Which counties are gaining health plan share fastest?
Growth gets reported two ways and they tell different stories. Share gained (percentage points) asks how much of the local market MA took from Original Medicare. Enrollment growth (percent) asks how many more members joined, which rises even in a county where MA is losing ground, simply because the county is ageing.
Share gained is the honest read on competitive momentum. Enrollment growth is the honest read on revenue. A county can post 8% enrollment growth and still be a share loser.
Small counties swing hardest, because a few hundred members move the percentage a long way. Filter by minimum Medicare population in the county explorer before you treat a big swing as a signal.
De Soto, LA is gaining MA & other health plan share fastest of the 1,883 US counties with at least 5,000 Medicare beneficiaries: +7.2 points year on year, to 56.1%. 2 of the top 10 are in LA. Counties with fewer than 5,000 Medicare beneficiaries are excluded, because a small base moves a percentage a long way.
These figures came from a dataset that has been withdrawn. They are not current production analytics.
Top 12 tracked counties by YoY penetration gain
Velocity = 2026 penetration minus 2025, in percentage points. “→ n%” shows the resulting 2026 penetration. All 3,199 US counties are ranked, from the April 2026 CMS Medicare Monthly Enrollment county file.
Data table — top 20 counties with 4-year penetration history
| County | St | MA & other health plan share | YoY change | Enrollment growth | Medicare beneficiaries |
|---|---|---|---|---|---|
| De Soto Parish | LA | 56.1% | +7.2 | +15.6% | 6,607 |
| Caddo Parish | LA | 55.3% | +4.4 | +8.9% | 52,706 |
| Pulaski | MO | 45.1% | +3.5 | +10.7% | 7,727 |
| Cattaraugus | NY | 66.8% | +3.5 | +6.3% | 19,640 |
| Mckinley | NM | 33.7% | +3.1 | +12.1% | 12,558 |
| Webb | TX | 64.9% | +3.1 | +7.7% | 39,200 |
| Lea | NM | 34.7% | +3.0 | +10.9% | 9,799 |
| Sanpete | UT | 52.5% | +3.0 | +8.3% | 5,663 |
| Scotts Bluff | NE | 35.6% | +2.8 | +9.9% | 8,829 |
| Calvert | MD | 18.6% | +2.7 | +20.1% | 19,318 |
| Fulton | IL | 47.2% | +2.6 | +6.5% | 8,437 |
| Luna | NM | 62.9% | +2.6 | +4.8% | 6,785 |
So what / Now what Interpretation
Momentum is concentrated in already-high-penetration Florida metros — growth there is switcher-driven and D-SNP-heavy, not conversion. Counties combining above-average velocity with sub-60% penetration are the genuine expansion plays.
- Market entry: filter the CSV for velocity ≥ 1.0pt and penetration < 60% to get the short list of true conversion markets.
- Competitive defense: in top-velocity counties, check which carrier is capturing the growth (top-carrier column) before committing AEP budget.
- Watch D-SNP share — several fast counties grow primarily through dual-eligible products; see the dual-eligible density page.
Provenance — MA and other health plan enrollment and share by county
- Source file
- CMS Medicare Monthly Enrollment (BENE_GEO_LVL=County)
- Vintage
- April 2026
- Last updated
- July 31, 2026
- Refresh cadence
- Monthly, following the CMS Medicare Monthly Enrollment release (CCW-derived, roughly three months in arrears)
- Method
- penetration_pct = MA_AND_OTH_BENES / TOT_BENES * 100, rounded to 1 decimal. Velocity is the year-on-year change in that percentage, in points.
- Known limitations
- The denominator includes Part A-only beneficiaries, who cannot enroll in Medicare Advantage, so an eligible-only basis reads higher. Small counties swing hardest: a few hundred members move the percentage a long way. Carrier and plan detail is not in this file.