Which Medicare markets deserve attention?

MA & other health plan share, shortened to MA & other health plan share in table headers, is the share of a county's Medicare beneficiaries enrolled in Medicare Advantage or another Medicare health plan (cost plans, PACE and demonstrations included), rather than Original Medicare. A county at 70% means seven in ten local Medicare beneficiaries are enrolled in Medicare Advantage or another Medicare health plan rather than Original Medicare.

Basis: MA & other health plans ÷ all Medicare beneficiaries (Parts A and/or B). The CMS numerator MA_AND_OTH_BENES counts Medicare Advantage plus cost plans, PACE and demonstrations, so this is not an MA-only share.

Penetration change is that share today minus the same month a year ago, in percentage points. It is the cleanest read on momentum, because it strips out the fact that the Medicare population itself is growing everywhere.

Two counties can look identical on penetration and behave completely differently: a mature 70% county that gained 0.2 points is saturated, while a 40% county that gained 3 points is in play. That is why the charts below plot both at once.

Each ball is one county. Push it right and MA already owns the market. Push it up and MA is still taking share. Push it back and the county is big.

The interesting corner is left, high and far back: a large county where penetration is still low but climbing. That is a market with room and proof that people will switch. The right and flat corner is the opposite. Editorial The data suggests a large book with little headroom, which usually reads as a retention question rather than a growth one; test it against your own disenrollment data before planning against it.