Growth of enrollment in zero-premium Medicare Advantage (MA) among US counties with high cancer burden.

227 Background: Zero-Premium MA plans have grown substantially in popularity. Previous research found these plans tend to have lower Centers for Medicare & Medicaid Services (CMS) quality star ratings and greater utilization management than other MA plans, with enrollment concentrated in counties of greater socioeconomic disadvantage and/or higher health needs. To understand its implications for cancer care access and quality, this study investigates whether enrollment trends in zero-premium MA plans differ across US counties by cancer mortality rates. Methods: This county-level analysis integrated CMS Medicare Advantage (MA) enrollment data (2019-2024) with National Center for Health Statistics five-year average cancer mortality data (2018-2022). We only included plans available to the general public (HMO, HMO-POS, PPO) excluding plans with 10 or fewer enrollees. County-level cancer burden was defined as the age-adjusted cancer mortality rate for Medicare age-eligible (65+) adults, categorized into quartiles. The primary outcome was the county-level proportion of MA enrollees in zero-premium plans. To quantify the evolving association between cancer burden and zero-premium MA plan enrollment, we performed annual Pearson correlation analyses and used Kruskal-Wallis tests to evaluate enrollment differences across county mortality quartiles. Results: Nationally, the market share of zero-premium MA plans surged from 47.1% in 2019 to 74.0% in 2024, with enrollment nearly doubling from 5.8 million to 11.7 million. A consistently strengthening positive correlation emerged between county-level cancer mortality rates and zero-premium MA enrollment over the study period (Pearson's r ranged from 0.04 in 2019 to 0.12 in 2024; p<0.05 for all years). From 2020 onward, enrollment in these plans was significantly higher in counties with greater cancer mortality. Between 2019 and 2024, counties in the highest cancer mortality quartile (Q4) saw a 28.9 percentage-point increase in zero-premium MA enrollment, compared to a 25.1 percentage-point increase in the lowest quartile (Q1). Conclusions: The rapid and disproportionate growth of zero-premium MA plans in counties with high cancer mortality raises critical concerns, as these plans' restrictive features may create barriers to quality cancer care. Urgent research is needed to evaluate the real-world impact of this market shift on care access, patient costs, and clinical outcomes for this vulnerable population. Zero-premium MA plans enrollment by county cancer burden and year. Cancer Burden 1 Quartile (Q) 2019 (%) 2022 (%) 2024 (%) Increment2024 v 2019 Overall 47.1 65.2 74.0 26.9 Q1 (lowest) 45.9 61.1 71.0 25.1 Q2 46.5 63.8 72.3 25.8 Q3 46.0 65.9 74.6 28.5 Q4 (highest) 50.0 70.0 78.9 28.9 1 Age-adjusted cancer mortality rates in population ≥ 65 years.