Contra Costa County's overall measles vaccination rate tops 95%, but a new Kaiser Permanente study warns that countywide numbers can hide dangerous gaps.

The study, published July 30 in JAMA Network Open, examined vaccination records for nearly 248,000 seven-year-old Kaiser members across Northern California between 2017 and 2024. Overall coverage was 97.6%. But researchers identified seven clusters of census tracts where rates fell below the 95% threshold federal health officials consider necessary for herd immunity, as Patch reported.

Those clusters spanned 242 census tracts in the North Bay, the Sacramento metropolitan area and Fresno County, covering about 25,800 children. Their combined vaccination rate was 94.6%. Some individual tracts recorded coverage as low as 75%, according to Kaiser Permanente's Division of Research.

The study's own summary named clusters only in the North Bay, Sacramento metropolitan area and Fresno County — Contra Costa wasn't listed among them, though the county falls outside that North Bay grouping. Lead author Dr. Karen Jacobson, a research scientist with the Kaiser Permanente Division of Research, cautioned against reading too much comfort into broad averages.

"One main message of our study is to not be falsely reassured by state or county-level vaccination coverage data, because there could be geographic pockets where much less than 95% of children are vaccinated," Jacobson said.

The warning resonates locally. In December 2025, Contra Costa Health confirmed the county's first measles case since 2023. The infected person was a healthy young adult who had been fully vaccinated abroad, making it a breakthrough infection, according to Dr. Lisa Rodelo, deputy health officer at Contra Costa Health, as reported by Richmondside.

That person visited several Broadway Plaza stores in Walnut Creek between Dec. 17 and Dec. 24, 2025, while contagious. Exposure sites included Macy's, the Apple Store, Anthropologie and ALO. The person also visited a Lafayette clinic and the Kaiser Permanente Walnut Creek Emergency Department during that period.

Dr. Meera Sreenivasan, deputy health officer for Contra Costa County, told ABC7 in December 2025 that the county had over a 95% vaccination rate.

The Kaiser study covered only Kaiser members, who make up about one-third of Northern California's population. Because Kaiser emphasizes childhood vaccination, actual community rates in the identified clusters may be lower than 94.6%.

Measles cases have surged nationally. As of Aug. 27, the Centers for Disease Control and Prevention (CDC) reported 2,903 confirmed cases across the United States, according to the Press Democrat. California had confirmed 59 cases as of Aug. 31.

The CDC recommends two doses of the measles-mumps-rubella (MMR) vaccine: the first between 12 and 15 months of age and the second between ages 4 and 6. An infected person can spread measles to nine out of 10 unvaccinated people nearby, and the virus can linger in the air for up to two hours.

Residents who develop symptoms such as high fever, cough, runny nose or a rash should stay home and call their health-care provider before seeking care. Contra Costa Health's immunization clinic can be reached at 925-608-5350.