On "State of the Union," Birx told CNN's Dana Bash that she has "tremendous respect" for Pelosi, but criticized a New York Times article last month that reported she had painted an optimistic view of the pandemic to the White House during a critical period in getting control of the virus.

"I have tremendous respect for the speaker, and I have tremendous respect for her long dedication to the American people," Birx said, adding, though, that she could have "brought forth the data" to back up her analysis had the Times spoken with her.

"I have never been called pollyannish, or nonscientific, or non-data driven," Birx said. "And I will stake my 40-year career on those fundamental principles of utilizing data to really implement better programs to save more lives."

In its article, the Times said Birx declined to be interviewed and the newspaper did not call Birx "pollyannish," as she had claimed during her CNN interview.

Pelosi doubled down Sunday morning on a comment she reportedly made behind closed doors on Thursday night accusing Birx of spreading disinformation about the coronavirus pandemic.

Politico reported Pelosi told White House chief of staff Mark Meadows and Treasury Secretary Steven Mnuchin that Birx was the "worst" and that they were in "horrible hands" with her.

Asked on ABC's "This Week" whether the account was true and if she has confidence in Birx, Pelosi said, "I think the President is spreading disinformation about the virus and she is his appointee, so I don't have confidence there, no."

Birx also told CNN on Sunday that the US has entered a "new phase" in the coronavirus pandemic, saying that the virus is "extraordinarily widespread" in urban and rural areas.

Asked by Bash if it was time to reset the federal government response to the pandemic, Birx said, "I think the federal government reset about five to six weeks ago when we saw this starting to happen across the South." Roughly six weeks ago, however, Vice President Mike Pence, who heads the coronavirus task force, declared in a Wall Street Journal op-ed that the US is "winning the fight" and there "isn't a 'second wave.'"

As CNN has previously reported, Birx, despite ties to the Obama administration, has been able to develop a close relationship with the Trump White House, which has tainted her reputation among some public health experts.

In 2014, President Barack Obama nominated Birx to her role at the US State Department as an ambassador-at-large and coordinator of the US Government Activities to Combat HIV/AIDS and US Special Representative for Global Health Diplomacy.

She served as the head of US Military HIV Research Program in the 90s -- a role in which she oversaw the Thai vaccine trial, the first HIV vaccine to show preventative results -- and later became director of the Centers for Disease Control and Prevention's Division of Global HIV/AIDS in 2005.