Nationwide measles cases are at a 33-year high, and a new study highlights varying levels of risk across California counties. The research, published Thursday in the journal Nature Health, suggests that declining vaccination rates are a key factor in the increased risk of contracting the highly contagious disease.
Rising Cases and Regional Risk
Investigators at Icahn School of Medicine at Mount Sinai and Boston Children’s Hospital surveyed 22,062 parents across 3,109 counties between July 2023 and April 2024. Counties were categorized by predicted MMR vaccination rates among children aged 6 months to 5 years: very high risk (under 60%), high risk (60-69%), medium risk (70-79%), low risk (80-84%), and lowest risk (85% or higher).
Across Southern California, risk levels vary. Santa Barbara and Imperial counties are considered “very high” risk, while Orange and San Diego counties are categorized as “medium” risk. Los Angeles County falls into the “high risk” category, with an estimated 66.5% vaccination rate.
County-Level Risk Estimates
- Santa Barbara County — very high risk (estimated 58.4% vaccination rate)
- Imperial County — very high risk (estimated 58.8% vaccination rate)
- San Bernardino County — high risk (estimated 61.9% vaccination rate)
- Ventura County — high risk (estimated 65.3% vaccination rate)
- Los Angeles County — high risk (estimated 66.5% vaccination rate)
- Riverside County — high risk (estimated 67.5% vaccination rate)
- Orange County — medium risk (estimated 70.3% vaccination rate)
- San Diego County — medium risk (estimated 70.6% vaccination rate)
Marin County has the highest estimated vaccination rate in California at 82.1%, placing it in the “low risk” category.
Data Collection and Future Outlook
The study’s researchers noted that current surveillance systems, often relying on phone surveys or school records, may not fully capture vaccination rates, particularly among homeschooled or uninsured children. Their approach aims to provide a more comprehensive picture. Model-based surveillance, they suggest, could help identify at-risk communities and guide targeted interventions.
Measles had been declared eliminated in the United States in 2000, but outbreaks occurred in 2019 and 2025. Last year saw nearly 50 outbreaks and 2,012 cases across 45 states – the highest level since 1992. Three deaths were reported, all among unvaccinated individuals.
As of January 13, 2026, the CDC has confirmed 171 measles cases in nine states, with the majority (145) in South Carolina. Nationally, MMR vaccination rates among kindergartners have declined from 95.2% in 2019 to an estimated 92.5% during the 2024-2025 school year. A vaccination rate of around 95% is considered necessary to achieve herd immunity.
Frequently Asked Questions
What makes measles so contagious?
Measles is one of the most contagious infectious diseases, spreading easily through the air when an infected person coughs or sneezes. The virus can remain in the air or on surfaces for up to two hours, and one person with measles can infect nine out of 10 unvaccinated people, according to the CDC.
How are counties categorized by risk?
Counties are grouped into five risk categories based on predicted vaccine status among children ages 6 months to 5 years of at least one dose of the MMR vaccine: very high risk (less than 60%), high risk (60% to 69%), medium risk (70% to 79%), low risk (80% to 84%), and lowest risk (85% or higher).
When was measles declared eliminated in the United States?
Measles was declared eliminated in the United States in 2000, but significant outbreaks have occurred in 2019 and 2025.
Given the current trends in vaccination rates and the highly contagious nature of measles, what steps do you think are most important to protect public health in your community?
