Dr. Rochelle Walensky, the head of the Centers for Disease Control and Prevention, submitted her resignation Friday, saying the waning of the COVID-19 pandemic was a good time to make a transition.

Walensky's last day will be June 30, CDC officials said, and an interim director wasn’t immediately named. She sent a resignation letter to President Joe Biden and announced the decision at a CDC staff meeting.

Walensky, 54, has been the agency's director for a little over two years. In her letter to Biden, she expressed “mixed feelings” about the decision and didn't say exactly why she was stepping down, but said the nation is at a moment of transition as emergency declarations come to an end.

“I have never been prouder of anything I have done in my professional career,” she wrote.

The World Health Organization said Friday that COVID-19 no longer qualifies as a global emergency, and the U.S. public health emergency declaration will expire next week. Deaths in the U.S. are at their lowest point since the earliest days of the coronavirus outbreak in early 2020.

The CDC, with a $12 billion budget and more than 12,000 employees. is an Atlanta-based federal agency charged with protecting Americans from disease outbreaks and other public health threats.

Walensky, previously an infectious-diseases specialist at Harvard Medical School and Massachusetts General Hospital, had no experience running a government health agency when she was sworn in on the first day of the Biden administration.

She came with a reputation as a prominent voice on the pandemic, sometimes criticizing certain aspects of how the government was responding. She was brought in to raise morale at the CDC, to rebuild public trust in the agency and to improve its sometimes-bumbling response to the pandemic.

She started a center for forecasting and outbreak analytics, took steps to modernize data and improve the public health work force. Last year, she began a reorganization designed to make the agency more nimble and to improve its communications with the public.

But there have been stumbles during her tenure too.

In the spring of 2021, Walensky said fully vaccinated people could stop wearing masks in many settings, only to reverse course as the then-new delta variant spread. And in December 2021, the agency’s decision to shorten isolation and quarantine caught many by surprise and caused confusion.

In a statement, White House Chief of Staff Jeff Zients praised her performance.

“Her creativity, skill and expertise, and pure grit were essential to our effective response and an historic recovery that made life better for Americans across the country,” Zients said.