Measles Study Finds Low Vaccination Hotspots In 8 CA Counties

NORTHERN CALIFORNIA — Despite Northern California’s high overall measles vaccination rate, Kaiser Permanente researchers identified seven pockets where coverage fell below expected levels, including a sizeable portion of Sonoma County and a section of Napa County.

The findings come as measles cases surge nationwide and vaccine skepticism drives federal policy.

The study, published July 30 in JAMA Network Open, examined vaccination rates among 7-year-old Kaiser patients. Researchers found seven clusters of U.S. Census tracts that collectively recorded lower-than-expected coverage for measles-containing vaccines.

The clusters crossed eight counties in the North Bay, greater Sacramento area, and Fresno County. They covered 242 census tracts and included 25,803 children, representing 10.8% of the children in the study.

Those areas recorded a combined vaccination rate of 94.6%. That is just below the vaccination rate necessary to establish herd immunity against measles. National health officials set that rate at 95%.

The study’s authors said the findings could help local health officials identify communities at heightened risk.

Measles on the rise

The research follows recent warnings that measles has regained ground domestically and globally.

Measles was declared eliminated, but not eradicated, in the United States in 2000. That meant the virus no longer spread continuously within the country, although travelers could still carry it into the United States.

During the first three months of 2024, health officials confirmed 58 cases across 17 states, including California — matching the total number of U.S. cases recorded during all of 2023.

The Centers for Disease Control and Prevention said 93% of those cases were connected to international travel. Austria, the Philippines, Romania, and the United Kingdom were experiencing outbreaks at the time.

California confirmed four cases by March 16, 2024, equaling its total for all of 2023.

Hundreds of people that year faced possible exposure at UC Davis Medical Center after staff treated an infected child, according to Sacramento County Public Health.

Officials also reported a possible exposure at an Alameda County restaurant in 2024.

In June this year, a fifth case of measles was confirmed in Los Angeles County and health officials warned that members of the public may have been exposed at LAX and several other places.

As of Aug. 31​​, California officials have confirmed 59 measles cases in the state.

Despite the low rate of immunization, Sonoma County’s last confirmed case of measles was recorded in 2011.

However, in January, Napa county officials recently reported the first case of measles diagnosed in a resident since 2012.

On March 13 and 16, the measles virus was also detected in city of Napa wastewater through routine sampling, indicating the virus could again be present in the county.

In the East Bay, Contra Costa Health notified the public about a measles case confirmed in late 2025, after the infected person visited several shops — including Macy's, Anthropologie and the Apple Store — in Lafayette and Walnut Creek while they were contagious.

The CDC recommends two doses of the measles-mumps-rubella vaccine, with the first administered between 12 and 15 months and the second between ages 4 and 6 or before a child starts school. The agency has also recommended vaccination for children 6 months and older who will travel internationally.

An infected person can transmit measles to nine out of 10 unvaccinated people nearby. The virus spreads through infectious droplets and the air when a person breathes, coughs, or sneezes, and it can remain infectious in the air and on surfaces for as long as two hours.

Early symptoms include a high fever, cough, runny nose, and pink eye. A rash typically appears about 14 days after exposure. Infected people can transmit the virus from four days before the rash begins through four days afterward.

Check your local health department for the latest measles cases, outbreaks, and exposure notices in your county.