Medicare Advantage vs. Original Medicare

Medicare operates through two structurally different delivery systems. Original Medicare — Parts A and B — is a federal fee-for-service program administered directly by the Centers for Medicare & Medicaid Services (CMS). Medicare Advantage, formally called Part C, is a parallel track in which a beneficiary's Part A and Part B benefits are delivered instead by a private insurer that has contracted with CMS. Understanding what each part of Medicare actually covers is the starting point for seeing how those two delivery systems diverge.

The divergence is not cosmetic. The two systems differ in how providers are paid, how out-of-pocket costs are structured, what supplemental benefits are available, and how a beneficiary's drug coverage is handled. This piece maps those structural differences as they exist in the program's current design.

How the Two Delivery Systems Are Built Differently

Under Original Medicare, a beneficiary may generally use any provider in the United States that accepts Medicare assignment. The federal program pays providers directly on a fee-for-service basis: each covered service generates a separate claim, and CMS pays a set amount for each. Part A covers inpatient hospital, skilled nursing facility, hospice, and some home health care. Part B covers outpatient services, physician visits, preventive care, and durable medical equipment. The two parts carry separate deductibles and cost-sharing obligations, and there is no built-in annual cap on a beneficiary's out-of-pocket spending under Original Medicare alone.

Medicare Advantage plans must cover everything Original Medicare covers — CMS requires this as a condition of the contract — but they deliver that coverage through their own network and cost-sharing structures. Most Medicare Advantage plans are Health Maintenance Organizations (HMOs) or Preferred Provider Organizations (PPOs). An HMO generally requires the beneficiary to use in-network providers and to obtain referrals for specialist visits. A PPO allows out-of-network use but at higher cost-sharing. Unlike Original Medicare, Medicare Advantage plans are required by law to impose a maximum out-of-pocket limit on Part A and Part B services, which CMS sets annually.

Medicare Advantage plans are also permitted — and in practice commonly do — bundle Part D prescription drug coverage into the same plan. This is called an MA-PD plan. A beneficiary enrolled in Original Medicare who wants drug coverage must instead enroll separately in a standalone Part D plan. The mechanics of why Medicare Part D exists as a separate structure trace back to the Medicare Modernization Act of 2003, which added outpatient drug coverage as a distinct benefit rather than folding it into Parts A or B.

Premiums work differently across the two systems as well. Every Medicare beneficiary pays the standard Part B premium regardless of which delivery system they use — Part B premiums scale with income through a surcharge mechanism called IRMAA. On top of that, a Medicare Advantage plan may charge its own additional premium, or it may charge zero additional premium and use its CMS capitation payment to cover costs. Some plans return a portion of the Part B premium to the beneficiary as a credit, a mechanism CMS permits under specific conditions.

Original Medicare has no network. Medicare Advantage plans do. That structural difference has downstream consequences for beneficiaries who travel frequently, live in rural areas where a plan's network may be thin, or require specialist care at academic medical centers that have not contracted with a given plan.

Who Administers Each System and What Role CMS Plays

Original Medicare is administered by CMS, a federal agency within the Department of Health and Human Services. CMS sets the coverage rules, maintains the fee schedules, processes claims through Medicare Administrative Contractors (MACs) — private companies operating under federal contract — and publishes the annual parameters for deductibles, coinsurance rates, and premium amounts. The Social Security Administration handles Medicare enrollment for most beneficiaries: enrollment in Medicare Part B is typically processed through SSA, and the Part B premium is most commonly deducted from a Social Security benefit payment. This is the operational link between Social Security and Medicare enrollment, though the two programs are governed by separate statutes.

Medicare Advantage is administered by private insurers that hold contracts with CMS under Part C of the Medicare statute. CMS pays each contracted insurer a risk-adjusted capitation rate — a per-member monthly payment — based on the expected health costs of that plan's enrolled population. The insurer then bears the financial risk of delivering all required Medicare benefits within that payment. CMS audits plan performance, enforces coverage requirements, and can terminate contracts with plans that fail compliance standards. The insurer, not CMS, handles claims processing, prior authorization decisions, and appeals at the first level.

Enrollment into Medicare itself — the prerequisite for either delivery system — is governed by CMS rules but processed largely through SSA. Beneficiaries who are already receiving Social Security retirement benefits are enrolled in Parts A and B automatically at age 65. Those who are not yet receiving Social Security must actively enroll. The 7-month initial enrollment window centered on a beneficiary's 65th birthday is the primary opportunity to enter the program without a late-enrollment penalty.

Where the Structural Differences Produce Unexpected Results

The most common friction point is prior authorization. Original Medicare does not require prior authorization for most covered services — a physician orders a service, it is rendered, and a claim is submitted. Medicare Advantage plans are permitted to require prior authorization for many services, including imaging, surgical procedures, and post-acute care. CMS has issued rules tightening the timeframes within which plans must respond to prior authorization requests, but the requirement itself remains. A beneficiary accustomed to Original Medicare's relatively direct access to care may encounter delays or denials under a Medicare Advantage plan that did not exist in the fee-for-service system.

Network adequacy is a second friction point. CMS sets minimum standards for the number and types of providers a Medicare Advantage plan must include in its service area, but those standards do not guarantee that a specific hospital or specialist a beneficiary prefers will be in-network. If a beneficiary moves to a different county or state, the Medicare Advantage plan may not operate in the new area at all, potentially triggering a special enrollment period but also a gap in established care relationships.

Supplemental benefits create a different kind of confusion. Medicare Advantage plans are permitted to offer benefits beyond what Original Medicare covers — dental, vision, hearing, fitness programs, and transportation — but these benefits vary widely by plan, by county, and by year. They are not guaranteed to remain in a plan from one year to the next because plan designs are re-filed with CMS annually. A supplemental benefit present in a plan in one year may be reduced or eliminated in the following year's plan design.

Disenrollment timing adds another layer. A beneficiary who enrolls in a Medicare Advantage plan and later wishes to return to Original Medicare can do so during the annual Open Enrollment Period (October 15 through December 7) or during the Medicare Advantage Open Enrollment Period (January 1 through March 31). However, returning to Original Medicare does not automatically restore access to a Medigap supplemental policy. In most states, Medigap insurers are not required to accept a returning beneficiary without medical underwriting outside of specific guaranteed-issue windows, which are tied to events like first enrollment at age 65 or loss of employer coverage.

What Plan Documents and Notices Show — and What They Leave Out

CMS requires Medicare Advantage plans to provide an Annual Notice of Change (ANOC) and an Evidence of Coverage (EOC) document each fall before the Open Enrollment Period. The ANOC summarizes changes from the prior year's plan — premium adjustments, changes to cost-sharing, network changes, and modifications to supplemental benefits. The EOC is the full contract document describing all covered services, prior authorization requirements, and appeals procedures. Both documents are required to be delivered before October 1 so that beneficiaries have time to compare plans during the enrollment window.

What these documents do not show is a direct comparison to Original Medicare. The ANOC and EOC describe the plan's own terms; they do not quantify how a beneficiary's costs would differ under the fee-for-service alternative. CMS's Medicare Plan Finder tool provides premium and cost-sharing data for comparison purposes, but the actual out-of-pocket exposure under either system in a given year depends on the services used, which cannot be known in advance.

For Original Medicare, the primary record document is the Medicare Summary Notice (MSN), issued quarterly to beneficiaries who have had claims processed. The MSN shows what was billed, what Medicare paid, and what the beneficiary owes under the standard cost-sharing rules. It does not show what a Medicare Advantage plan would have charged for the same services, nor does it reflect any Medigap payments that may have covered the beneficiary's share.

Medicare Advantage plans issue Explanation of Benefits (EOB) documents after claims are processed. The EOB shows the plan's allowed amount, what the plan paid, and the member's cost-sharing responsibility under the plan's specific schedule. Because Medicare Advantage cost-sharing structures vary by plan, two beneficiaries receiving the same service in the same year may see very different cost-sharing figures on their EOBs depending on which plan they are enrolled in.

The choice between Original Medicare and Medicare Advantage is built into the program's structure as an annual, revocable election — a design that reflects the program's dual nature as both a federal entitlement and a regulated private market. The two systems cover the same statutory benefits through mechanisms that are, in nearly every operational detail, distinct.

Note: This explains how a retirement system works. It is not financial, tax, or legal advice, it is not specific to any individual's retirement, and it is not a substitute for a licensed financial, tax, or legal professional. Rules, ages, and dollar limits change by year — check the cited sources.