Rural Implications of Increased Medicare Beneficiary Enrollment in ACOs and MA Plans

This brief explores the interplay of Medicare Advantage (MA) enrollment and beneficiary attribution to Centers for Medicare & Medicaid Services (CMS) Medicare Shared Savings Program (MSSP) Accountable Care Organizations (ACO). MA enrollees are not eligible for such ACO attribution, and both payment models are drawn from the same pool of Medicare beneficiaries. As the number of beneficiaries not participating in either is shrinking, it is important to understand how the growth (or shrinkage) in one model influences participation in the other.

Key Findings:

• Significant growth in both MSSP ACO assignment and MA enrollment occurred between 2013 and 2023. ACO assignment grew from 3.2 million to 9.9 million beneficiaries, more than tripling (206 percent growth), while MA enrollment increased from 14.0 million to 30.7 million beneficiaries, more than doubling (120 percent growth).
• Between 2013 and 2023, rural (nonmetropolitan) areas experienced faster growth in both ACO assignment and MA enrollment than metropolitan areas. The percentage of Medicare beneficiaries assigned to an MSSP ACO increased from 5.1 percent to 18.6 percent in micropolitan counties and from 4.9 percent to 18.4 percent in noncore counties, compared with 7.5 percent to 16.5 percent in metropolitan counties. Similarly, the percentage of Medicare beneficiaries enrolled in MA increased from 20.9 percent to 45.8 percent in micropolitan counties and from 17.9 percent to 44.3 percent in noncore counties, compared with 32.9 percent to 54.0 percent in metropolitan counties.
• Despite faster ACO growth rates, MA plans maintained a broader reach due to a higher starting point (30.4 percent enrollment/penetration vs. 7.0 percent attribution for ACOs in 2013).
• In 2023, 30.7 percent of eligible Medicare beneficiaries were not enrolled in an MA plan or assigned to an ACO, with noncore counties comprising the largest share.
• Counties with low uptake of both MA plans and MSSP ACOs were more rural and underserved, often with small populations and limited infrastructure, but more likely to include a Critical Access Hospital.