Medicare: Parts, Coverage, and Enrollment

Medicare covers roughly 66 million Americans — people 65 and older, plus younger adults with certain disabilities or end-stage renal disease — making it one of the largest health insurance programs in the world. Its structure is genuinely unusual: four lettered parts that function almost like separate programs, each covering a different slice of medical life. Getting them right matters because the enrollment windows are narrow, the penalties for missing them are permanent, and the coverage gaps can be surprisingly large.

Definition and scope

Medicare is a federal health insurance program administered by the Centers for Medicare & Medicaid Services (CMS), established under Title XVIII of the Social Security Act. It is not means-tested — eligibility is based on age or qualifying disability, not income — which distinguishes it sharply from Medicaid, its income-based sibling.

The program is divided into four parts:

  1. Part A (Hospital Insurance) — Covers inpatient hospital stays, skilled nursing facility care following a qualifying hospital stay, hospice, and limited home health services. Most people pay no Part A premium if they or a spouse have 40 or more quarters of Medicare-taxed employment (CMS, Medicare & You 2024).
  2. Part B (Medical Insurance) — Covers outpatient care, physician services, preventive screenings, and durable medical equipment. The standard Part B premium in 2024 is $174.70 per month (CMS, 2024 Medicare Parts A & B Premiums).
  3. Part C (Medicare Advantage) — Private insurance plans approved by CMS that bundle Part A, Part B, and usually Part D into a single product. These plans must cover everything original Medicare covers, but they operate within networks and may require referrals.
  4. Part D (Prescription Drug Coverage) — Standalone drug coverage for people in original Medicare, or integrated into a Part C plan. Premiums and formularies vary by plan.

How it works

Original Medicare — Parts A and B — functions as fee-for-service insurance. Medicare pays its share of approved costs directly to providers, and the beneficiary is responsible for deductibles, coinsurance, and copayments. There is no out-of-pocket maximum under original Medicare alone, which surprises a fair number of people who assume federal insurance works like employer coverage.

That gap is typically addressed one of two ways: a Medigap (Medicare Supplement) policy, sold by private insurers and standardized into lettered plan types (A through N) by federal regulation, or enrollment in Medicare Advantage (Part C), which replaces original Medicare with a private plan that does carry an annual out-of-pocket maximum.

The Inflation Reduction Act of 2022 introduced a significant Part D change: beginning in 2025, out-of-pocket prescription drug costs under Part D are capped at $2,000 per year (CMS, Inflation Reduction Act and Medicare). That is the first hard annual cap in Part D's history.

Common scenarios

Turning 65 with employer coverage: Someone still working at 65 with coverage through a large employer (20 or more employees) can delay Part B without penalty. The employer plan is considered primary. Enrolling in Part B later triggers a Special Enrollment Period. Waiting too long after that window closes results in a 10% premium surcharge for each 12-month period of delay — permanently.

Retiring before 65: A gap often exists between employer coverage loss and Medicare eligibility. COBRA, marketplace plans, or other coverage options can bridge that window, but Medicare itself does not begin before 65 in most cases.

Low-income beneficiaries: Extra Help (the Low Income Subsidy program) can reduce Part D premiums and cost-sharing significantly. Beneficiaries who qualify for both Medicare and Medicaid — called "dual eligibles" — receive coordinated benefits through Dual Special Needs Plans (D-SNPs) or other integrated models. The Medicare Rights Center maintains detailed guidance on these intersections.

Decision boundaries

The choice between original Medicare with a Medigap supplement and Medicare Advantage is the central decision most beneficiaries face. A side-by-side comparison:

Feature Original Medicare + Medigap Medicare Advantage (Part C)
Provider network Any Medicare-accepting provider nationwide Usually network-restricted (HMO/PPO)
Out-of-pocket maximum None under Medicare alone; Medigap may cover gaps Required by law; varies by plan
Monthly premium Part B + Medigap premium Part B + plan premium (sometimes $0)
Prescription drugs Separate Part D plan required Usually included
Referrals Not required Often required (HMO plans)

Medigap policies generally offer more predictable costs and broader access — relevant for people with chronic conditions who see specialists regularly, or those who travel frequently. Medicare Advantage plans tend to carry lower upfront premiums but restrict care to networks and require more active navigation.

Enrollment timing is non-negotiable. The Initial Enrollment Period runs for 7 months centered on the month of turning 65 (3 months before, the birthday month, and 3 months after). Missing it without a qualifying Special Enrollment Period triggers late enrollment penalties that do not expire.

For a broader picture of how Medicare fits within the full landscape of U.S. coverage programs, the National Healthcare Authority homepage provides context across public and private insurance structures.

References