Uncertainty surrounding implementation of health care reform soured Moody's Investors Service Inc.'s economic outlook this year for U.S.-based health insurers, which it downgraded to a negative outlook from stable.

The New York-based financial research and rating firm said in a report published Wednesday that it revised the managed health care industry's 2014 outlook due primarily to the proportion of high-risk individuals who have enrolled in health insurance plans offered by federal and state exchanges established under the Patient Protection and Affordable Care Act.

Preliminary enrollment statistics indicate that only 24% of individuals who purchased coverage through public insurance exchanges are 18 through 34 years old, well below the 40% benchmark Moody's said is needed to offset the added costs insurers will face with an influx of high-risk enrollees as well as those with pre-existing medical conditions.

Moody's said its negative outlook on the U.S. health insurance industry also was motivated by uncertainty surrounding the industry assessment tax, which takes effect this year. Health insurers will collectively be charged a nondeductible assessment of $8 billion in 2014, increasing to $14.3 billion by 2018.

Moody's noted in its report that although some insurers have already adjusted their premium calculations in anticipation of the additional taxes, those adjustments may not entirely offset the added costs.

“While we've had industry risks from regulatory changes on our radar for a while, the ongoing unstable and evolving environment is a key factor for our outlook change,” Stephen Zaharuk, New York-based senior vice president and author of the Moody's report, said Thursday in a statement. “The past few months have seen new regulations and announcements that impose operational changes well after product and pricing decisions were finalized.”

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Moody's report predicts an average industrywide net profit margin of approximately 2% in 2014 compared with 3% in 2013.

Average membership growth will slow, the report said, to just 1% in 2014 compared with 3% the previous year, Moody's said.

Aside from the health care reform law, Moody's said its revised outlook for the health insurance industry also reflects increasing competition, particularly the recent boom in private health insurance exchanges that it said may have a disruptive effect on the employer group market.

“We continue to expect that these dynamics will have an uneven effect on insurers, as the impact of these factors will vary by market segment and geography,” Mr. Zaharuk wrote in the report. “Our view continues to be that the larger and more diversified insurers will be better positioned, both financially and strategically, to meet the challenges facing the sector.”