WASHINGTON — Dr. Anthony Fauci is back.

In truth, the nation’s leading infectious-diseases expert never really went away. But after enduring nearly a year of darts and undermining comments from former President Donald Trump, Fauci now speaks with the authority of the White House again.

He called it “liberating” Thursday to be backed by a science-friendly administration that has embraced his recommendations to battle COVID-19.

“One of the new things in this administration is, If you don’t know the answer, don’t guess,” Fauci said in one pointed observation during a White House briefing. “Just say you don’t know the answer.”

Fauci’s highly visible schedule on Thursday, the first full day of President Joe Biden’s term, underscored the new administration’s confidence in the doctor but also the urgency of the moment.

His day began with a 4 a.m. virtual meeting with officials of the World Health Organization, which is based in Switzerland, and stretched past a 4 p.m. appearance at the lectern in the White House briefing room.

The breakneck pace showcased the urgent need to combat a pandemic that has killed more than 400,000 Americans and reached its deadliest phase just as the new president comes to office.

Fauci made clear that he believed the new administration would not trade in the mixed messages that so often came from the Trump White House, where scientific fact was often obscured by the president’s political agenda.

“The idea that you can get up here and talk about what you know and what the science is … it is something of a liberating feeling,” Fauci told reporters. White House press secretary Jen Psaki had invited Fauci to take the podium first at her daily briefing.

While choosing his words carefully, Fauci acknowledged that it had been difficult at times to work for Trump, who repeatedly played down the severity of the pandemic, refused to consistently promote mask-wearing and often touted unproven scientific remedies, including a malaria drug and even injecting disinfectant.

“It was very clear that there were things that were said, be it regarding things like hydroxychloroquine and other things, that really was uncomfortable because they were not based in scientific fact,” Fauci said. He added that he took “no pleasure” in having to contradict the president, a move that often drew Trump’s wrath.

Biden, during his presidential campaign, pledged to making Fauci his chief medical adviser when he took office, and the 80-year-old scientist was immediately in motion.

Fauci was up well before dawn Thursday for the virtual meeting with WHO, which Biden had rejoined the previous day after Trump withdrew the U.S. from the group out of anger over how it dealt with China in the early days of the pandemic. Fauci told the group that the United States would join its effort to deliver coronavirus vaccines to poor countries.

In the afternoon, the doctor stood alongside Biden and Vice President Kamala Harris in the White House as they unveiled a series of executive orders aimed at slowing the spread of the virus, which is killing more than 4,000 Americans a day, as well as bolstering the nation’s sluggish vaccine distribution program.

Fauci had chatted amiably with reporters while awaiting the tardy new president. He acknowledged it was a long day and said that while he’d prefer to go for a run, he planned to powerwalk a few miles Thursday evening.

It was all a stark contrast after being kept on a tight leash by the Trump administration. Their West Wing press shop had tightly controlled Fauci’s media appearances — and blocked most of them.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, laughs while speaking in the James Brady Press Briefing Room at the White House, Thursday, Jan. 21, 2021, in Washington. (AP Photo/Alex Brandon)

istration should strive for two or three times that number. Even with the slow pace of vaccinations, the U.S. is already closing in on 1 million shots a day.

“It’s a disappointingly low bar,” said Dr. Leana Wen, a public health expert and emergency physician.

Asked about that at the White House on Thursday, Biden told a reporter: “When I announced it, you all said it’s not possible. Come on, give me a break, man.”

Fauci told reporters the U.S. could return to “a degree of normality” by the fall if the vaccination campaign goes well. Achieving widespread or “herd” immunity would require vaccinating as many as 280 million people.

Biden has directed the Federal Emergency Management Agency to begin setting up vaccination centers, aiming to have 100 up and running in a month. He’s ordering the Centers for Disease Control and Prevention to begin a program to make vaccines available through local pharmacies starting next month, building on a plan devised by the Trump administration. And he’s launching an effort to train more people to administer shots.