Comparing ACO REACH vs MSSP comes down to one operational question: how a beneficiary becomes part of an ACO. Managing a Medicare ACO becomes more difficult when the rules shift from program to program, and errors in alignment carry direct consequences: outreach that does not map to CMS requirements, beneficiary communications that fall out of compliance, and survey or notification data that is not organized when a submission deadline arrives.

The comparison carries near-term weight because ACO REACH is scheduled to end on December 31, 2026, with the new LEAD model beginning January 1, 2027.

This article examines how ACO REACH and MSSP differ operationally, where LEAD fits, and how organizations keep outreach, survey, and notification work organized. Mabel offers the infrastructure to support beneficiary engagement across all 3 models.

Where the Three Models Stand Right Now

MSSP remains CMS’s permanent ACO program.

ACO REACH runs on a fixed timeline: per the American Health Law Association’s summary of CMS’s transition plans, its first performance year began January 1, 2023, and the model ends December 31, 2026. LEAD takes its place starting January 1, 2027, as a 10-year model.

Standard ACOs need 5,000 aligned beneficiaries in Performance Year 1 to participate, per CMS’s LEAD model FAQ, which has prompted growth teams to track alignment volume early.

LEAD is often described as eliminating benchmark rebasing; CMS’s payment documentation is more precise. The CMS LEAD Model Payment Fact Sheet gives experienced ACOs an equally weighted three-year benchmark average, while new ACOs phase in at 10%, 30%, and 60% weighting across their first three base years. The structure mitigates rebasing rather than removing it.

The Size Gap Between These Two Programs

MSSP covers 480 ACOs, more than 10.8 million assigned beneficiaries, and over 634,000 participating providers, nearly half of Traditional Medicare, according to CMS’s CY2025 Medicare Physician Fee Schedule final rule fact sheet.

ACO REACH covered 115 ACOs and about 2.5 million beneficiaries in Performance Year 2024.

REACH’s PY2024 results resist a single financial interpretation: gross spending fell 2.2% ($706.1 million), while net spending, after shared savings and loss payments, rose 0.2% ($55.3 million) model-wide. High Needs ACOs, despite a small share of total ACOs, posted a substantially higher gross reduction rate than the rest of the model and accounted for a disproportionate share of that net increase, per CMS’s ACO REACH PY2024 evaluation.

Alignment Is the Real Dividing Line

The most consequential operational distinction between the models is alignment rather than payment structure.

MSSP relies almost entirely on claims-based assignment: a beneficiary is attributed based on which providers delivered primary care. MSSP allows for Electronic Voluntary Alignment (EVA), but requires a beneficiary to log in to medicare.gov to select their provider — a burdensome and unrealistic process for most Medicare beneficiaries.

ACO REACH also includes EVA, but also allows Signature-Based Voluntary Alignment (SVA), where a beneficiary signs a form designating a provider as their main source of care. That attestation is submitted to CMS on a quarterly basis, and allows ACOs to grow their alignment numbers.

LEAD expands on the REACH model with two channels: Electronic Voluntary Alignment (EVA) through Medicare.gov (now called Medicare.gov Voluntary Alignment (MVA)), which designates an individual provider, and Signature-Based Voluntary Alignment (SVA), which lets a beneficiary designate a Participant TIN — a practice site — on paper or electronic forms, per CMS’s LEAD Alignment and Finance Paper. That TIN-level option under SVA is what makes LEAD’s voluntary alignment infrastructure more flexible than REACH’s.

Mabel already runs SVA-based campaigns and audit-logged notifications for ACO REACH; that infrastructure serves as a readiness foundation for ACOs joining LEAD in 2027.

Where These Models Get Complicated

ACO REACH carries a reporting obligation MSSP does not have in this form: beneficiary-reported Social Determinants of Health (SDoH) data. For PY2026, REACH ACOs must collect SDoH data using one of three CMS-approved screening tools (the Accountable Health Communities tool, the North Carolina SDOH screening tool, or NACHC’s PRAPARE tool), and beneficiaries surveyed in a prior year must be re-surveyed in PY2026 to receive credit, per CMS’s PY2026 Quality Measurement Methodology Report. Complete SDoH reporting can add up to five percentage points to an ACO’s Initial Quality Score, a direct financial incentive tied to field-collected data. A separate mechanism, the Health Equity Benchmark Adjustment (HEBA), adjusts REACH’s financial benchmark itself, not the Quality Score, adding up to $30 per beneficiary per month for beneficiaries in the highest-deprivation areas, per CMS’s Performance Year 2024 model update, as reported in Crowell Health Solutions’ summary of the ACO REACH health equity changes.

Most REACH ACOs are networks of independent practices on different EMR systems, and few have a routine workflow for administering SDoH screeners. That combination of disparate systems, no standing process, and a requirement limited to three specific tools explains why SDoH collection tends to stall mid-network. A workable collection strategy does not depend on any single EMR: administrative data pulls where available, online or paper forms where they are not, and completion tracked by practice so gaps are visible before the submission window closes.

MSSP ACOs face a different administrative obligation: the Beneficiary Information Notice (BIN), timed to assignment methodology. It is due at or before the first primary care visit for preliminary prospective assignment, and during the performance year for prospective assignment, with a follow-up communication required within 180 days, per NAACOS’s beneficiary notification guidance; that window was simplified effective January 1, 2025, replacing an older visit-based rule. ACOs must be able to produce evidence that each notice went out, a significant recordkeeping obligation at network scale. MSSP ACOs separately report clinical quality through the APM Performance Pathway, now expanding to “APP Plus” with up to eleven measures by 2028, a distinct compliance track from BIN distribution, per CMS’s CY2025 Medicare Physician Fee Schedule final rule fact sheet.

The most reliable practice across all three models is mapping outreach to CMS’s calendar rather than internal convenience: MSSP’s BIN timing tied to assignment methodology, REACH’s annual SDoH re-collection window, and LEAD’s beneficiary minimums that reward early alignment growth. Centralizing campaign execution, response processing, and CMS-formatted reporting in one system gives a network real-time visibility into collection status. Mabel’s ACO REACH service is built around this kind of multi-channel SDoH and HEDR collection, aggregating data across EMRs, online forms, and paper submissions into one CMS-formatted report.

Mabel in Practice

Across nine ACO REACH organizations in PY23, Mabel’s HEDR and SDoH collection product helped each capture more than 90% of its adjustment target, collectively close to $2 million in additional revenue, per Mabel’s PY24 HEDR strategy overview. The result came from a multi-channel collection strategy tailored to each ACO’s provider mix, an approach relevant to any REACH ACO’s PY2026 SDoH targets. Furthermore, with the change in guidelines for PY26, Mabel helped more >35 ACO REACHs achieve >95% SDoH data reporting and the quality withhold that comes with it. 

A second ACO REACH organization faced a roughly 50% CMS rejection rate on voluntary alignment forms, driven by errors such as mistyped Medicare Beneficiary Identifiers and TIN-NPI mismatches, per Mabel’s ACO REACH case study. After consolidating outreach, form collection, and processing onto Mabel’s platform, the organization grew net-new alignments by 30% over 12 months while gaining real-time submission visibility. The pattern holds across beneficiary-facing CMS workflows: processing accuracy at the back end determines results as much as outreach volume.

Where This Leaves Your Organization

This comparison is not a recommendation on which model to join. That decision is ultimately financial and actuarial, dependent on benchmark methodology and risk tolerance best modeled with actuarial support.

This article focused on the operational side: MSSP runs on claims-based assignment and BIN compliance, REACH carries a PY2026 SDoH reporting obligation with direct financial consequences, and LEAD combines voluntary alignment with a beneficiary minimum that rewards organizations building aligned lives now. Whichever model applies to your organization, outreach, survey, and notification data needs to stay organized, trackable, and ready to hand to CMS on demand.

Mabel supports that layer of the work, whether that means helping a REACH ACO complete its PY2026 SDoH collection or helping an MSSP ACO keep its BIN workflow audit-ready. If your team is weighing how to handle either, reach out to Mabel to discuss what that looks like for your network.

Frequently Asked Questions

What happens to ACO REACH organizations after the model ends on December 31, 2026?

Organizations generally have three paths: apply to the new LEAD model, apply to MSSP, or exit federal ACO participation. For a January 1, 2027 MSSP start, CMS’s Phase 1 application window runs June 9 to 23, 2026, according to CMS’s Application Types and Timeline page. The transition involves operational changes beyond the application itself: REACH aligns beneficiaries prospectively at the TIN-NPI level, while MSSP attributes at the TIN level and, under the retrospective option, finalizes the population only after the year closes, per Wakely’s white paper on transitioning from REACH to MSSP.

What distinguishes a “High Needs” ACO from a “Standard” ACO in REACH?

High Needs Population ACOs serve smaller Medicare populations with complex health care needs, while Standard ACOs are organizations with substantial experience serving Traditional Medicare beneficiaries, according to CMS’s ACO REACH fact sheet. Of the 115 PY2024 participants, CMS’s evaluation counted 97 Standard, 3 New Entrant, and 13 High Needs ACOs among those it could rigorously analyze. The financial results diverge considerably: Standard ACOs generated a 2.0 percent gross spending reduction, New Entrant ACOs generated 6.2 percent, and High Needs ACOs generated 8.9 percent, per CMS’s PY2024 performance data.

Are the beneficiary minimums the same for every organization applying to LEAD?

No. Standard ACOs need 5,000 aligned beneficiaries in Performance Year 1, high-needs ACOs can qualify with as few as 800, and newly entering ACOs need 1,000, according to CMS’s LEAD model FAQ. A separate set of Base Year minimums applies earlier: 3,000 claims-based beneficiaries for Standard ACOs, 500 for High Needs, and 600 for New Entrant. The widely cited 5,000 figure describes one participant type at one point in the model’s timeline, so applicants should check both thresholds against their own participant type.

What information must a Beneficiary Information Notice include?

A compliant BIN must disclose the ACO’s participation in the Shared Savings Program, the beneficiary’s data-sharing options, applicable voluntary alignment processes, and details of any beneficiary incentive program, according to NAACOS’s beneficiary notification guidance. ACOs must maintain evidence of distribution for each individual beneficiary regardless of delivery channel. NAACOS has linked confusion over BIN language to opt-out rates in the 2 to 5 percent range, some driven by beneficiaries mistaking a legitimate notice for a scam, per NAACOS’s write-up on improving the beneficiary notification letter.

When is the deadline to apply for the LEAD model, and how does the commitment compare to MSSP?

CMS’s Request for Applications deadline for LEAD is May 17, 2026, ahead of the model’s January 1, 2027 launch, according to CMS’s LEAD model application announcement. LEAD is a 10-year commitment, double MSSP’s standard 5-year agreement period under 42 CFR § 425.200. The same date also covers a separate, non-binding Letter of Interest for organizations considering a future cohort, so submitting only the letter does not commit an organization to the first cohort.

Can an MSSP ACO fall below its beneficiary minimum without losing its agreement?

Yes, as of the CY2025 Medicare Physician Fee Schedule final rule. An ACO must still meet the 5,000-beneficiary minimum to start a new agreement period, but dropping below it mid-term no longer triggers automatic termination, according to CMS’s CY2025 final rule fact sheet. The rule replaces the previous hard cutoff with a recovery window: the ACO has until its next agreement renewal to rebuild its count.