Measles Outbreak Fails to Push MMR Rates to Herd Immunity Levels, Study Reveals

Despite major measles outbreaks in the United States this year, new data show measles-mumps-rubella (MMR) vaccination coverage remains far below the level needed to prevent sustained transmission.¹ A repeated cross-sectional study published in JAMA Network Open followed more than 149,000 children across a central Ohio pediatric care network for 20 months after a large 2022 postelimination outbreak and found no meaningful improvement in vaccination rates.¹

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“Measles is a highly contagious, vaccine-preventable disease that requires high population-level immunity to prevent sustained transmission,” the authors wrote. “Among children older than 12 months, the MMR vaccine is estimated to be 93% effective after 1 dose and 97% effective after 2 doses. In the World Health Organization (WHO) Region of the Americas, measles vaccination coverage must exceed 93% to maintain herd immunity.”¹

Findings

At the onset of the Ohio outbreak in October 2022, only 53.6% of eligible children had received a timely first dose of MMR, a rate that remained unchanged at 20 months (74,157 of 138,301 children). Second-dose coverage increased slightly from 57.9% to 60.2%, while receipt of at least one valid dose by 84 months rose from 77.3% to 77.9%. However, despite modest upticks, all measures remained far below the 93% herd immunity threshold.¹

Investigators found that children of Somali descent, who accounted for many early cases, consistently showed lower first-dose coverage. At 20 months, 33.5% of Somali children received MMR1 on time compared with 55.6% of non-Somali peers, a gap of 22.1 percentage points. However, differences narrowed for MMR2 and overall vaccine receipt, which the authors suggest was related to age-based catch-up, prekindergarten visits, or school-entry requirements.¹

Children were eligible for the analysis if they were younger than 15 years and had at least one well-child visit in the prior 2 years, reflecting the age group affected by the outbreak. At the start, the cohort included 133,476 children; this number grew to 143,720 at 12 months and 149,092 at 20 months. The median age was 7.96 years, 51.3% were boys, and 8.6% were of Somali descent. Investigators reconstructed the analytic cohort at each time point, with age-based criteria applied to determine eligibility for each vaccination outcome.¹

Broad National Trend

The 90 confirmed pediatric cases of measles during the Ohio outbreak appear to be part of a broader national trend, the authors stressed. As of mid-2025, the United States has reported more than 1,300 measles cases, marking the highest annual total in the country in more than 3 decades.³ There were 3 confirmed deaths from the virus this year, the first measles-related deaths in a decade, according to data the authors cited. The outbreak in Texas that began in January this year and now accounts for more than 760 cases,⁴ is projected to exceed 12 months, raising concern that US measles elimination status could be reversed, according to the researchers.

At the same time, state and local health departments face shrinking resources and reduced federal support, even as they remain central to prevention and outbreak response. KFF Growing public skepticism about vaccine safety and waning trust in health authorities have further eroded vaccination rates, complicating efforts to control the spread of measles.⁴

Indeed, the study authors noted, "In response to the outbreak, public health officials implemented a range of interventions, including outbreak notifications, quarantines, daycare closures, and walk-in vaccination clinics. However, widespread vaccine hesitancy limited MMR vaccine uptake." ¹

"Underimmunized and Susceptible"

The authors caution that the persistence of the low coverage metrics suggest the central Ohio pediatric population is at risk for future measles outbreaks. "With more than 149 000 children in care, the PCN represents a substantial population in which even minor lapses in herd immunity can result in widespread transmission," they wrote. These risks are not limited to any single community. Although children of Somali descent remain a priority for outreach, the data clearly show that the broader population is also underimmunized and susceptible."¹

They concluded that their findings underscore an urgent need for "targeted, data-informed public health strategies and ongoing surveillance to close immunity gaps and sustain measles elimination in the US.”¹

References

1. Martoma RA, Martoma JC, Majumder MS. Measles vaccination coverage after a postelimination outbreak. JAMA Netw Open. 2025;8(9):e2533732. doi:10.1001/jamanetworkopen.2025.33732

2. Measles cases and outbreaks. Centers for Disease Control and Prevention. Updated July 16, 2025. Accessed September 30, 2025. https://www.cdc.gov/measles/data-research/index.html

3. Michaud J. Measles elimination status: what it is and how the US could lose it. KFF. July 28, 2025. Accessed September 31, 2025. https://www.kff.org/other-health/measles-elimination-status-what-it-is-and-how-the-u-s-could-lose-it/

4. Measles outbreak - August 12, 2025. Texas Health and Human Services. August 12, 2025. Accessed September 30, 2025. https://www.dshs.texas.gov/news-alerts/measles-outbreak-2025