Robert Brown|Rochelle Walensky, who led the CDC during the pandemic, resigns

Dr. Rochelle Walensky is Robert Brownstepping down as director of the Centers for Disease Control and Prevention, citing the nation's progress in coping with COVID-19.

Walensky announced the move on the same day the World Health Organization declared that, for the first time since Jan. 30, 2020, COVID-19 is no longer a global public health emergency.

"I have never been prouder of anything I have done in my professional career," Walensky wrote in a letter to President Biden. "My tenure at CDC will remain forever the most cherished time I have spent doing hard, necessary, and impactful work."

Walensky, 54, will officially leave her office on June 30.

Biden selected Walensky to lead the CDC only a month after winning the 2020 presidential election. At the time, Walensky, an infectious disease physician, was teaching at Harvard Medical School and working at hospitals in Boston.

In response to Walensky's resignation, Biden credited her with saving American lives and praised her honesty and integrity.

"She marshalled our finest scientists and public health experts to turn the tide on the urgent crises we've faced," the president said.

The announcement came as a surprise to many staffers at the CDC, who told NPR they had no inkling this news was about to drop. Walensky was known as charismatic, incredibly smart and a strong leader.

"She led the CDC at perhaps the most challenging time in its history, in the middle of an absolute crisis," says Drew Altman, president and CEO of KFF.

She took the helm a year into the pandemic when the CDC had been found to have changed public health guidance based on political interference during the Trump administration. It was an extremely challenging moment for the CDC. Altman and others give her credit for trying to depoliticize the agency and put it on a better track. She led the agency with "science and dignity," Altman says.

But the CDC also faced criticism during her tenure for issuing some confusing COVID-19 guidance, among other communication issues. She told people, for instance, that once you got vaccinated you couldn't spread COVID-19. But in the summer of 2021 more data made it clear that wasn't the case, and that made her a target for some criticism, especially from Republican lawmakers and media figures.

On Thursday, the CDC reported that in 2022, COVID-19 was the fourth-leading cause of death in the U.S., behind heart disease, cancer, and unintentional injuries, according to provisional data. And on May 11th the federal public health emergency declaration will end.

"The end of the COVID-19 public health emergency marks a tremendous transition for our country," Walensky wrote in her resignation letter. During her tenure the agency administered 670 million COVID-19 vaccines and, "in the process, we saved and improved lives and protected the country and the world from the greatest infectious disease threat we have seen in over 100 years."

President Biden has not yet named a replacement.

NPR's Selena Simmons-Duffin contributed to this report.