Measles Passenger Traveled Amtrak Routes Across Six California Counties

SACRAMENTO, Calif. — The Centers for Disease Control and Prevention estimates measles’ basic reproduction number ($R_0$) at between 12 and 18. A single infected individual can infect up to 90% of susceptible, non-immune people in close proximity. One dose of the measles, mumps, and rubella (MMR) vaccine is roughly 93% effective at preventing infection, while the standard two-dose regimen confers approximately 97% lifelong protection.

The case involves an unvaccinated passenger diagnosed with measles who traveled thousands of miles across the U.S. rail network before being hospitalized. The individual began the journey in Chicago and arrived at Los Angeles Union Station on Sept. 12 after a two-day cross-country rail trip.

After spending several hours in Los Angeles, the traveler used Amtrak trains and connecting Thruway buses, with connections and station stops across Los Angeles, Kern, San Joaquin, Butte, Sacramento, and Alameda counties. The route included Bakersfield, Stockton, Oroville, Sacramento, and Emeryville before the traveler departed California and arrived in Denver, where the individual was admitted to a hospital for medical treatment.

Measles is an airborne virus that can remain suspended and infectious in enclosed airspaces for up to two hours after an infected person leaves. Public health officials warned that people present on the same trains, buses, or station platforms during the itinerary may have been exposed.

In California, health authorities have recorded 59 confirmed measles cases spanning 13 counties this year. More than 90% of those infections occurred in people who were either completely unvaccinated or had an unverified immunization history, as the nation experiences a heightened year for measles transmission.

The California Department of Public Health (CDPH) issued clinical guidance based on the time elapsed since possible exposure. People exposed within the past seven days who are unvaccinated or under-vaccinated are urged to contact a physician immediately to determine whether they qualify for post-exposure prophylaxis, such as the MMR vaccine or an intramuscular injection of immune globulin. When administered promptly, these measures can prevent or reduce the severity of illness.

Dr. Rita Nguyen, assistant state public health officer, said, “Measles is one of the most contagious infections that can lead to severe, lifelong consequences, including permanent brain damage, and can also be fatal, especially for children. The measles-mumps-rubella (MMR) vaccine is safe and provides long-lasting protection against measles. Vaccination protects both our own families and those who are too young to be fully vaccinated.”

People exposed between one and three weeks ago who lack documented immunity must self-quarantine at home and restrict interactions with others for 21 days from the date of possible exposure. During that observation window, they must monitor for high fever, cough, runny nose, red watery eyes, and a developing maculopapular rash.

Anyone exhibiting symptoms should telephone healthcare facilities before arriving so staff can implement airborne infection isolation procedures and prevent exposure to other patients in waiting rooms. State and local epidemiologists continue working with Amtrak and interstate health departments to notify passengers and transit employees identified on manifests for the affected train and bus legs.