When Medicare began in 1965, the program operated on a straightforward logic: the federal government paid doctors and hospitals directly for care delivered to Americans over 65. No private intermediary. No network. No prior authorization. No shareholder. A beneficiary went to a Medicare-participating provider, the provider billed Medicare, and Medicare paid. The administrative overhead ran around two percent of program expenditures.
That is still how traditional Medicare works today. But more than half of the people enrolled in Medicare are no longer in traditional Medicare. They are in Medicare Advantage — a program that routes their Medicare benefits through a private insurance company, paid a fixed monthly sum by the federal government to cover whatever care they need. The private insurer manages the coverage, builds the network, decides what requires prior authorization, and keeps the margin between what the government pays and what care actually costs.
Understanding what Medicare Advantage is — and what it is not — is the starting point for everything else this hub documents.
Two Programs, One Name
Medicare Advantage is often described as a Medicare plan. The name encourages that reading. So does the marketing, which typically presents MA plans as Medicare with additional benefits — dental, vision, fitness memberships — things traditional Medicare does not cover.
That framing is inaccurate in a way that matters.
Traditional Medicare is a public program. The federal government is the insurer. There are no networks — any provider who accepts Medicare accepts you. There is no prior authorization for most services. There is no private company making coverage decisions between you and your physician. The program pays claims and you receive care.
Medicare Advantage is a privatization of that program. Congress authorized the federal government to pay private insurance companies a fixed monthly amount — a capitated rate — to cover Medicare beneficiaries. The insurer steps into the role that traditional Medicare would otherwise play. It pays the claims, but before it does, it manages the care. It builds a network of participating providers. It determines which services require prior authorization. It designs the benefit structure. And it keeps what remains after paying for care out of the capitated payment it receives from the government.
The financial structure is not a detail. It is the defining feature of what Medicare Advantage is and what it produces. A traditional Medicare beneficiary’s physician is paid by a program whose financial interest is indifferent to how much care that beneficiary receives — Medicare pays for what is needed. A Medicare Advantage beneficiary’s insurer is paid a fixed sum regardless of what care is delivered — the insurer’s margin increases when less care is delivered and decreases when more is. That structural misalignment between insurer interest and beneficiary interest runs through every mechanism this hub documents.
The Scale of the Shift
Medicare Advantage was not always the dominant form of Medicare enrollment. In the early years following its current legislative form — established by the Medicare Modernization Act of 2003 — it covered a relatively small share of Medicare beneficiaries. Enrollment grew steadily as insurers built marketing infrastructure, as broker commissions created financial incentives to steer enrollment toward MA plans, and as the program’s extra benefits attracted new retirees during their initial Medicare eligibility period.
By 2024, Medicare Advantage enrollment had crossed 33 million beneficiaries — more than 50 percent of all people enrolled in Medicare. The federal government paid approximately $450 billion to private MA insurers that year to cover those beneficiaries.
To put that in context: $450 billion is a program larger than the entire Medicaid budget. It is larger than the defense procurement budget. It is routed through four private companies — UnitedHealth Group, CVS Health/Aetna, Humana, and Elevance Health — that collectively dominate the MA market.
UnitedHealth Group, the largest MA insurer, posted $447.6 billion in total revenue in 2025. A company with revenue approaching the entire federal MA program budget is not an administrative vendor processing government claims. It is a financial entity of a scale that gives it structural influence over the program it is paid to administer.
The growth of Medicare Advantage from a niche alternative to the majority enrollment vehicle for Medicare beneficiaries is not evidence that the program is working as intended. It is evidence that the enrollment machinery — the marketing, the broker commissions, the extra benefits targeting newly eligible 65-year-olds — has been effective at capturing enrollment. What that enrollment produces for beneficiaries is a separate question. The evidence on that question is what the rest of this hub documents.
How the Payment Works
The capitated rate the federal government pays a Medicare Advantage insurer is not a flat fee applied uniformly to all beneficiaries. It is adjusted based on documented health status — a system called risk adjustment. Sicker beneficiaries, in theory, are worth higher payments; healthier beneficiaries are worth lower ones. The adjustment is calculated using a diagnosis-coding system that translates documented conditions into a risk score, and that risk score into a payment multiplier applied to the base capitated rate.
The risk adjustment mechanism is explained in full in the fourth article in this hub. What matters here is the structural implication: the MA insurer’s revenue is determined not just by how many beneficiaries it enrolls, but by what diagnoses are documented for those beneficiaries. The more conditions documented, the higher the risk score, the higher the government’s payment.
Treating those conditions does not increase payment. Documenting them does.
That distinction — between revenue driven by documented diagnosis and revenue driven by care delivered — is the origin of the $84 billion annual overpayment the Medicare Payment Advisory Commission reported to Congress in March 2025. The mechanism by which that overpayment is generated is documented in Article 05 of this hub. The payment architecture that makes it possible is documented here.
What Traditional Medicare Covers
Traditional Medicare covers hospital care under Part A and physician and outpatient services under Part B. It covers any provider who participates in Medicare — which is to say virtually any licensed physician or accredited hospital in the country. It does not require prior authorization for most services. It does not restrict beneficiaries to a network. It does not require a referral from a primary care physician to see a specialist.
What it does not cover — and this is the opening that MA marketing exploits — is dental care, vision care, hearing aids, and routine services that fall outside the Part A and Part B definitions. A traditional Medicare beneficiary who needs a crown, a hearing aid, or glasses pays out of pocket, or purchases a supplemental insurance policy — called Medigap — to cover cost-sharing and additional benefits.
Medicare Advantage plans are permitted to offer benefits beyond what traditional Medicare covers. Dental, vision, hearing, fitness memberships, transportation to medical appointments — these are the extra benefits that appear in MA advertising and that drive enrollment decisions, particularly for healthy 65-year-olds for whom the prior authorization constraints and network restrictions matter less than the dental allowance.
Those extra benefits are real in the sense that traditional Medicare does not offer them. What they are in practice — their dollar limits, their network restrictions, their durability after enrollment — is addressed in Article 10 of this hub. The short version is that their primary function is to attract healthy enrollees, not to deliver comprehensive supplemental coverage.
The Four Companies
Medicare Advantage is nominally a competitive market. There are hundreds of MA plans available across the country. But the market is dominated by four companies that control the overwhelming majority of MA enrollment nationally.
UnitedHealth Group operates MA plans through its UnitedHealthcare subsidiary while simultaneously owning Optum, the largest health services company in the country — providing pharmacy benefit management, care delivery, data analytics, and physician practice management. The integration of insurance and care delivery within a single corporate structure means that the company adjudicating coverage claims and the entity receiving payment for care delivered are subsidiaries of the same parent. The implications of that vertical integration for the medical loss ratio — the share of the capitated payment that reaches actual care — are addressed in Article 06.
CVS Health/Aetna is the product of CVS Health’s 2018 acquisition of Aetna. Like UnitedHealth, it combines an MA insurer with a pharmacy and care services infrastructure — CVS pharmacies, MinuteClinics, and the Caremark pharmacy benefit management operation.
Humana is the second-largest MA insurer by enrollment, with particular concentration in the southeastern United States. It has faced False Claims Act litigation over its risk adjustment practices, settling for $90 million in 2023.
Elevance Health — formerly Anthem — operates Blue Cross Blue Shield plans in multiple states and is among the largest MA insurers by enrollment.
These four companies receive the majority of the $450 billion in annual federal MA payments. They are not utilities. They are publicly traded corporations with shareholders, executive compensation at nine-figure levels, and financial incentives oriented toward margin — the gap between the capitated payment received and the cost of care delivered.
What This Hub Documents
The remaining articles in this hub document the mechanisms by which the Medicare Advantage payment structure produces its documented outcomes.
The political history explains how a program created in 2003 with payment rates deliberately set above traditional Medicare costs — to induce private insurer participation — has grown into a $450 billion annual transfer of public funds to private insurers over three decades.
The financial structure articles document the risk adjustment payment system, the $84 billion annual overpayment generated by the upcoding of diagnoses without treatment, and the profit extraction model that takes administrative overhead and shareholder returns from the capitated payment before care is reached.
The beneficiary experience articles document what MA enrollees actually encounter: prior authorization denials for care OIG found met Medicare coverage criteria; a denial and appeals system that produces high reversal rates when appealed but is practically inaccessible to most of the Medicare-eligible population; network gaps that become coverage failures at the point of greatest need; extra benefits that are enrollment tools rather than comprehensive coverage; and the disenrollment pattern at the end of life, when MA enrollees who develop serious illness return to traditional Medicare at documented rates that transfer the highest-cost patients back to the public program after years of capitated payment collection.
The policy debate articles present the full range of reform proposals — from audit reform to elimination — and what the evidence shows about the adequacy of each.
The closing piece documents what the evidence across the hub resolves. The core factual questions about what Medicare Advantage is and what it produces are resolved. The political questions about what to do belong to the forum.
A Note on the Name
Medicare Advantage is the program’s marketing name. It was originally called Medicare+Choice, then briefly Medicare Part C, before the Medicare Modernization Act of 2003 rebranded it. The current name — Medicare Advantage — implies an improvement on Medicare, a version with advantages the original lacks. The name is in the enrollment advertising, in the broker pitch, and in the legislation.
This hub uses the name because it is the name. It does not treat the name as accurate. Whether Medicare Advantage produces advantages for beneficiaries relative to traditional Medicare is a question the evidence in this hub addresses. The evidence does not support the name.
The complete Medicare Advantage series
01 — What Medicare Advantage Actually Is — and How It Replaced Traditional Medicare
02 — The Political History: How Private Insurers Got Into Medicare
03 — The Marketing Machine: How Enrollment Works and Who It Targets
04 — Risk Adjustment: The Payment System That Rewards Diagnosis, Not Treatment
05 — The $84 Billion Overpayment: How Upcoding Works at Scale
06 — The Profit Extraction Model: What Insurers Take Before Care Is Delivered
07 — Prior Authorization in Medicare Advantage: What OIG Found
08 — The Denial and Appeals Record: What Happens When Enrollees Push Back
09 — Network Adequacy and the Coverage Gap
10 — The Extra Benefits Myth: Dental, Vision, and What the Fine Print Says
11 — When Medicare Advantage Fails: Disenrollment at the End of Life
12 — The Reform Proposals: From Audit Reform to Elimination
13 — What the Evidence Resolves — and What It Doesn’t
This article was researched and drafted with AI assistance under human review. See our full AI and editorial practices.