As the fifth anniversary of the signing of the president's landmark health insurance reform law nears, the Obama administration is boasting about what it says is the law's major success: reducing the number of uninsured.

More than 16 million adults have gained health insurance coverage due to the Patient Protection and Affordable Care Act, the Department of Health & Human Services reported this week.

That count includes 14.1 million previously uninsured adults who have been added to the health insurance rolls since October 2013, the beginning of the first public health insurance exchange open enrollment period.

The total also includes 2.3 million young adults ages 19 through 25, who have gained coverage due to a provision in the law that requires employers to offer coverage to employees' adult children.

“Since the passage of the Affordable Care Act almost five years ago, about 16.4 million uninsured people have gained health coverage — the largest reduction in the uninsured in four decades,” HHS Secretary Sylvia M. Burwell said in a statement.

The big gains in coverage are the result of several provisions in the health care law, which the president signed on March 23, 2010.

For example, the law gave states hefty financial incentives to expand their Medicaid programs. More than two dozen states did so, changing their laws to allow coverage to uninsured individuals earning up to 138% of the federal poverty level, or $16,242 in 2015. Previously, individuals earning more than 100% of the federal poverty level, or $11,770, were not eligible for Medicaid.

In addition, the health care act provides federal premium subsidies to the lower income uninsured — those making between 100% and 400% of the federal poverty level — to obtain coverage in public exchanges.

The law also requires employers to offer coverage to employees' adult children until age 26. Prior to the reform law, employers typically ended coverage to employees' children at age 18 or 19, or age 23 or 24 in the case of full-time college students.

The administration did not break out how many of the 14.1 million recently insured adults gained coverage through the public exchanges the law created and how many received coverage through Medicaid.

However, HHS earlier reported that nearly 11.7 million individuals chose health care plans in the exchanges before the close of the 2015 open season. It isn't known, though, how many enrollees were previously uninsured.

In the last year, the health care reform law, though, has not had much effect on enrollment in employer-sponsored health care plans.

A Mercer L.L.C. survey of nearly 600 employers released Tuesday found that on average 83% of employees eligible for coverage enrolled in plans this year, down from 84% in 2014.

One factor holding down growth in employer plans was the move of some lower-paid employees into newly expanded Medicaid programs, Mercer said.