Most U.S. infections are now caused by a contagious new virus variant, the C.D.C. says.

Scientists have been predicting another rise in infections, as states relax their public health restrictions and restive Americans go back to school and work. But they are hoping that vaccination will blunt any potential fourth surge.

On Tuesday, President Biden moved up his vaccination timetable by two weeks, calling on states to make every American adult eligible for coronavirus vaccination by April 19, a goal nearly all states have already met or expect to beat after he initially asked they do so by May 1.

“It is in our power to minimize death, disease, and misery,” Andy Slavitt, a White House pandemic adviser, said Wednesday. “If all of us do our part, we can help save lives in April, May, and June. Wear a mask. Socially distance. Get vaccinated when it’s your turn.”

In February, a study that analyzed half a million coronavirus tests and hundreds of genomes predicted that in a month this variant could become predominant in the United States. At that time, the C.D.C. was struggling to sequence the new variants, which made it difficult to track them.

But those efforts have substantially improved in recent weeks and will continue to grow, in large part because of $1.75 billion in funds for genomic sequencing in the stimulus package that President Biden signed into law last month. By contrast, Britain, with a more centralized health care system, began a highly touted sequencing program last year that allowed it to track the spread of the B.1.1.7 variant.

“We knew this was going to happen, this variant is a lot more transmissible, much more infectious than the parent strain and that obviously has implications,” said Dr. Carlos del Rio, a professor of medicine and infectious disease expert at Emory University. In addition to spreading more efficiently, he said, the B.1.1.7 strain appears to cause more severe disease, “so that gives you a double whammy.”

The White House also announced an expansion to its vaccination program at community health centers, bringing the total to nearly 1,400 community centers across the country where people can get vaccinated. Mr. Slavitt said most of these community centers are in underserved neighborhoods with many uninsured patients. Last week, Mr. Biden promised that 90 percent of adults in the country will have a vaccine site within five miles of their home.

Eileen Sullivan contributed reporting.