A California health authority has warned that a single unvaccinated passenger who travelled on Amtrak trains and a state bus service could have exposed people to measles across six counties. The warning, issued on Saturday, comes after the passenger was confirmed positive for measles and had been on the route that links San Diego, Los Angeles, San Bernardino, Riverside, Orange and Santa Ana counties.

The exposed passenger, who had travelled from San Diego to San Bernardino on an Amtrak line on the 14th of September, was found to have measles on the 18th. According to the state Department of Public Health, the patient had been on the train for around four hours and then boarded a bus that connected to the Orange County transit system, spending additional time in the Santa Ana area before the outbreak was identified.

"The potential for spread is real, especially given the close contact on public transport," said a spokesperson for the California Department of Public Health.

Health officials are actively identifying the 400–500 passengers who shared the train and bus compartments during the relevant time windows. They have begun contacting those individuals through a combination of email alerts, telephone calls and local media. The Department advises anyone who travelled on the Amtrak service between 12:00 and 16:00 on the 14th, or on the connecting bus between 18:00 and 22:00 the same day, to check their vaccination records.

Measles, a highly contagious virus, spreads through respiratory droplets and can linger in the air for up to two hours. The disease is preventable with a two‑dose MMR (measles‑mumps‑rubella) vaccine, which provides protection for about 97 % of people who receive it. In the United States, the last major measles outbreak occurred in 2014‑15, when an unvaccinated traveller sparked a cluster in Florida. Since then, California has maintained a vaccination rate of 92 % for children under 12, but pockets of low coverage remain in some communities.

Officials urge anyone who may have been exposed to seek medical advice. A 23‑year‑old man from San Bernardino, who was on the train during the passenger's journey, told the Department that he had no history of measles and had not yet been vaccinated. He is currently monitoring himself for symptoms such as fever, rash or swollen lymph nodes.

State authorities have also notified the California Department of Transportation and the San Diego County Health Department, and are coordinating with the CDC’s regional office to track potential secondary cases. The Department has opened a hotline that operates 24 hours a day for people to report symptoms or to verify their immunisation status. The public can also use the state health portal to register their vaccination records if they are unsure.

Experts say that early notification and rapid vaccination are key to containing a measles outbreak. Dr. Linda Chen, an infectious disease specialist at the University of California, San Francisco, told the Department that a single missed dose can create a vulnerable chain of transmission, especially on crowded public transport.

The current alert follows a broader national trend of increasing measles cases, driven by vaccine hesitancy and travel. The CDC estimates that measles cases in the United States have risen by 20 % in the past two years, with a significant proportion linked to unvaccinated travellers. California’s warning is part of a larger effort by public health agencies across the country to monitor and respond to potential measles exposure events in real time.

Residents who were on the affected routes are encouraged to review their vaccination history and to seek a booster if necessary. The Department has set up a mobile vaccination clinic in downtown Los Angeles that will run through the weekend. Those who are unsure of their immunisation status should contact the Department’s hotline or visit a local health clinic for a quick test and a possible MMR dose.

Health officials are monitoring the situation closely and will provide further updates if additional cases emerge. The Department’s latest guidance will be available on its website and through local news outlets in the coming hours.