What is a closed block?

First, a plain warning about the term

“Closed block” and “death spiral” are industry and actuarial usage. They are not legal categories, they are not defined by Medicare, and no regulator maintains a list of them. If a website tells you a specific carrier is “officially in a death spiral”, it is inventing an authority that does not exist.

What is documented is every piece of the mechanism: that policies are guaranteed renewable, that rate increases apply to filed classes rather than to individuals, and that insurers must file those increases with the state. Put those together and the pattern follows. We will show you the parts, cite each one, and let you draw the conclusion.

The mechanism, in order

1. An insurer stops selling a policy form. Maybe it launched a replacement, maybe it left the market, maybe the pricing stopped working. Nothing about this is improper, and it does not have to be announced to you.
2. No new customers enter that group. Existing holders keep their coverage — the policy is guaranteed renewable and the insurer cannot cancel it while you pay.^{[Medicare.gov]} But nobody younger joins.
3. Everyone in it ages together. A group with no new entrants gets older every year, in lockstep. Claims per person rise accordingly.
4. Premiums follow claims. Federal law requires a Medigap policy to be expected to return at least 65% of premium as benefits for individual policies, and 75% for group policies.^{[Social Security Administration]} When claims climb against a fixed premium, the arithmetic has one release valve: file for more premium.
5. The healthiest people leave. Here is the turn. Anyone healthy enough to pass medical underwriting can shop elsewhere — and the people who can pass are, by definition, the ones with the lowest claims.^{[Medicare.gov]}
6. Which makes the average worse. Removing the cheapest people from a pool raises the average cost of everyone remaining. That drives the next increase, which prompts the next healthy exit. This self-reinforcing loop is what the “spiral” in the nickname refers to.

Guaranteed renewability is doing exactly what it was designed to do throughout: nobody is cancelled, nobody is singled out for their health. The model regulation states adopt simply does not extend that protection to price — it says plainly that guaranteed renewability does not prohibit rate increases otherwise authorised by law.^{[Centers for Medicare & Medicaid Services]}

Why the same brand runs two different books

This is the part that makes carrier-level advice useless.

Rates are filed by a legal entity, for a specific policy form, in a specific state. A national brand may sit on top of several legal entities, each with its own NAIC number. One of those entities may hold an older, closed book. Another may hold the current one, still selling, still taking in 65-year-olds.

Our own research corpus shows this clearly. In Ohio, two entities selling under the same well-known brand, on the same plan letter, with increases effective the same day each June, ran like this over three consecutive years: one at roughly 10%, then 20%, then 36%; the other at roughly 11%, then 14%, then 15%. A customer of the first would reasonably conclude the brand raises rates aggressively. A customer of the second would reasonably conclude the opposite. Both would be generalising from their own block, and both would be wrong about the other.

So “which company is best?” has no answer. “Which block am I in, and what has it filed?” has a precise one.

A closed block is not the same as a discontinued plan letter

These get conflated constantly, and they are different things.

Plan C and Plan F are not available to people who turned 65 on or after 1 January 2020, because federal law stopped new policies from covering the Part B deductible for people newly eligible from that date.^{[Medicare.gov]} That is a statutory change to what can be sold, not an insurer's business decision.

Crucially, it did not close the existing books. People who were eligible for Medicare before 1 January 2020 keep their Plan C or Plan F, and may still buy one.^{[National Association of Insurance Commissioners]} Those blocks remain open to a shrinking but real pool of new entrants.

The practical upshot: being on Plan F does not by itself mean you are in a closed block. It means your pool of possible new entrants is limited to people who reached Medicare eligibility before 2020, and that pool only shrinks. Worth watching, not worth panicking about.

How to check whether your block is closed

You can do this yourself in about fifteen minutes.

1. Get the exact entity name from your policy schedule. Not the brand on the envelope — the legal entity, and the NAIC number if it is printed. This is the only identifier that maps to a filing.
2. Ask the insurer the direct question: “Is this policy form still being sold to new applicants in my state?” They know. Ask for it in writing.
3. Look at the shape of the history, not one number. Increases that hold steady in the mid single digits describe a functioning block. Increases that step up year over year — 10, then 20, then 36 — describe one under pressure.
4. Check the loss ratio. A block paying out more in claims than it collects has a problem it must eventually solve through price. What a loss ratio is and how to read one.
5. Compare against the same carrier’s current offering. If the entity is quoting a much lower premium to a new 65-year-old for the same plan letter, you are likely in the older book.

If you are in one

Being in a closed block is not an emergency and it is not a reason to do something hasty. It is a reason to find out what your options are before you need them, because the options narrow as your health changes.

The honest constraint: leaving requires somewhere to go, and outside your one-time open enrollment window that usually means passing medical underwriting.^{[Medicare.gov]} Some states give you standing rights the federal floor does not. What applies where you live.

And if you do move, move on the destination’s filed history, not its opening price. A cheaper policy in a worse block is a slower version of the same problem.