WASHINGTON — Marilyn Tavenner is resigning as administrator of the Centers for Medicare and Medicaid ­Services.

She will step down at the end of this month, she told CMS staff members in an e-mail.
Principal deputy administrator Andrew Slavitt, who joined the agency last year to help fix HealthCare.gov, will be the acting administrator.

Department of Health and Human Services Secretary Sylvia Burwell hailed Tavenner’s efforts in launching the federal online exchange, notwithstanding its troubled debut.
“Marilyn will be remembered for her leadership in opening the health insurance marketplace,” Burwell said. “In so doing, she worked day and night so that millions of Americans could finally obtain the security and peace of mind of quality health insurance at a price they could afford.”

“It’s a measure of her tenacity and dedication that after the tough initial rollout of HealthCare.gov, she helped right the ship, bringing aboard a systems integrator and overseeing an overhaul of the website,” the secretary added. “She is a big part of the reason why, as of this past spring, roughly 10 million Americans had gained health coverage since last year — the largest increase in four decades.”

Sen. Orrin Hatch (R., Utah), chairman of the Senate Finance Committee, said, “Marilyn has done a great job in a very difficult position under near impossible circumstances. She has proven herself to be a strong leader and a straight shooter who brought much-needed private sector sensibility into the agency.”

Tavenner had taken criticism in connection with the opening of the online health insurance exchange, including for CMS’ role in the incorrect count of the number of U.S. residents who signed up for plans during the first open enrollment period.

The Obama administration reported in September that plans obtained through the Affordable Care Act covered 7.3 million people, after saying in April that 8 million people had enrolled. Tavenner had told a House committee that some of those may have gotten insurance through work or discovered they were eligible for Medicaid while others may have not paid their premiums.

The Senate confirmed Tavenner as CMS administrator in May 2013 after she was nominated by President Obama that February. She became the first permanent head of the agency in more than six years.