Advisory issued after tens of thousands of commuters may have been exposed to disease by Bay Area’s first confirmed case since outbreak began in December

Tens of thousands of commuters may have been exposed to measles after an infectious LinkedIn employee – apparently en route from vaccination-averse Silicon Valley – traveled by train into and out of San Francisco last week, county health officials said on Wednesday.

Contra Costa Public Health officials issued an advisory to riders of San Francisco’s Bay Area Rapid Transit (Bart) system on Wednesday after confirming the area’s first case of measles since a massive outbreak began in California in December and quickly spread across the US. The infected employee rode the Bart to and from work for three days last week, officials said. The infectious employee also went to a restaurant and bar in San Francisco last week.

Since the beginning of the year, 121 people from 17 states and Washington DC are reported to have contracted measles, a majority of which are linked to the outbreak that began with an infected guest at the Disneyland theme park in southern California. But one rider in the north had a major metropolis and its tech economy on alert Thursday.

“Measles is circulating in the Bay Area and we don’t know yet where this person was exposed,” Erika Jenssen, communicable disease program chief with Contra Costa Public Health, said in a statement. “The ongoing measles outbreak in California highlights the need for people to be vaccinated, and this is just another example of how interconnected our region is and how important it is for everyone to be up to date on their immunizations.”

Officials said they are tracing the infected rider’s movements and contacting all those who may have been potentially exposed. The rider is recovering, and has not been hospitalized.

According to the San Francisco Chronicle, the rider worked at LinkedIn. Officials at the social networking site – based in the Silicon Valley suburb of Mountain View, which is also home to Google – told the Chronicle they were notified on Tuesday that one of their workers had been infected with measles. The company said it notified all employees that day, alerting them of the risk of exposure.

Children who attend daycare facilities run by top Silicon Valley tech companies seem to have comparatively low vaccination rates, according to an investigation by Wired using data from the California department of public health. Wired found that half of the 12 companies it surveyed – including Google, Yahoo and IBM – had a level of measles vaccination lower than required to provide “herd immunity”, which is critical to preventing the spread of this disease.

Christopher Mims (@mims) Vaccination rate at Google's and Pixar's daycare is less than 50% http://t.co/6GFxs6VDI2 pic.twitter.com/NHrrW3ys6H

Public health officials stressed that the risk of infection is, in general, low because most people are inoculated against the virus, and the vaccine is highly effective. Still, officials urged any rider who believes they have symptoms of measles to contact a doctor immediately, especially as those who are not vaccinated against the virus are at heightened risk.

Measles is known to be so contagious that as high as 90% of people who are not immune and come into contact with an infectious person will themselves become infected, according to the Centers for Disease Control and Prevention.

“Although the risk of contracting measles by being exposed on Bart is low, Bay Area residents should be aware of the situation,” the county public health department said in its statement.

Measles symptoms begin to appear between one and three weeks after exposure to the virus. Symptoms can include high fever, runny nose, coughing and watery red eyes. The tell-tale sign of measles: a rash that appears on the face and neck, that spreads over the body.

Health officials declared measles eradicated in the United States in 2000, due in large part to the widespread use of the vaccine. Though the vaccines are said to be 99% effective, a recent anti-vaccination movement in the US – linking autism to the vaccines, even though that link was debunked – has seen an increase in the number of parents who choose not to vaccinate their children.