Although 100 million people in the U.S. are fully vaccinated, concern grows over reaching those who haven’t gotten shots.

Federal health officials said Friday that more than 100 million people in the United States had been fully vaccinated against the coronavirus, a milestone that represents almost 40 percent of the nation’s adults. But concern continues to build among health officials over reaching more people who have not received shots.

The 100 million mark is almost double what the nation had registered at the end of March. The federal government also shipped its 300 millionth dose this week, according to Jeffrey D. Zients, the White House’s Covid-19 response coordinator.

The Biden administration is beginning a critical stage of its vaccination campaign that requires finding ways to inoculate those who remain hesitant or skeptical, or have had trouble accessing a vaccine. Officials repeatedly emphasized at a White House news conference on Friday that the “next phase,” as Mr. Zients referred to it, called for targeted, local and personalized efforts. President Biden recently called on employers to give workers paid time off to get vaccinated.

Polls have shown resistance to the vaccine is more deeply rooted among white people who live in rural areas, especially those who vote Republican, or describe themselves as evangelical Christians. Campaigns aimed at Black and Latino communities have made striking gains, though public health experts have said obstacles to access deserve much of the blame for disparities seen in their vaccination rates.

As of Friday, providers were administering about 2.55 million doses per day on average, about a 25 percent decrease from the peak of 3.38 million reported on April 13, according to data from the Centers for Disease Control and Prevention. And some regions of the country — particularly the South — were lagging well behind others.