SAN MATEO (CBS SF) — Bay Area Rapid Transit riders may have been exposed to measles when an infected San Mateo County resident commuted last week from the Peninsula to San Francisco.

San Mateo County health officials confirms this is the fourth measles case since the outbreak began in December. People in San Mateo County and neighboring counties may have also been exposed to measles.

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The resident commuted Feb. 20 at about 4:30 p.m. from the Millbrae station to Civic Center in San Francisco on a Richmond-bound train, according to health officials.

While the risk of contracting measles by being exposed on BART is low, health officials warn Bay Area residents to watch for symptoms if they are not vaccinated.

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San Mateo County Health System’s Communicable Disease team is compiling a list of all people the person was knowingly in contact with while contagious and a list of other public places where the person may have come into close contact with other people.

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Earlier this month, BART riders were also warned of possible measles exposure from a passenger traveling between the East Bay and San Francisco.

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VACCINATION RATES: All 2014-15 Kindergarten Classes
LATEST CASES:  CA Public Health Tracking