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Despite strong measles vaccination rates in Northern California, the region hosts a handful of “hot spots” with vaccination rates below what is considered herd immunity, including two in the North Bay, Kaiser Permanente researchers found in a newly published study.
The report, which indicated shortfalls in vaccine uptake in one sizeable part of Sonoma County and a slice of Napa County, comes as measles cases are on the rise in the U.S., underscoring a resurgent public health threat as vaccine skepticism now grips the highest levels of U.S. government.
The study, published in JAMA Network Open, examined childhood vaccination rates for 7-year-olds who are Kaiser patients, found seven “clusters” with U.S. Census tracts that, when considered as a group, had lower-than-expected measles vaccine coverage. The authors of the study said the findings can help local health officials zero in on communities at risk.
“This information can be used … to inform the design and implementation of vaccination campaigns and surveillance for vaccine-preventable infections,” Dr. Karen Jacobson, a Kaiser Permanente research scientist, said in an email.
The Sonoma County cluster includes census tracts that take in west Santa Rosa, Sebastopol, Graton, Forestville and Fulton. The cluster in Napa County is defined by a single tract on the county’s border with Sonoma, extending from the rural outskirts of American Canyon north to at least Napa.
Jacobson said cluster analysis was used to identify geographic areas where census tracts with lower-than-expected vaccine coverage are concentrated near one another. Cluster analysis helps distinguish between an isolated census tract with low coverage — which might occur by chance — from a broader geographic “hot spot” of lower vaccination coverage.
Jacobson said in her email that medical professionals who practice in these areas could use the study’s findings to “more accurately counsel patients about the risks and benefits of vaccination, and to consider vaccine-preventable infections in the differential diagnosis when patients present with compatible symptoms.”
Kaiser has 4.4 million members across its Northern California region, which includes the Bay Area, the Greater Sacramento Area, the Central Valley down to Fresno, Madera, Kings and Tulare counties. There are 867,000 children in the region who are part of the Kaiser health care system.
Kaiser said its members comprise 33% of the total population in that region, making them “sociodemographically representative” of the region. Measles vaccine coverage for Kaiser’s entire Northern California region was 97.6%.
Overall, the study found 248,048 Kaiser members who were 7 years old and were fully vaccinated between between January 2017 and December 2024.
There were 276 census tracts, home to 24,308 Kaiser 7-year-olds, with coverage less than 95%. The seven hotspot clusters included 242 census tracts with a combined measles vaccine coverage of 94.6%, although some of those census tracts had significantly lower rates.
The seven clusters with combined measles vaccine coverage of less than 94.6% had 25,803 children who were part of the Kaiser system.
Dr. Mia Shim, Sonoma County’s health officer, said the study helps identify “localized gaps in protection” that are “masked” by surrounding communities with high vaccination rates. Shim pointed out that Northern California’s strong overall measles vaccination rate of 97.6% in some ways hides the fact that there are local communities that are at higher risk of measles spread.
“When unvaccinated children gather in school settings, these pockets of lower coverage increase the local risk of measles transmission,” Shim said.
Shim pointed out that the study, while providing valuable insight, has some key limitations. The study’s findings rely entirely on Kaiser Permanente Northern California records, which reflect only a third of Northern California’s population.
Shim said the study excludes uninsured families and those using other healthcare systems. It also does not include analysis of more rural areas across Kaiser’s territory or racial and ethnic demographics, she added.
While the study does not represent total population immunity, Shim called the findings a “helpful tool.” She said that school immunization data used by the state and counties can provide a comprehensive look at vaccination status among students at each school. “This study adds value by tracking 7-year-olds by their home neighborhoods,” she said.
Jennifer Henn, public health manager at Napa County Health and Human Services Agency, stressed that overall measles vaccination rates among kindergarten school kids between 2017 and 2024 ranged from 96.4% to 99%, showing high levels of protection against the virus. Henn, however, said the articles make an important point.
“Smaller areas within the county with lower vaccination rates can still pose a risk for measles transmission and potential outbreaks, even when the overall population is highly vaccinated,” Henn said in an email.
Henn said vaccine rates in schools, published by state public health officials, are also used to help identify settings that may be at greater risk for measles transmission. Henn said measles vaccines, such as the MMR shot, “provide the best defense against measles and its complications.”
Napa County earlier this year reported its first case of measles since 2012. The county’s public health team has been working closely with schools and healthcare providers to ensure families are educated about the importance of the measles vaccination.
“Vaccination helps protect children from illness while also protecting people who cannot be vaccinated, including infants,” Henn said.
As of August 27, 2026, there have been 2,903 confirmed measles cases reported in the United States this year, according to the federal Centers for Disease Control and Prevention. There were a total of 2,289 confirmed measles cases in the United States in all of 2025.
Shim, the Sonoma County health officer, said her team is taking a “proactive, data-driven approach” in supporting local families where resources are most needed. Shim said county public health staff work closely with the Sonoma County Office of Education to do deliver vaccine education directly to schools and the families in communities with low vaccine rates.
“I urge pediatricians and healthcare providers to proactively screen for and reach out to families whose children may be unvaccinated or under-vaccinated for measles and other vaccine-preventable diseases,” Shim said.
Staff Writer Martin Espinoza can be reached at martin.espinoza@pressdemocrat.com.