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 (n = 46,924) with primary care clinic activity who can be matched to potential parents with exclusively Democratic (n = 19,449) or exclusively Republican (n = 27,475) party affiliation. We statistically test for increasing polarization over time across three outcomes: whether or not a patient had a documented vaccination refusal for any childhood vaccination, whether a patient received a timely first MMR dose, and the time delay to first MMR vaccination.

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 (p < 0.01), on average. Refusals are 3.3 [0.29–6.22] percentage points (p = 0.03) more polarized in states allowing non-medical vaccine exemptions. Political polarization in MMR vaccinations has also increased over time; the difference in on-track coverage between children of Republican and Democratic parents has grown, on average, by 0.52 [0.42–0.62] percentage points per year (p < 0.01), while the partisan gap in delays has widened, on average, by 2.0 [1.0–3.0] days per year (p < 0.01), or approximately 2.5 months over the study period.

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.