CHICAGO — The student union had been converted into a vaccination center. The doses had arrived on campus. The first appointments were minutes away. Then, at 7:23 a.m. on Tuesday, news of the pause in Johnson & Johnson vaccinations reached Youngstown State University.

“We were ready to go,” said Shannon Tirone, an associate vice president at the university, in eastern Ohio, who instead started calling students to tell them they would not be able to get the vaccine after all.

Similar scenes played out across the country as the abrupt halt in the use of the Johnson & Johnson vaccine because of concerns about potential blood clots upended plans to vaccinate some of the country’s hardest-to-reach populations.

In California, mobile vaccine clinics in rural areas were canceled. In Chicago, vaccination events for restaurant employees and aviation workers were postponed indefinitely. And at colleges in Ohio, New York and Tennessee, where the one-dose vaccine offered a chance to quickly inoculate students before they left campus for the summer, appointments were called off en masse.

“It really fit into this kind of tight deadline we were doing,” Ms. Tirone said of the Johnson & Johnson vaccine. She said she hoped to offer another on-campus clinic in Youngstown with one of the two-dose alternatives, but she worried that students would not be eligible for a second dose until finals week or later.

At the White House, Biden administration officials played down the effect of the Johnson & Johnson pause and pledged to help states with the logistics involved in rescheduling patients to receive shots from Pfizer and Moderna, the two other coronavirus vaccine manufacturers authorized by the Food and Drug Administration.

Federal health officials said the Johnson & Johnson vaccine could be returned to use in days after a review by the Food and Drug Administration and the Centers for Disease Control and Prevention.

“The most important thing is that the supply exists to continue to vaccinate three million Americans a day, and there’s enough supply to actually accelerate that,” Jeffrey D. Zients, the Covid response coordinator at the White House, told reporters on Tuesday.

In much of the country, public health officials said they were able to offer other vaccines to people who had been scheduled to receive a Johnson & Johnson shot.

The Albany County Health Department in New York said it would provide Pfizer doses for a Johnson & Johnson clinic on Tuesday at a local university. The chief public health officer in Detroit said people who had appointments for a Johnson & Johnson vaccine at a city-run site would be allowed to keep their times and receive a Pfizer or Moderna shot. And officials in New Hampshire, who had planned to use the Johnson & Johnson vaccine on Tuesday at clinics and for homebound patients, said they were working to find Pfizer or Moderna doses to use instead.

“This news will not slow down New Hampshire,” Gov. Chris Sununu said in a statement. “While the federal government has directed a brief pause in the J&J vaccine, the state is already working with our partners to ensure that they have an alternative supply of Pfizer or Moderna to help continue their efforts today.”

But in some places, there was no immediate alternative. In Aurora, Ill., a mass vaccination clinic planned for Tuesday was called off, leaving 1,000 patients without appointments. In Riverside County, Calif., mobile clinics that had planned to vaccinate about 400 people in less populous areas on Tuesday were canceled. And in rural Jefferson County, in southeast Iowa, a Johnson & Johnson clinic targeting manufacturing workers was scrapped at the last minute.

“It was so heartbreaking to me,” said Christine Estle, the county’s public health nurse administrator, who said she and her colleagues had encouraged the roughly 140 people scheduled to attend to make appointments at local pharmacies or hospitals.

The Johnson & Johnson vaccine had long been seen as a key to the country’s vaccination effort because it requires only one shot, unlike the two-dose Moderna and Pfizer regimens, and because it can be stored more easily. In cities around the country, public health experts had begun using the vaccine in places where hesitancy about one shot — much less two — is high.