UnitedHealthcare ACOs: NexusACO, Medicare Advantage, and Results

A UnitedHealthcare ACO is an accountable care organization — a group of doctors, hospitals, and other providers under contract with UnitedHealthcare to coordinate care for a defined population and share financial accountability for both the cost and quality of that care. As of November 2025, UnitedHealthcare manages 113 ACOs in its commercial book of business, and 68% of commercial spending flowing through those ACOs sits in risk-sharing arrangements where providers can earn bonuses for hitting cost and quality targets or owe money back when spending overshoots. The company projects that nearly 40% of its total commercial spending will run through value-based contracts by 2026.¹

The program has two main faces. For employer-sponsored commercial plans, the flagship is a tiered benefit product called NexusACO. For seniors, UnitedHealthcare has built ACO-style partnerships inside its Medicare Advantage business, which covered 7.8 million people at the end of 2024.²

How the Shared-Accountability Model Works

In any ACO, a group of providers agrees to be measured against a projected spending benchmark for its attributed patients. Beat the benchmark while meeting quality standards, and the group shares in the savings. In two-sided risk arrangements, the group also pays money back if spending comes in over target.

UnitedHealthcare’s commercial ACO contracts follow that general shape but run on the company’s own benchmarks rather than the federal Medicare Shared Savings Program rules. Providers are scored against two yardsticks: quality metrics compared to national rates for similar patients, and cost efficiency compared to risk-adjusted costs of peers in the same specialty and local market. The top-tier contracts are limited to organizations that outperform competitors on both dimensions in a given market.³

Each participating ACO is paired with what UnitedHealthcare calls an activation team — a market medical director, an ACO account manager, and a clinical transformation consultant. They share patient data with the ACO’s physicians, monitor referral patterns to keep care inside the high-performing network, and flag patients for proactive outreach. If the ACO’s network has a gap in specialty coverage, it is expected to work with UnitedHealthcare to fill it.³

NexusACO for Employer Plans

NexusACO is a national ACO-based health plan product sold to employer groups. It uses a tiered benefit design to steer members toward the high-performing providers inside the ACO. Members who pick a Tier 1 doctor or hospital pay lower copays and coinsurance, typically 20 to 30 percentage points less than the cost-sharing applied to non-Tier 1 providers.³

Where a featured ACO operates, its providers make up Tier 1. In areas without a featured ACO, Tier 1 defaults to physicians who carry UnitedHealthcare’s Premium Care designation based on individual quality and efficiency scores.⁴

The plan comes in two flavors:

NexusACO R requires members to get a referral from their primary care physician before seeing a specialist.
NexusACO OA is open-access with no referral requirement.⁴

Both require members to pick a primary care physician to coordinate their care.⁴

NexusACO is available to employer groups of any size but is not offered in every state or market.⁵ As of early 2018 the product was in 22 markets, and UnitedHealthcare said in February 2025 that it “continues to expand around the country” without publishing a complete market list.⁴ Confirmed operating markets include Massachusetts, Texas, and the Chicago metropolitan area.⁶

What Tier 1 Looks Like in Practice: Advocate Health Care in Chicago

The Chicago-area NexusACO is built around Advocate Health Care. Within a core service area covering several northeastern Illinois counties, every Advocate primary care physician and specialist is Tier 1. In the surrounding plan footprint, roughly 70% of physicians qualify as Tier 1.⁷ Advocate contributes nearly 400 care sites, 12 hospitals, and 6,300 affiliated physicians to the arrangement. To enroll, an employer must be based in one of 14 Chicagoland counties, and employees must live in those counties or in three adjacent Northwest Indiana counties.⁸

Medicare Advantage ACO Partnerships

On the Medicare side, UnitedHealthcare has built similar arrangements with large health systems. The most prominent is a partnership with Intermountain Healthcare announced in December 2020, covering roughly 136,000 UnitedHealthcare Medicare Advantage members in Utah. Physician reimbursement is tied to health outcomes for patients who get care from Intermountain primary care physicians.⁹

UnitedHealthcare feeds Intermountain’s physicians patient-level data on chronic conditions, past treatments, medications, and gaps in care so they can identify patients at high risk for hospitalizations or ER visits.¹⁰ Intermountain’s subsidiary Castell handles care coordination, network services, and documentation.⁹ Castell has posted strong results in federal value-based programs as well: $46.8 million in Medicare savings in 2022, the second-highest total among ACOs in its program, with $20.6 million shared back to participating primary care groups.¹¹

What the Numbers Say About Results

On the commercial side, UnitedHealthcare has projected that employers using NexusACO can save up to 15% compared with broad-access health plans, with a trended claims discount of 2% in year one, 5% in year two, and 8% in year three.¹² The company has also reported that NexusACO members had 10% fewer hospital admissions and emergency room visits.¹³ UnitedHealthcare’s own analysis of high-performing ACO-centric networks more broadly has attributed up to 9% lower total cost of care and 47% fewer inpatient admissions to those designs.

Outcome data on the Medicare Advantage side comes from studies involving Optum, UnitedHealth Group’s care delivery and analytics arm. A study published in JAMA Network Open in February 2023 found that patients in Optum’s fully accountable Medicare Advantage model were 18% less likely to be admitted to a hospital, 11% less likely to visit an emergency department, and 9% less likely to be readmitted within 30 days than patients in traditional Medicare fee-for-service.¹⁴ A separate study by researchers from Optum, America’s Physician Groups, and Harvard, drawing on 2019 data for more than one million Medicare Advantage enrollees, found that patients in value-based arrangements were up to 22% less likely to be hospitalized for chronic conditions and 13% less likely to be readmitted within 30 days.¹⁵ Both studies were conducted by or with the company and look at its own patients.

How UnitedHealthcare ACOs Differ From Medicare’s ACO Programs

UnitedHealthcare’s ACOs are private contracts. They are distinct from the federal ACO programs run directly by the Centers for Medicare and Medicaid Services, the largest of which is the Medicare Shared Savings Program. In 2026, that federal program includes 511 participating ACOs serving an estimated 12.6 million beneficiaries, and those ACOs earned $4.1 billion in shared savings in performance year 2024 while saving Medicare $2.5 billion.¹⁶

Research has flagged structural differences between the two kinds of ACO. A 2016 Health Affairs study found that commercial ACOs tend to be larger, more integrated with hospitals, more often physician-led, and generally report higher quality-of-care scores than Medicare ACOs. They also used more disease-monitoring tools and patient satisfaction data. Shared electronic medical records were present in only about a third of commercial ACOs and 15% of public ones at the time.¹³

The federal side is moving further toward downside risk. CMS finalized changes in its 2026 Physician Fee Schedule to push more Shared Savings Program ACOs into two-sided risk, and 82.8% of them participate at risk levels that qualify as Advanced Alternative Payment Models in 2026. A new Long-term Enhanced ACO Design (LEAD) model is scheduled to launch in 2027 to broaden participation to independent, rural, and specialist practices.¹⁶

The Regulatory Backdrop

UnitedHealth Group’s Medicare business, which includes the Medicare Advantage ACO relationships, is under federal scrutiny. In July 2025, the company disclosed that it had contacted the Department of Justice after media reports surfaced about investigations into its Medicare program participation, and confirmed it is complying with “formal criminal and civil requests” from the DOJ.¹⁷ Reporting indicates the inquiry focuses on practices for recording diagnoses that trigger extra payments to UnitedHealth’s Medicare Advantage plans, including oversight of physician groups owned by the company.¹⁸ UnitedHealth Group has said it has “full confidence in its practices” and has launched third-party reviews of its risk-coding, managed care, and pharmacy practices.

The investigation is not directed at the ACO program itself, but it sits over the same Medicare Advantage business that UnitedHealthcare’s provider partnerships depend on, and vertical integration — insurer, provider network, pharmacy benefit manager, and data analytics under one corporate roof — is both what makes those ACO arrangements possible and what has drawn regulators’ attention.

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