
She said ten percent of the population carries the bacteria without symptoms -- but they can spread it to others.
The CDC is not recommending that students everywhere get the B-strain vaccine. Meyer said that's a local decision made by individual colleges.
Others do advocate for more universal vaccination for this strain, including the founder of the Kimberly Coffey foundation, created after the death of a high school senior.
Meyer said bacterial meningitis can cause death or disability in up to 20 percent of patients. Symptoms include fever, neck stiffness, light sensitivity and headaches.