Partisan differences in childhood measles vaccination and general refusals: a retrospective cohort study of electronic health records in the United States, 1988-2024.

Background: Vaccination remains one of the most effective public health tools globally. Yet recent years have seen increasing political polarization around vaccination, particularly in the United States, as well as measles outbreaks among unvaccinated children. Using individual-level data, we characterize the national evolution of parental political polarization in childhood vaccination refusal, timely receipt of a child's first measles, mumps, and rubella (MMR) dose, and delay in first MMR vaccination. Methods: In this retrospective cohort study, using electronic health records from a primary care clinic registry and voter registration data with representative coverage across all 50 U.S. states, we identify children born from 1988 to 2024 ( Findings: Partisan polarization in childhood vaccine refusals spiked for children born during the COVID-19 pandemic-up to a gap of 9.2 [4.2-14.1, 95% confidence interval] percentage points for children born in 2021-but has been increasing since approximately 2000, with Republican parents more likely to refuse vaccinations than Democratic parents by an additional 0.25 [0.16-0.34] percentage points per year ( Interpretation: This study provides the first nationwide, individual-level evidence linking parental political affiliation to vaccine refusal rates, MMR vaccination rates, and MMR vaccination delays. Despite various data limitations, our finding of the emergence and widening of partisan gaps over time suggests that political identity has become an increasingly important factor in childhood health decision-making. Funding: This research was supported in part by the Stanford Institute for Human-Centered Artificial Intelligence and the Robert Wood Johnson Foundation.