Metropolitan and Nonmetropolitan Medicare Shared Savings Program Assigned Beneficiaries Among Original Medicare Enrollees

Since 2018, the number of Medicare beneficiaries assigned to a Medicare Shared Saving Program (SSP) Accountable Care Organization (ACO) has been relatively stable – approximately 60M in 2024. Although the number of beneficiaries assigned to an ACO has changed little, it must be noted that the number of beneficiaries available for assignment has shrunk significantly over the same time period. There are several circumstances that can preclude a beneficiary from being attributed to an ACO with the biggest of those being enrollment in a Medicare Advantage (MA) plan. MA enrollees cannot be assigned to an ACO, and the growth in MA enrollment means that the pool of ACO-assignable beneficiaries has gotten smaller.

In 2018, the total pool of "available" Medicare beneficiaries (i.e., those enrolled in both Part A and Part B) was 54.2M with 38.8 percent of those in an MA plan and 29.7 percent of those assigned to a Medicare SSP ACO. By 2024, the proportion of the 60.3M available Medicare beneficiaries enrolled in an MA plan had increased to 54.6 percent, but the proportion attributed to a Medicare SSP ACO had also increased – to 36.5 percent. The rate of growth in both programs was substantially higher in nonmetropolitan counties than in metropolitan counties.