California public health officials are warning of a possible measles exposure spanning multiple counties after an infected and unvaccinated passenger traveled through the state via Amtrak trains and thruway buses.

The warning comes as the US faces its largest measles resurgence in more than three decades, fueled largely by growing vaccine hesitancy.

The California department of public health said the traveler passed through at least six counties – Alameda, Butte, Kern, Los Angeles, Sacramento and San Joaquin – while making multiple transfers.

Officials said the passenger entered California on an Amtrak train that arrived in Los Angeles on 10 September. From there, the traveler continued to Bakersfield before heading through Stockton, Oroville, Sacramento and Emeryville, eventually leaving California for Denver.

Health officials said passengers who were onboard the same trains or buses, as well as people who were inside the stations along the route on the dates involved, have been exposed to measles.

The state said it is working with federal partners and local health departments to identify and notify people who may have been exposed.

“This case, along with previous measles outbreaks in California this year, serve as reminders of the importance of vaccination against measles,” assistant state public health officer Rita Nguyen said in a statement. “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.”

Pennsylvania is currently dealing with the country’s worst active measles outbreak. The Pennsylvania health department announced on Wednesday that 731 cases have been confirmed across 38 counties so far this year. At least four people in the state have died from measles-related illness.

Pennsylvania is now seeking emergency assistance from the US Centers for Disease Control and Prevention (CDC) as the outbreak continues to spread. State officials have warned, however, that the request could be withdrawn unless the CDC publicly recognizes the four measles-associated deaths, something the US health secretary, Robert F Kennedy Jr, previously instructed the CDC not to do.

And as the death toll from the outbreak continues to rise, Kennedy headlined the Washington DC conference of the anti-vaccine organization Children’s Health Defense earlier this week, an group he led for nearly a decade.

He only addressed the measles outbreak in Pennsylvania once during his speech, instead peddling unfounded fears about vaccines at a time when health experts are urgently encouraging people to vaccinate against measles.

“In our country we care about rare illnesses and deaths. We care about every child who dies from measles. They get headlines,” he said. “I’d ask you to care about children who are dying or suffering or being injured by vaccine injuries.”

The California warning also follows another recent mass-exposure incident involving Amtrak.

Earlier this week, it was reported that a person infected with measles traveled onboard an Amtrak train from Sedalia, Missouri, to Chicago, Illinois, resulting in more than 250 people being exposed to the virus in Jackson county alone.

This month, it was also revealed that the Trump administration changed how measles deaths are counted during the outbreak. The CDC is now relying on data from the National Center for Health Statistics, which is used to track mortality trends over time. It is unclear why this change was made.