About 89% of Medicare beneficiaries in 2024 lived in a county where at least half of beneficiaries were enrolled in Medicare Advantage plans sponsored by only one or two insurers, making the market share highly concentrated, according to a recent analysis. About 4% of beneficiaries lived in a county considered a very highly concentrated market, defined as one in which a single insurer held at least 50% of Medicare Advantage enrollment.

The researchers analyzed data from the Centers for Medicare & Medicaid Services (CMS) Medicare Advantage Enrollment, Benefit, and Landscape files for 2010 to 2024. They used the Herfindahl-Hirschman Index (HHI) to categorize counties by market concentration. HHI scores range from 0 to 10,000, with the highest score defining a pure monopoly. Counties scoring less than 1,000 were classified as unconcentrated. Those scoring between 1,000 and 1,800 were classified as moderately concentrated. Those scoring between 1,800 and 5,000 were classified as highly concentrated. Those scoring more than 5,000 were classified as very highly concentrated.

• In 2024, 79% of counties were highly concentrated Medicare Advantage markets.
• 18% of counties were very highly concentrated markets.
• Fewer than 1% of counties scored as moderately concentrated markets.
• The remaining 2% of counties did not have sufficient Medicare Advantage enrollment to classify the level of market concentration.
• Urban counties were less likely to have a very highly concentrated Medicare Advantage market than rural counties.
• About 6% of urban counties were very highly concentrated.
• About 15% of rural counties near urban areas were very highly concentrated. About 39% of the most rural counties were very highly concentrated in 2024.

Among payers:

• UnitedHealthcare had the highest market share in 41% of counties for the 2024 plan year.
• Humana had the highest market share in 25% of counties.
• Blue Cross Blue Shield had the highest market share in 11% of counties.
• CVS Health had the highest market share in 8% of counties.
• Elevance Health had the highest market share in 4% of counties.
• In 12% of counties, other insurers had the highest market share, or the county had insufficient Medicare Advantage enrollment to identify a dominant insurer.

These findings were reported in Most Medicare Advantage Markets are Dominated by One or Two Insurers, by Nicole Zhu, Jeannie Fuglesten Biniek, Nolan Sroczynski, and Tricia Neuman, with KFF, a non-profit organization focused on health policy. The goal was to examine the competitiveness of Medicare Advantage markets. Beneficiary enrollment shares were based on total Medicare Advantage enrollment of 32.4 million. 

The full text of Most Medicare Advantage Markets are Dominated by One or Two Insurers was published on July 14, 2025, by KFF. A free copy is available (accessed August 6, 2025). 

For more information, contact: Craig Palosky, Director of Communications, KFF, 1330 G Street Northwest, Washington, District of Columbia 20005; Email: cpalosky@kff.org; Website: https://www.kff.org/

October 2025     00US25EUA0040