So far, about 43 percent of Americans have received at least one dose of a Covid-19 vaccine, and 29 percent have received both doses of the two vaccines requiring double shots. The United States is averaging around 55,000 new cases a day, a roughly 20 percent drop from two weeks ago, according to a New York Times database.

“I know the quarantine and shutdowns throughout the pandemic have been exhausting,” Dr. Walensky said. “I know we all miss the things we used to do before the pandemic, and I know we all want to do the things we love and soon. Today is another day we can take a step back to normalcy of before.”

Her remarks, and those of the president, got a welcome reception even from some of the Biden administration’s fiercest Republican critics in Congress, many of whom have complained that coronavirus restrictions were an intrusion on their personal freedoms.

“It’s about time,” said Representative Jim Jordan, Republican of Ohio, who recently excoriated Dr. Anthony S. Fauci, the federal government’s top infectious disease specialist, at a hearing on Capitol Hill. “Now when do we get the rest of our liberties back?”

Senator Ron Johnson, Republican of Wisconsin, who has spread fringe theories and given a platform to vaccine skeptics, called the guidance “long overdue.”

Senator Ted Cruz, Republican of Texas, who quit wearing masks indoors after he was vaccinated, said he was “glad the C.D.C. finally acknowledged what has been obvious for a long time, which is that wearing a mask outside is silly and not remotely justified by the science.”

In fact, the science behind the C.D.C.’s new guidance is not comprehensive. A growing body of research indicates that the odds of the virus spreading outdoors are far lower than they are indoors, but that the risk is not zero and is hard to quantify.