• Jeffrey Marr, Mark K Meiselbach, and Daniel Polsky.
• Department of Health Policy and Management, Johns Hopkins Bloomberg School of Public Health, 624 N Broadway, Baltimore, MD 21205. Email: jmarr5@jhu.edu.
• Am J Manag Care. 2023 Oct 1; 29 (10): e317e319e317-e319.

ObjectivesCommercial health insurers can participate in the rapidly growing Medicare Advantage (MA) market, which may affect network formation and prices in traditional commercial insurance markets. We aim to quantify the prevalence and growth of commercial insurers participating in MA within the same state.Study DesignRepeated cross-sectional analysis of Clarivate's Interstudy enrollment data comprising the universe of insurers in the United States from 2015 to 2021.MethodsWe calculated the share of employer-sponsored insurance (ESI) enrollees covered by an insurer offering MA in their state in 2015, 2017, 2019, and 2021. We documented this share across states, years, and the state's 2015 tercile.ResultsBetween 2015 and 2021, the share of ESI enrollees covered by an insurer offering MA in the same state increased from 83.5% to 95.3%. This growth was concentrated in states with initially low rates in 2015 (lowest 2015 state tercile, ≤ 70.5%), in which the share grew from 47.6% to 87.9%. In 2015, 23.5% of states had a share greater than 90.0% compared with 74.5% in 2021.ConclusionsBy 2021, almost all ESI enrollees were covered by insurers who participated in MA in the same state. Future research should investigate how insurer participation in MA affects network formation and prices in commercial markets.

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