The announcement arrived at a critical moment, as the steep decline in coronavirus cases seems to have plateaued and millions of Americans are on waiting lists for shots.

Johnson & Johnson has pledged to provide the United States with 100 million doses by the end of June. When combined with the 600 million doses from the two-shot vaccines made by Pfizer-BioNTech and Moderna slated to arrive by the end of July, there will be more than enough shots to cover any American adult who wants one.

But federal and state health officials are concerned that even with strong data to support it, some people may perceive Johnson & Johnson’s shot as an inferior option.

The new vaccine’s 72 percent efficacy rate in the U.S. clinical trial site — a number scientists have celebrated — falls short of the roughly 95 percent rate found in studies testing the Moderna and Pfizer-BioNTech vaccines. Across all trial sites, the Johnson & Johnson vaccine also showed 85 percent efficacy against severe forms of Covid-19 and 100 percent efficacy against hospitalization and death.

“Don’t get caught up, necessarily, on the number game, because it’s a really good vaccine, and what we need is as many good vaccines as possible,” Dr. Anthony S. Fauci, the government’s top infectious disease expert, said in an interview on Saturday. “Rather than parsing the difference between 94 and 72, accept the fact that now you have three highly effective vaccines. Period.”

If Johnson & Johnson’s vaccine would have been the first to be authorized in the United States instead of the third, “everybody would be doing handstands and back flips and high-fives,” said Dr. James T. McDeavitt, dean of clinical affairs at the Baylor College of Medicine.

On Sunday a committee of vaccine experts who advise the Centers for Disease Control and Prevention will meet to discuss whether certain population groups should be prioritized for the vaccine, guidance that state health officials have been eagerly awaiting in anticipation of the F.D.A.’s authorization.

One administration official familiar with the distribution of the vaccine said that shipments would begin on Monday and deliveries could arrive as soon as Tuesday.

Johnson & Johnson has said it will ship nearly four million doses as soon as the F.D.A. authorizes distribution and another 16 million or so doses by the end of March. That is far fewer than the 37 million doses called for in its $1 billion federal contract, but the contract says that deliveries that are 30 days late will still be considered timely.

The federal government is paying the firm $10 a dose for a total of 100 million doses to be ready by the end of June, substantially less per dose than it agreed to pay Moderna and Pfizer, which developed its vaccine with a German partner, BioNTech.

Johnson & Johnson’s one-dose vaccine will allow states to rapidly increase the number of people who have been fully inoculated. Unlike the other two vaccines, it can be stored at standard refrigeration temperatures for at least three months.

Dr. Danny Avula, the vaccine coordinator for Virginia, said the Johnson & Johnson shipments would boost the state’s allotment of vaccine next week by nearly one-fifth.

“I’m super-pumped about this,” he said. “A hundred percent efficacy against deaths and hospitalizations? That’s all I need to hear.”

He said the state was planning mass vaccination events specifically for the Johnson & Johnson vaccine, partly to quell any suspicion that it is a lesser product targeted to specific groups.

“It will be super clear that this is Johnson & Johnson, here’s what you need to know about it. If you want to do this, you’re coming in with eyes wide open,” he said. “If not, you will keep your place on the list.”

Michele Roberts, the assistant secretary of Washington State’s health department, said that it would be difficult to explain the technical aspects of how Johnson & Johnson’s vaccine trials differed from those of other drug makers. Because the studies were conducted at different times and with different protocols, precise comparisons can be problematic. All three trials showed the vaccines provided strong protection against Covid-19, especially for severe disease.