MSSP: A Proven Program With More Than a Decade of Success

The Medicare Shared Savings Program (MSSP) applies to people with traditional, fee-for-service Medicare. It is not a Medicare Advantage program, an HMO, or an insurance plan of any kind. A patient attributed to an ACO keeps the right to see any provider who accepts Medicare, with no referral required, and the patient’s Original Medicare benefits do not change.¹ For the provider, billing for a traditional Medicare patient does not change either. ACO attribution doesn't change how a claim is billed for that patient.

MSSP sometimes gets talked about like it is a recent experiment. It is not. It was written into law as part of the Affordable Care Act in 2010, and CMS spent the following year and a half building out the regulations before the program went live. Even so, the MSSP program moved fast in its first year: in April 2012, CMS approved the initial 27 ACOs, covering nearly 375,000 beneficiaries across eighteen states, and by that July had added 88 more, covering over 2.4 million Medicare patients. For 2026, CMS approved 134 applications, including 72 new participants and 62 returning ones, bringing total MSSP participation to 511 ACOs, up from 476 the year before. Those ACOs now include more than 700,000 providers and are projected to serve 12.6 million traditional Medicare beneficiaries in 2026, a 12.3 percent increase over 2025 and the largest number the program has ever served.

MSSP is now the nation's largest value-based payment program. Unlike many CMS Innovation Center initiatives, which are authorized as temporary model programs that can be modified, narrowed, or ended, MSSP is not a CMS Innovation Center initiative. It was established directly by statute and has continued as a permanent, standing part of Medicare for over a decade.

The results back that stability up. For Performance Year 2024, 75 percent of the 476 participating ACOs, representing 80 percent of the 10.3 million assigned beneficiaries, earned performance payments totaling $4.1 billion, while Medicare saved $2.5 billion relative to benchmarks. CMS's own data also shows meaningful year-over-year improvements among ACO-assigned beneficiaries in blood pressure control, diabetes management, and depression screening with follow-up.² In practical terms, that means patients in ACOs tend to do better, CMS spends less than it otherwise would have, and primary care physicians participating in an ACO have a path to additional revenue beyond standard fee-for-service billing alone.

One structural feature is worth highlighting: MSSP offers a one-sided model, where an ACO can earn shared savings without ever owing money back if spending comes in above target, in addition to two-sided models where greater risk may be assumed by participating providers. That stands apart from many CMS Innovation Center models, which are often built around mandatory two-sided risk and run as time-limited models rather than permanent programs.

Fourteen years in, MSSP is not a pilot program anymore. It has established its merit, and continues to grow, both in ACO participation and in the number of Medicare beneficiaries receiving coordinated, accountable care. That kind of longevity and consistent growth sets MSSP apart from other value-based initiatives.

This is one in a series of posts looking at MSSP from the ground up: what it is, why a properly managed ACO with the right participating providers is well-positioned to meet the purposes of the MSSP to coordinate care, improve quality, lower cost, and general shared savings, and what should actually matter to a provider when evaluating whether to join a given ACO.

Nothing in this post constitutes financial or legal advice. No specific financial outcome is guaranteed or implied. References to benchmark dynamics or shared savings potential are illustrative and based on publicly available CMS data. Actual results will vary by ACO and provider.