She has deep relationships with health officials around the world, Dr. Frieden said, but she is less familiar with the public health system in this country.

Tough and disciplined — she walks several miles to work each day to “clear her head,” one associate said — Dr. Birx is most often described as “data driven.” She is running what amounts to a coronavirus war room from the vice president’s office, meeting with government and public health officials and pharmaceutical industry executives to shore up beleaguered local health departments, scale up the production of coronavirus test kits and encourage research into antiviral medicines and vaccines.

“I think this administration is realizing that her credibility and the years of sweat equity and trust she has built up working with people on both sides of the aisle is arguably her best asset,” said Senator Ron Wyden, Democrat of Oregon.

For the past six years at the State Department, Dr. Birx has, among her responsibilities, overseen the Presidents’s Emergency Plan for AIDS Relief, or PEPFAR, created in 2003 by President George W. Bush when antiretroviral drugs saving lives in developed countries were not available in other nations.

In its first decade, the initiative focused on “priority countries,” offering broad public health programs to slow the spread of H.I.V., said Paul Zeitz, who worked for Dr. Birx for three years.

Dr. Birx, he said, concluded that it would be more effective to concentrate specifically on H.I.V. prevention and the treatment of infected people in areas where the epidemic was disseminating swiftly. She made difficult decisions to take money from “low transmission zones” and beef up spending in areas where the disease was spreading rapidly, using statistics as a guide.

“I saw her be very tough with country teams,” Dr. Zeitz said. “She wanted the facts about exactly what was happening with their epidemic, and if people did not have data in a way that it could be used, where it could be disaggregated by gender, by geography — and then she got all the way down to statistics for each medical clinic — they would have to go back to the drawing board.”