Fueled by growth in the large-employer market, overall enrollment in health savings accounts linked to high-deductible health plans leaped nearly 15% to 15.5 million this year, according to an annual census released Wednesday.

In the large-employer market — defined in the survey by Washington-based America's Health Insurance Plans as employers with at least 51 employees — 9.6 million people were enrolled in HSA-linked plans at the start of the year, a 21.5% increase from 2012.

Enrollment growth in the small-group and individual markets fell, though AHIP said actual enrollment was higher than what was reported because “some responding companies were not able to categorize enrollment by market.”

In the case of the small-group market, reported enrollment fell 13.3% to 2.6 million, down from 3 million in 2012. Reported enrollment in the individual market declined 20% to 2 million, down from 2.5 million in 2012.

“HSA plans have given individuals and employers a valuable coverage option,” AHIP President and CEO Karen Ignagni said in a statement. “HSA plans encourage individuals to take an active role in their health care decisions while stretching their health care dollars.”

HSAs, authorized under a 2003 law that added a prescription drug benefit to the Medicare program, became available on Jan. 1, 2004. Enrollment has risen steadily since then.

AHIP said 1 million people were enrolled in HSAs in 2005, 3.2 million in 2006, 4.5 million in 2007, 6.1 million in 2008, 8 million in 2009, 10 million in 2010, 11.4 million in 2011 and 13.5 million in 2012.

States with the highest percentage of HSA enrollees younger than 65 with private health insurance at the start of this year were:

• Vermont, 15.4%

• Minnesota, 13.5%

• Utah, 12.0%

States with the lowest percentage of HSA enrollees at the start of this year were:

• Hawaii, 0.2%

• Mississippi, 1.1%

• Alabama, 1.7%

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The key factor driving HSA growth is that premiums for high-deductible health plans, which the law requires be linked to HSAs, tend to be lower than more traditional health plans, according to numerous surveys.

For example, a survey by Mercer L.L.C. found that the cost of medical coverage through a consumer-driven health plan linked to an HSA averaged $7,833 per employee in 2012. That's about 20% less than the average of $10,007 per employee for medical coverage through a preferred provider organization.

This year, the minimum deductible for single coverage through an HSA-linked health plan is $1,250. The minimum deductible for family coverage is $2,500.

AHIP's 2012 HSA census is based on 91 insurers and their subsidiaries offering HSA-linked health insurance plans.