August 19, 2025  – A new analysis from KFF reveals that the majority of Medicare Advantage beneficiaries in the U.S. live in counties with highly concentrated insurance markets, where one or two insurers dominate coverage options. The study, based on 2024 enrollment data from the Centers for Medicare & Medicaid Services (CMS), raises important questions about competition, consumer choice, and the potential implications for plan quality and cost.

According to the report, approximately 89% of Medicare Advantage enrollees reside in counties where at least half of all enrollment is held by just one or two insurers. Even more striking, 18% of counties are considered “very highly concentrated,” meaning a single insurer controls at least 50% of the local Medicare Advantage market.

To determine concentration levels, researchers used the Herfindahl-Hirschman Index (HHI), a standard measure of market concentration where scores range from 0 (perfect competition) to 10,000 (monopoly). In 2024, 79% of U.S. counties were classified as highly concentrated (HHI between 1,800 and 5,000), and another 18% as very highly concentrated (HHI above 5,000). Only 1% of counties were considered moderately concentrated, while the remaining 2% lacked enough enrollment data to assess.

The extent of concentration varies by geography. Urban areas are less likely to be dominated by a single insurer, with just 6% of urban counties considered very highly concentrated. However, the situation is notably different in rural areas. In rural counties near urban areas, 15% fell into the very highly concentrated category, while the figure jumped to 39% in the most rural counties.

UnitedHealthcare emerged as the top player, holding the largest market share in 41% of U.S. counties. Humana followed, leading in 25% of counties. Blue Cross Blue Shield led in 11%, CVS Health in 8%, and Elevance Health in 4%. A remaining 12% of counties either had another dominant insurer or too little enrollment to determine a clear leader.

The high concentration levels raise concerns for consumers, as reduced competition can impact plan premiums, benefits, and service quality. In markets dominated by a single insurer, beneficiaries may have fewer options and less leverage to switch plans that better meet their healthcare needs.

While the Medicare Advantage program continues to grow in popularity, with more beneficiaries choosing managed care over traditional Medicare, the findings suggest that this growth is occurring in an increasingly consolidated market. The implications for access, affordability, and innovation remain uncertain as policymakers and stakeholders evaluate the future of Medicare Advantage.