San Mateo County Measles Case Highlights Risk for Unvaccinated, Expert Says

The California Department of Public Health confirmed that there have been eight cases of measles in the state since February, with at least one of those infections in San Mateo County.

San Mateo County Health officials told KQED that the patient is a county resident whose infection earlier this year was related to travel. They confirmed that their investigation and contract tracing found that no other infections had occurred as a result but declined to share the exact date of when the infection occurred or what the patient’s status is.

Across the country, there have been more than 300 confirmed measles cases across 18 jurisdictions, far outpacing the number of infections that occurred in all of 2024. In West Texas, where the first measles outbreak was recorded in early January, there has been one confirmed fatality.

According to the Centers for Disease Control and Prevention, 90% of the confirmed cases this year are outbreak-associated, as opposed to only 69% in 2024.

“Measles virus is the most contagious of any virus that causes human disease,” said John Swartzberg, clinical professor emeritus at the UC Berkeley School of Public Health. “One person who has measles can infect up to 15 to 18 other people.”

A person who is infected with measles can spread the virus four days before a rash appears and is contagious for four days after, Swartzberg said. In that time, the airborne disease spread is considered highly infectious.

Symptoms include a runny nose, swollen eyes and a cough. People with measles may also experience intense headaches, body aches and fever, similar to COVID-19 and influenza. In many cases, however, the symptoms associated with measles tend to be more severe and also include rashes that spread throughout the body.

Measles can also lead to serious complications, Swartzberg said, noting that one in a thousand measles cases in the U.S. results in death. For people who are immunocompromised or malnourished, mortality rates can go up to 5% to 10%, he said. Contracting measles can also lead to secondary infections such as pneumonia and subacute sclerosing panencephalitis, a type of rare brain inflammation that is especially fatal among young children.

Measles prevention requires a high level of immunization — around 95% — in order to prevent high levels of outbreaks, Swartzberg said. With just two doses of the measles vaccine, a person can have lifetime protection against the virus.

In the U.S., vaccination rates have plummeted over the last 20 years, he said. Even in California, which has mandates in place requiring school-age children to receive certain vaccinations, there are pockets of the state where vaccination rates are below 95%. In West Texas, where some of the largest outbreaks occurred, communities have immunization rates that dip below 50%.

Swartzberg said getting vaccinated is the most effective defense against getting sick.

“Measles harms not only the individual but also the community,” Swartzberg said.

“This vaccine has proven to be incredibly safe and unbelievably effective because it lasts so long and it works so well … People have a responsibility to themselves, to their children and to their community.”