As many as 1,500 BART riders may have been exposed to measles when an infected San Mateo County resident rode BART from Millbrae to San Francisco's Civic Center station last week, BART and San Mateo County health officials said Thursday.
The patient boarded a Richmond-bound train at Millbrae at about 4:30 p.m. last Friday and exited at the Civic Center station at about 5 p.m., according to county health officials.
The trip was against the typical commute direction on a "very isolated line," so the number of people potentially exposed to the disease was limited, BART spokeswoman Alicia Trost said.
"You might only have a dozen or so riders on one train car," she said.
By contrast, the number of BART riders who were potentially exposed when a Contra Costa County resident commuted to San Francisco's LinkedIn office between Feb. 4 and 6 was as many as 25,000 because it was a busier line and time, Trost said.
But San Mateo County health officer Dr. Scott Morrow stressed Thursday afternoon that the lesson is not to limit exposure but to make sure you are immune through vaccination.
If all BART riders were immune, "there'd be no issue at all," Morrow said.
County health officials have so far been unable to determine the vaccination status of the infected person, Morrow said.
It is the fourth measles case reported in San Mateo County since a statewide outbreak began at Disneyland in Southern California in December. Morrow said county health officials have so far been unable to determine if this case is connected to the Disneyland outbreak.
Despite the U.S. declaring measles eradicated in the country since 2000, a growing number of people declining to vaccinate themselves or their children has led to the disease spreading from international travelers, Morrow said.
The disease, spread through the air, is highly contagious and can be dangerous, particularly for children. Initial symptoms typically include a high fever, cough, runny nose, and red eyes. Three to five days after the appearance of symptoms, a red flat rash develops on the body.
County health officials said they are compiling a list of people the patient had contact with and places visited while contagious. The person's employer has been cooperative in discovering which other employees may have been exposed, county officials said.
Anyone determined to have been exposed to the disease is being contacted and informed of the risks and having their immunization status determined.
"The measles vaccine is safe and being vaccinated not only protects you and your family, but every child and person in your community. I strongly urge everyone to get vaccinated and help stop the spread of measles," Morrow said.