INTRODUCTION: Insurer ownership of primary care practices has expanded rapidly in recent years, but its magnitude, geographic distribution, and market drivers remain unclear.

METHODS: Using corporate filings, M&A databases, and insurer directories, we identify medical groups operated by UnitedHealth's Optum, Humana, Elevance, Aetna-CVS Health, and Cigna from 2016 to 2023 and calculate each payer's share of the Medicare primary care market-encompassing both Traditional Medicare and Medicare Advantage-nationally and by county. We then compare primary care market penetration by payers in counties above vs below the population-weighted median for hospital and insurer concentration, Medicare Advantage penetration, UnitedHealth's share of the Medicare Advantage and employer insurance markets, and hospital-physician integration.

RESULTS: Payer-operated practices account for 4.2% of the national primary care market by service volume in 2023, up from 0.78% in 2016. Optum, the largest payer-affiliated entity, held 2.71% nationally and over 35% in 3 large counties. The prevalence of payer-operated primary care was positively associated with Medicare Advantage penetration and negatively associated with concentrated hospital and employer-based insurance markets.

CONCLUSION: Insurer control of primary care is expanding, concentrated in areas with robust Medicare Advantage enrolment and less concentrated hospital markets. Further research should examine its impact on care delivery, spending, and competition.