The president of Mexico has the coronavirus. Travel from South Africa to the U.S. will be barred.

Dr. Vivek Murthy, President Biden’s nominee for surgeon general, spoke in Wilmington, Del

Four Biden officials tried on Sunday to manage public expectations about vaccine distribution, as frustration grows among Americans over long lines, canceled appointments and other daunting issues.

The officials also tried to smooth over confusion about President Biden’s goal of 100 million shots in the first 100 days of his administration. Dr. Anthony Fauci, the nation’s top infectious disease specialist, called the figure “a floor, not a ceiling,” on the CBS program “Face the Nation.”

Dr. Fauci clarified that Mr. Biden was talking about doses, not fully vaccinated people. Within those 100 days, he said, some 67 million people might have received the second of their two required vaccine doses, with another 37 million having gotten just the first dose.

By Inauguration Day last Wednesday, nearly 36 million doses of the Pfizer and Moderna vaccines had been distributed to state and local governments. However, only about 16.5 million shots had been administered by that time.

Along with Dr. Fauci, the four officials included Dr. Vivek Murthy, the president’s nominee for surgeon general; Xavier Becerra, his nominee for secretary of Health and Human Services; and Ron Klain, Mr. Biden’s chief of staff. In appearances on the Sunday talk shows, each deflected questions about whether Mr. Biden’s goal was too ambitious or too modest.

Instead, Mr. Klain said that the vaccine distribution infrastructure inherited from the Trump administration needed to be fortified, and that many bottlenecks had to be cleared. “We need more vaccine, we need more vaccinators, we need more vaccination sites,” he said on the NBC program “Meet the Press.”

Public health officials are eagerly awaiting late-stage trial results for the one-dose vaccine under development by Johnson & Johnson. On the ABC program “This Week,” Dr. Murthy said that while the 100-million-dose goal could be achieved with the vaccines from Moderna and Pfizer that are already authorized, he was hopeful about adding the Johnson & Johnson vaccine as well.

Mr. Klain batted away the possibility that frustrated governors would purchase vaccine for their own states directly, rather than relying on the federal government. “I don’t think that’s possible,” Mr. Klain said, because the emergency authorization for the vaccines means that federal oversight is required.

Dr. Murthy said that the government also wanted to increase research on virus treatments. And he mentioned dispatching mobile vaccine units to remote communities. Until now, vaccines have been given mainly to captive populations like nursing home residents and health care workers, or in urban settings.

Mr. Becerra compared the current pandemic situation, with a severely hampered vaccine rollout and the country hurtling toward 600,000 deaths, to a stricken airplane. He said that the administration would be able to change its trajectory, but that it wouldn’t happen overnight. “You got to give us a chance to figure out what’s going on in the cockpit, causing this plane to nosedive so severely,” he said on the CNN program “State of the Union.”

Pressed for a specific timetable with realistic goals, Mr. Becerra demurred, saying his nomination had yet to be confirmed.

Dr. Fauci and other health officials have been trying to reassure the public that the vaccines that are now available would be effective against new variants of the virus that were first identified in Britain and South Africa. The vaccines could be modified if a particular variant posed a risk, Dr. Fauci said on Sunday, but there was no indication yet that modifications would be needed.