On average, there was a net gain of one issuer per county in the 35 states served by HealthCare.gov in both years, the report said. In 2014, there was an average of 2.6 Obamacare issuers per county on HealthCare.gov; this year, that average rose to 3.5.
Just 8 percent of counties on HealthCare.gov saw a net reduction in the number of insurance companies selling plans. One-third of all counties saw no change in the number of issuers.
The overall increase in issuers is "good news for consumers," said Richard Frank, assistant secretary for planning and evaluation at HHS. "Because there's more choice—and diverse choice—we have a better chance of getting consumers' needs met."
Frank added that "premiums are typically driven lower" with an increase in the number of insurance companies offering plans, because existing issuers seek "to protect their market share" from new entrants.
As evidence of that, he pointed to several findings in the report. Among them: The average price increase in important benchmark plans sold on HealthCare.gov in 2015 was just 2 percent, based on weighed enrollment from the prior year.
The benchmarks are the second lowest-priced plans in the "silver" tier of Obamacare plans, and are important because they play a key role in determining how much all eligible customers of an Obamacare exchange get in the way of federal subsidies.
The growth in premiums for benchmark plans between 2014 and this year was 8.4 percentage points lower in counties that experienced a net gain in issuers, the report said.
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What's more, each net gain in issuer was associated with a 2.8 percentage point reduction in the growth rate in premiums of the benchmark plans, the report said.
Silver plans, as a group, have moderate prices compared to other "metal" group plans—they're not the cheapest or the priciest. Silver plans, which are the most popular in terms of enrollment, cover 70 percent of medical costs, on average, with the rest of the costs being the responsibility of customers.
In counties that saw a net gain in the number of issuers offering plans, the average premiums for all silver plans—not just the benchmark plan—was 1.5 percentage points lower than in other counties, the analysis found.
"It shows how competition keeps premium prices moderate," Frank said.