CDC Director Dr. Rochelle Walensky on Friday announced she will resign at the end of June, ending a tenure marked by repeated change to adapt to an evolving coronavirus crisis.

Walensky did not provide a specific reason for her departure, but noted in a letter to President Joe Biden that the U.S. is transitioning out of the emergency of the Covid-19 response.

“The end of the COVID-19 public health emergency marks a tremendous transition for our country, for public health, and in my tenure as CDC Director,” Walensky wrote in the letter.

“I took on this role, at your request, with the goal of leaving behind the dark days of the pandemic and moving CDC – and public health – forward into a much better and more trusted place,” she said.

The U.S. public health emergency will end Thursday. The World Health Organization on Friday declared an end to the global Covid health emergency.

Biden, in a statement, thanked Walensky for her service.

“Dr. Walensky leaves CDC a stronger institution, better positioned to confront health threats and protect Americans,” the president said.

Walensky acknowledged in August 2022 that the CDC’s response to the pandemic was inadequate. She launched a reorganization that sought to make the agency faster at responding to disease threats and improve its communication of health guidance to the public.

But the CDC still struggles to respond to public health threats due to limited authority in the face of a fragmented health-care system. The agency will have less data to track Covid and new variants when the public health emergency expires because it cannot compel states to report this information.

Walensky assumed leadership of the battered agency in early 2021 as the U.S. was rolling out its Covid vaccination campaign. She helmed the public health agency while the national pandemic response faced repeated setbacks from the emergence of the delta and omicron variants.

Walensky also helped lead the U.S. response to the sudden outbreak of mpox in the summer of 2022.

She led the infectious disease division at Massachusetts General Hospital and was a professor of medicine at Harvard Medical School before joining the Biden administration. Walensky is an expert on HIV.