Vaccinated commuter train passengers helped prevent a measles outbreak after an infected student rode during rush-hour.

High levels of vaccinated people are being touted as the reason why a person infected with measles didn’t create an outbreak in the San Francisco Bay Area.

Last Thursday, officials announced that an unvaccinated University of California, Berkeley, student who lived off campus traveled through the area’s largest public transportation system between Feb. 4 and Feb. 7, potentially exposing thousands of people to the highly infectious virus.

The student traveled from home in Contra Costa County to class via the Bay Area Rapid Transit System, or BART, the fifth most-used transit system in the U.S. with nearly 422,000 riders a week. The student, who was not named, rode during busy morning and evening commute hours.

Health officials say the student most likely contracted measles while traveling abroad. Symptoms of the virus can take up to a week to emerge.

Vicky Balladares, a spokeswoman for Contra Costa Health Services, said that despite an infected passenger riding on the train system, an outbreak was avoided because a high percentage of Bay Area residents are vaccinated.

“Things seem very quiet,” she said, adding that no new measles cases have been reported as of Tuesday.

The MMR vaccine, which is recommended for nearly all children beginning at 12 to 47 months, can effectively protect against measles infection. The only people who should not get an MMR vaccination are those with compromised immune systems and babies younger than 12 months.

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An infected passenger on a major urban transportation system is a serious concern when it comes to potential outbreaks. Due to the highly infectious nature of the virus, BART and other authorities issued an alert as soon as the passenger was diagnosed.

Jason McDonald, a spokesman for the U.S. Centers for Disease Control and Prevention (CDC), told Healthline that measles can be spread to others between four days before and four days after the characteristic skin rash appears.

Besides direct contact with an infected person’s mucus, the virus can also live on surfaces for up to two hours and spread to others that way. The virus “spreads so easily that people who are not immune will probably get it when they come close to someone who is infected,” McDonald said.

While those vaccinated against measles won’t contract the virus from riding with an infected person on the train, those who haven’t received the necessary vaccination typically aren’t so lucky.

“Measles is so contagious that if one person has it, 90 percent of the people close to that person who are not immune will also become infected with the measles virus,” McDonald said.

Symptoms of a measles infection are a fever followed by a cough, runny nose, and red eyes. Red spots soon develop on the head and spread to the rest of the body. About 40 percent of children under the age of five who contract measles require hospitalization, according to the CDC.

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Measles was declared ‘eliminated’ from the U.S. in 2000. But it continues to appear in clusters throughout the country, mainly among people who avoid vaccinations.

While the U.S. typically sees about 60 measles cases per year, most contracted outside the U.S., 189 Americans contracted measles in 2013, according to the CDC.

Of those cases, 91 percent occurred among people who were unvaccinated or had unknown vaccination status, and the majority of those people—79 percent—chose not to be immunized due to “philosophical objections,” according to the CDC.

A measles outbreak in North Carolina that involved 23 people was tied directly to individuals who chose not to be vaccinated because of personal religious beliefs, and another 20 cases were linked to a Texas megachurch with an anti-vaccination stance.

Outbreak clusters often appear when a member of an unvaccinated group travels abroad, becomes infected, and returns home to infect other non-vaccinated people.

A recent study published in the Journal of the American Medical Association showed that from 2009 to 2012, legislators in 18 states tried 31 times to expand the scope of immunization exemptions for parents of public school children. None of the bills passed.

Photo courtesy of Jon Davis via Wikimedia Commons.