CALIFORNIA — The passenger passed through at least six California counties: Alameda, Butte, Kern, Los Angeles, Sacramento, and San Joaquin. The American intercity passenger rail operator transported the individual across portions of the state during the two-day window identified in the public health alert.

"This case, along with previous measles outbreaks in California this year, serve as reminders of the importance of vaccination against measles," Nguyen said. "The measles-mumps-rubella (MMR) vaccine is safe and provides long-lasting protection against measles." She added that "Vaccination protects both our own families and those who are too young to be fully vaccinated."

Officials urged people to monitor for symptoms such as fever and/or unexplained rash from seven to 21 days after exposure. Unvaccinated individuals are being urged to quarantine, avoid large gatherings, and monitor for signs of illness for up to three weeks. People who are not vaccinated against measles or are unsure of their status are recommended to contact their doctor about receiving the vaccine.

Officials recommended that anyone who suspects they have measles call their health care provider before visiting any health care facility. Two doses of the MMR vaccine are needed to be fully immunized, according to officials. This guidance aims to prevent further transmission within communities where the virus may have been present during the specified travel dates.

Timeline

The passenger departed Chicago at 1:30 p.m. on September 10. The passenger arrived in Los Angeles on an Amtrak train from Chicago on September 12. The passenger departed Bakersfield at 2:02 p.m. and arrived at Stockton—San Joaquin St Station at 6:29 p.m. on September 12. The passenger departed Stockton at 6:15 p.m. and arrived at the Oroville bus/thruway stop at 6:05 p.m. on September 12.

The passenger departed Oroville at 4 p.m. and arrived at Sacramento Valley Station at 6:05 p.m. on September 13. As of September 14, 2026, a total of 59 measles cases have been reported in 13 counties in California. The passenger departed Emeryville at 8:25 a.m. and arrived in Denver at 6:59 p.m. on September 14.

What's New

Further data indicates that patients were unvaccinated or had unknown vaccination status in 94 percent of California measles cases. Health officials emphasized that "Measles is one of the most contagious infections that can lead to severe life-long consequences including permanent brain damage and can also be fatal, especially for children." Background information notes that the California Department of Forestry and Fire Protection is an agency in California.

Why It Matters

The exposure event involves a highly contagious pathogen moving through major transportation hubs and multiple counties. Amtrak serves as a critical transit link across the state, meaning hundreds of individuals may have shared enclosed spaces with the infected passenger. The California Department of Public Health warned people who were on Amtrak trains and Thruway buses in six California counties between September 12 and 13, 2026, that they may have been exposed to measles.

This incident occurs within a broader context of measles activity in the state. The warning shows the ongoing risk posed by low vaccination coverage and the ease with which the virus can spread in public transit environments.