California issues exposure warning as unvaccinated passenger spreads measles across six counties

Health officials in California are advising residents to check for potential exposure following an unvaccinated individual travelling through six counties, highlighting ongoing concerns over rising measles cases and vaccination coverage.

California health officials are warning residents to check whether they may have been exposed to measles after an unvaccinated passenger who travelled across the state on Amtrak trains and thruway buses was hospitalised. The California Department of Public Health said the person passed through at least six counties – Alameda, Butte, Kern, Los Angeles, Sacramento and San Joaquin – as the state works with county agencies and the US Centres for Disease Control and Prevention to identify anyone who may have come into contact with the virus.

The traveller arrived in California on an Amtrak service into Los Angeles on 10 September, then moved on through a series of rail and bus connections before leaving the state for Denver, according to public health notices and local reporting. Officials said the schedule reflects planned departure and arrival times, which may differ from the actual journey, and urged passengers or station visitors along the route to review whether they were present at the relevant times.

Dr Rita Nguyen, the state’s assistant public health officer, said the episode, together with earlier outbreaks in California this year, underlined the importance of vaccination. Measles is among the most contagious infections and can lead to serious complications, including permanent brain damage and death, particularly in children, she said. State guidance says two doses of the measles-mumps-rubella vaccine provide the best protection, and public health officials note that unvaccinated people are far more likely to become infected after exposure.

The warning comes as measles activity remains elevated in California and across the United States. CDPH said 59 cases had been reported in 13 counties statewide as of 14 September, with 94% of patients unvaccinated or of unknown vaccination status. Alameda County health officials said the national toll had climbed sharply this year, with outbreaks accounting for the vast majority of confirmed cases, while local health departments have continued to stress that the virus can linger in the air and spread before symptoms are obvious.

Public health advice for people who may have been exposed depends on vaccination status and timing. Those who are not fully immunised, or who are unsure, are being told to contact a healthcare provider about the MMR vaccine. In some cases, immune globulin may be recommended for infants under 12 months, pregnant people without immunity and those with weakened immune systems. Anyone who develops fever or an unexplained rash in the 7 to 21 days after exposure is being urged to call a doctor before visiting a clinic or hospital, to avoid exposing others.