2 measles cases test whether the Bay Area can keep the virus from spreading

The Bay Area has now recorded two measles cases in the first weeks of 2026 — an early test of whether one of the nation’s most highly vaccinated regions can keep the virus from gaining ground as the United States approaches the loss of its measles-free status.

The latest infection, reported in an unvaccinated person in San Mateo County, has not led to additional cases. Health experts say that’s largely because vaccination rates across much of the Bay Area remain high enough to give the highly contagious virus few opportunities to spread.

“Vaccination goes hand in hand with the amount of the disease that’s being seen,” said Vidya Mony, a pediatric infectious disease physician at Santa Clara Valley Healthcare.

More than 2,140 measles cases were confirmed nationwide last year — the highest total in decades. California accounted for just 25 of those cases, according to early data from the Centers for Disease Control and Prevention.

While last year’s Bay Area numbers are not yet fully available, cases remain rare locally, health officials said. Last month, a contagious person with measles visited four Walnut Creek stores, but county officials reported no subsequent spread.

California law requires most school-age children to receive two doses of the measles, mumps and rubella vaccine, allowing exemptions only for specific medical conditions. The 2015 law, signed by then-Gov. Jerry Brown, eliminated exemptions based on personal beliefs. Parents who object to vaccination can instead homeschool their children.

“We are very lucky in the Bay Area and California that we have mandates for school entry,” Mony said. “You create a cocoon effect for everyone around you who can’t get vaccinated. It’s called herd immunity.”

That protection is visible in local data. In the 2023-24 school year, 98% of kindergarten students in both Santa Clara and Alameda counties were vaccinated against measles, mumps and rubella — well above the 95% threshold needed to prevent outbreaks, state figures show.

California has not been immune to the vaccine skepticism spreading nationally. Mony and other physicians said they are fielding more questions from parents about vaccine safety and effectiveness as U.S. Health Secretary Robert F. Kennedy Jr. has moved to scale back parts of the childhood vaccination schedule despite objections from major medical groups and state health officials.

Pediatrician Lynne Rosen of La Clinica de la Raza, which serves about 80,000 patients annually in the East Bay, said it remains rare for families to refuse vaccines outright.

“But it’s more common than it was a few years ago,” she said. “I don’t think it’s specific to measles. I think it’s the idea of vaccines, in general.”

Even with its high vaccination rates, California has seen outbreaks in recent years. In 2019, there were 73 measles cases. International travel was linked to five outbreaks, accounting for dozens of cases, according to the California Department of Public Health. And during the winter of 2014 through the following spring, an outbreak linked to Disneyland ultimately infected more than 130 Californians.

The CDC still recommends two doses of the measles, mumps and rubella vaccine by age 6. In practice, that may be combined with a shot against chickenpox for older children. The two-shot series is 97% effective at preventing measles if a person is exposed, said Nathan Lo, an assistant professor of infectious diseases at Stanford Medicine. Adults do not need boosters.

Almost everyone who is unvaccinated will become sick if exposed to measles, Lo said. The virus typically causes fever and rash, but complications can be severe: about 20% of patients are hospitalized, and roughly 3 in 1,000 die.

“It’s totally preventable for the most part,” Lo said.

Part of what makes measles so dangerous is how long it can linger in the air, said Meera Sreenivasan, deputy health officer for Contra Costa County. The virus can remain airborne for up to two hours after an infected person has left a room.

That risk prompted Contra Costa Health’s contact-tracing team to move quickly over the holidays after learning that a contagious person had shopped at a mall in Walnut Creek and visited health clinics while infectious.

“Because it is relatively rare, one case is considered an emergency response,” Sreenivasan said. “Depending on where that person was, the potential for it to become a much bigger response is always possible.”

As of Wednesday, contact tracers had not identified a single additional case. An outbreak appears to have been averted — a result health officials attribute largely to the region’s high vaccination rates.