Medicare Advantage in Sonoma County, California
66,260 people in Sonoma County, California were enrolled in Medicare Advantage in October 2026, 53.7% of 123,474 eligible beneficiaries (−1.1 pts since October 2025). Kaiser Foundation Health Plan, Inc. is the largest parent organization with 79% of reported enrollment; 15 parent organizations have 11 or more members and the HHI is 6,289 (highly concentrated). HRSA classifies the county as partly rural. For 2027, 1,561 members here are in plans that terminate or leave the county, and Health Plan of San Mateo leaves. MA plans offered go from 6 to 7.
Parent organizations in Sonoma County
| Parent organization | Members | Change since October 2025 | Share |
|---|---|---|---|
| Kaiser Foundation Health Plan, Inc. | 51,999 | +2,195 | 78.6% |
| Imperial Health Plan of California | 4,954 | +4,837 | 7.5% |
| Alignment Healthcare USA, LLC | 3,630 | +1,166 | 5.5% |
| UnitedHealth Group, Inc. | 1,902 | −3,771 | 2.9% |
| Elevance Health, Inc. | 1,687 | −737 | 2.6% |
| California Physicians' Service | 1,247 | +38 | 1.9% |
| CVS Health Corporation | 336 | −3,515 | 0.5% |
| Humana Inc. | 138 | +68 | 0.2% |
| AgeWell PACE | 112 | −7 | 0.2% |
| Centene Corporation | 32 | −5 | under 0.1% |
| Blue Cross Blue Shield of Michigan Mutual Ins. Co. | 27 | +1 | under 0.1% |
| SCAN Group | 20 | new | under 0.1% |
| Health Care Service Corporation | 17 | +2 | under 0.1% |
| Center For Elders Independence | 14 | new | under 0.1% |
| Health Plan of San Mateo | 11 | −2 | under 0.1% |
CMS suppresses county and contract counts of 10 or fewer; shares are of reported enrollment.
Value-based care alongside MA
In PY 2024, 14,205 beneficiaries in Sonoma County were assigned to 139 Medicare Shared Savings Program ACOs, 37.3% of assignable Original Medicare beneficiaries. Together, roughly 71% of all Medicare beneficiaries in the county are in Medicare Advantage or an MSSP ACO. See the ACOs in the MSSP Explorer.