(Bloomberg) — Marilyn Tavenner, head of the U.S. Centers for Medicare & Medicaid Services (CMS), plans to step down at the end of February, she told her staff in an e-mail.

Tavenner didn’t say why she was leaving.

Andy Slavitt, the former chief executive officer of the Ingenix health data unit at UnitedHealth Group Inc. (NYSE:UNH), who has been the principal deputy administrator at CMS, will move into Tavenner’s job on a temporary basis, Health and Human Services (HHS) Secretary Sylvia Mathews Burwell said in a separate e-mail to staff.

“It goes without saying that Marilyn will be remembered for her leadership in opening the Health Insurance Marketplace,” Burwell wrote on the e-mail. “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.”

Burwell said Tavenner “helped right the ship” after the tough initial rollout of the HealthCare.gov Patient Protection and Affordable Care Act (PPACA) exchange enrollment system.

Tavenner “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,” Burwell said.

In November 2014, Tavenner acknowledged that her agency had made a mistake in its calculation of the number of people enrolled in PPACA public exchange coverage for 2014. About 393,000 individuals with both health and dental coverage were “inadvertently counted twice,” she said in a letter to Rep. Darrell Issa, R-Calif., whose committee discovered the error.

See also: PPACA data inflation prompts HHS chief to demand review.

Tavenner had been acting administrator of CMS since 2011 and became the official head in May 2013.

See also: Marilyn Tavenner wins confirmation for CMS post.

In Tavenner’s e-mail today, she focused on the agency’s progress in expanding Medicaid enrollees and providing insurance through the PPACA exchange system.

“We came together and faced the challenges and are now providing millions of people with access to an affordable, high quality — and in some cases a lifesaving — health-care plan,” she said.

 

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