The study by the Milliman consulting firm did not look at the impact of federal health care overhaul on costs for most adults who receive coverage through their employer. Instead, it analyzed how the federal law would impact individual premium rates next year.

Covered California is hoping the study will guide health insurers in pricing their plans as the companies submit bids to sell on the state's insurance marketplace. The exchange, as it's called, launches Oct. 1.

The report estimated that extending coverage to the uninsured will result in a 26.5 percent average increase for individual plan premiums. However, the report said much of that increase will be offset by other factors, such as special payments to insurers to attract an outsize share of the sick, and the price-cutting effect of competition and more effective contracting.

Premium prices will vary greatly depending on an individual's age, income and where they live. For example, a person earning less than $28,725, or 250 percent of federal poverty level, will see rates drop an average of 85 percent because they will be eligible for larger tax credits.

A person earning between $28,725 and $45,960, or between 250 percent and 400 percent of federal poverty level, will pay an average of 45 percent less due to partial subsidies.

Individuals making more than $45,960 would likely see an average premium increase of 30 percent because they won't receive any subsidies.

Larry Levitt, a private health insurance expert with the nonprofit Kaiser Family Foundation, which analyzes health policy issues, said the projections illustrate flaws in the current individual insurance market system. He said rates are misleadingly low because people who are sick or have pre-existing conditions are excluded from medical underwriting.

"You may be paying a low premium because you're healthy but there's no guarantee that if you get sick, that that premium is going to stay low," Levitt said. "By fixing the market, people are getting more security and stability in their coverage over time."

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