The GAO studied the large group, small group and individual insurance markets.
Three or fewer insurers held 80 percent or more of the market in at least 46 of 49 Affordable Care Act exchanges and on 42 of 46 exchanges for small employers, the GAO said.
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A concentrated market can mean less competition and higher premiums for that area, the GAO said. There may be several companies selling health insurance in a given market, but most people generally enroll with one of a few companies.
Ten states had one insurer which dominated over 75 percent of the large group health insurance market as of 2016. In seven of these states, Blue Cross Blue Shield was the insurer, including in Alabama, Alaska (Premera Blue Cross Group), Arkansas, Nebraska, South Carolina and Vermont. Wyoming Blue Cross came close at 74.4 percent.
In Iowa, Wellmark had 77.7 percent of the market; in Mississippi, Mississippi Insurance Group had 82.5 percent; in Montana, HCSC Group had 82.4 percent;
and in West Virginia, Highmark had 78.3 percent.
States in which the three largest issuers in 2016 had a large group market share of over 90 percent (though all but the largest insurer are named) are Alabama, Alaska, Arkansas, Delaware, Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Minnesota, Mississippi, Montana, Nebraska, New Hampshire, New Mexico, North Carolina, North Dakota, Rhode Island, South Carolina, Vermont, West Virginia and Wyoming.
Blue Cross Blue Shield, Highmark, Wellpoint, Wellmark, CareFirst, HCSC, the Hawaii Medical Service Association, the Louisiana Health Service Group and Health Partners, had the largest share of the three insurers in these states.
Other states where the largest insurer had a relatively low market share, less than 50 percent of the market include California and Colorado, where the Kaiser Foundation had a 45.7 and 48.5 percent market share in 2016; Arizona, where BCBS had a 38.4 percent share; the District of Columbia, where Aetna had a 37.4 percent share and Connecticut where Wellpoint had a 29.8 percent share, among other states.
UnitedHealth, the nation's largest insurer, was listed as a state's largest insurer for the large group market only in Nevada, where it had a 65.4 percent market share in 2016.
Historically, the market for health insurance sold by private issuers has been highly concentrated.
Enrollment in private health insurance plans was concentrated among a small number of issuers in 2015 and 2016, the GAO report said. The trends are similar to what GAO previously reported for 2011 through 2014.
The GAO also found that within the overall individual and small group markets in each state, the health insurance exchanges established by the ACA were concentrated from 2015 to 2017.
Further, the largest issuers increased their market share in about two-thirds of exchanges. The number of issuers participating in a market and their market shares can affect concentration, and many individual exchanges generally had a decreasing number of participating issuers over time, the study said.
On average, there were 16 issuers participating in each state in 2016. The three largest held 80 percent or more of the market--an indicator of high concentration--in 37. The remaining issuers in each state often had significantly smaller market shares--on average, 12 of the 16 issuers in each state held less than 5 percent market share.
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