Publisher Health

Dr. Anthony S. Fauci said on Thursday that he hoped the nation would soon be able to resume use of the Johnson & Johnson vaccine, as a “pause” that now looks indefinite threatened to upend vaccination efforts overseas and in some of the most marginalized communities in the United States.

Dr. Fauci, President Biden’s chief medical adviser for the pandemic, made the comment during a hearing of the House Select Subcommittee on the Coronavirus Crisis, where he and other top federal health officials implored Americans to continue to get vaccinated and sought to reassure the nation that all three federally authorized vaccines are safe.

“Hopefully we’ll get a decision quite soon, as to whether or not we can get back on track with this very effective vaccine,” Dr. Fauci told the panel. With cases rising in the Midwest, he added, the nation is in a “precarious situation” and it is imperative to vaccinate “as many people as quickly and as expeditiously as we possible can.”

But it was increasingly clear that a suspension that initially was to last two to three days would be considerably longer, as officials wrestled with reports of at least six rare cases of blood clotting among women immunized with the Johnson & Johnson shot. Federal health agencies are trying to decide whether to recommend that doctors resume use of the vaccine, possibly with new restrictions.

The vaccine has not been a major component of the Biden administration’s vaccination campaign. About 7.7 million Americans have received the Johnson & Johnson shot, accounting for fewer than 4 percent of the more than 198 million doses administered across the country.

But while those numbers are small, many of the people the vaccine has been targeted at are vulnerable: homeless people in Baltimore, homebound residents in the District of Columbia, the poor and uninsured in Massachusetts, rural residents in a number of states. All are populations easier to reach with a single dose than the two-dose regimen of the Pfizer and Moderna vaccines.

About 10 million doses shipped to the states are now on shelves awaiting a decision. And many people who might not seek out a shot but could have been reached with mobile clinics and concerted outreach may be left behind, at least for now.

“There is a great deal of work going on with all of our vaccinators, providers around the state and with our team at the Minnesota Department of Health trying to help plan and shifting things around. It is a series of dominoes, obviously,” said Jan Malcolm, Minnesota’s health commissioner. “We very much regret the clinics that had to be canceled on short notice because of the pause in J&J, and that will continue to be an issue in the coming couple of weeks.”

The White House press secretary, Jen Psaki, again calibrated President Biden’s promise that he would secure enough vaccine for all American adults by the end of May. Now, the White House is emphasizing that all adults who want a vaccine will get one.

“That means that by the end of May, for about 80 percent of the population, by the end of July about 90 percent of the population,” Ms. Psaki told reporters.

In Lexington, Ky., a Johnson & Johnson clinic for Friday was canceled, leaving around 400 people without appointments. All were offered a last-minute option to get a Moderna vaccine, but only about 65 accepted the offer, said Kevin Hall, a spokesman for the Lexington-Fayette County Health Department.

“This was a major hit to our planning,” said Mr. Hall, whose agency had already used the Johnson & Johnson vaccine for inmates and homeless people, and had planned to offer neighborhood clinics in the coming weeks. “The logistics of pulling off a first-dose and second-dose clinic become much, much more difficult.”

Dr. Letitia Dzirasa, the health commissioner in Baltimore, said the city had received only about 1,400 doses of the Johnson & Johnson vaccine to date, but many were targeted at homeless residents. The city may have to rethink plans it had been making for outdoor pop-up clinics and efforts to reach homebound residents with the one-dose vaccine this summer, she said.

“We were thinking the J&J would be the ideal candidate to support that work,” she said.

In Washington, D.C., officials are delaying a program to reach homebound residents with Johnson & Johnson shots. The city canceled appointments for about 1,200 people this week, but by now all should have received an invitation for a new appointment for one of the other two federally authorized vaccines, developed by Pfizer-BioNTech and Moderna.

Other state and local health officials said they were getting by with those two vaccines. In the Flint, Mich., area, which has some of the country’s highest recent case rates, officials said they had been able to cover all planned clinics even without the Johnson & Johnson vaccine.

“It has been smooth so far,” Dr. Pamela Hackert, the medical health officer for the Genesee County Health Department, said in an email.

At Des Moines Area Community College in Iowa, administrators have been able to set up three on-campus clinics this month where students will be able to receive a two-dose vaccine. Rob Denson, the college president, said he had been pleased and surprised at his ability to arrange those future clinics so quickly.

“I think we’re going to be awash in vaccine within a relatively short amount of time,” he said.

But an extended break in Johnson & Johnson availability will begin to pinch, especially in poorer states with harder-to-reach populations. A spokeswoman for Dr. José Romero, the secretary of health in Arkansas, said that “the pause should be a sufficient length to answer safety questions, but not extended any longer than necessary.

“His concern is that an overlong pause will increase hesitancy and decrease confidence,” said the spokeswoman, Danyelle McNeill. Dr. Romero leads the advisory committee to the Centers for Disease Control and Prevention that will ultimately recommend how to proceed with the Johnson & Johnson vaccine.

The suspension in the United States may have more profound consequences overseas, where only a fraction of the rest of the world has so far been vaccinated. Dr. Luciana Borio, a former acting chief scientist at the Food and Drug Administration who also served on the National Security Council in the Trump White House, said the Johnson & Johnson vaccine was a crucial tool for stopping the spread of the virus around the world.

“It’s a vaccine that can be manufactured quickly in very large scale and has much easier distribution procedures,” she said. “The world needs more companies like J&J supplying their vaccine.”

Officials were counting on both Johnson & Johnson and another easy-to-distribute vaccine made by AstraZeneca to get inoculations to hard-to-reach parts of the globe. But recent reports of rare blood clots in recipients of the AstraZeneca vaccine have led a number of nations to reconsider its use.

Some regions decided to shift to Johnson & Johnson; two weeks ago, the African Union acquired 400 million doses even as wealthier nations have shied away. The European Union said it would not buy more from Johnson & Johnson, and Australia announced it would not purchase any doses.

In the United States, members of the C.D.C. advisory panel suggested on Wednesday that it would be a week to 10 days before they had enough information to assess the vaccine’s risks and could make a decision about its future in the United States. Dr. Fauci and two other officials — Dr. Rochelle Walensky, the C.D.C. director, and Dr. David Kessler, who runs the Biden administration’s vaccine effort — urged Americans to continue to get vaccinated.

“I hope we can all come together and send that message,” especially amid the spread of worrisome variants, Dr. Kessler said, adding that the three federally authorized vaccines have “an excellent safety profile.”

The reports of blood clots were the second recent blow to the Johnson & Johnson vaccine. Earlier this month, an ingredient mix-up at a Baltimore manufacturing plant owned by Emergent BioSolutions ruined up to 15 million doses of the vaccine. The F.D.A. is now inspecting the plant to see whether any doses manufactured there can be released to the public.

Yet during a hearing that lasted more than two hours, just one lawmaker — Representative Mark E. Green, Republican of Tennessee, who is a doctor — asked about the Johnson & Johnson vaccine. He urged the doctors to be careful when they talked about the investigation, saying he worried that they would stoke fears that would discourage people from getting vaccinated.

And there is early evidence that the concern is well taken. A Survey Monkey poll released on Thursday by Boston Children’s Hospital found that a willingness to get the Johnson & Johnson shot dropped 26 percentage points in two days among Americans who wanted to get vaccinated. The drop among women who wanted to get vaccinated was 31 percentage points, according to the survey, a project from the hospital’s Outbreaks Near Me initiative, a crowdsourced infectious disease surveillance system.

“Anytime there is a concern or red flag raised at the federal level, that’s going to spark concern for the general public,” Dr. Dzirasa said.

Thursday’s session was subtitled “A Science-Driven Approach to Swiftly and Safely Ending the Pandemic.” But whatever science was discussed was overshadowed by partisan posturing and bickering.

Democrats worried about the Fox News host Tucker Carlson, who has used his show to wrongly assert that the vaccines do not work.

Republicans, fresh off a trip to the nation’s southern border, used the session to attack the Biden administration’s handling of the immigration crisis. They waved photographs of migrants living in crowded conditions, while complaining about testing rules for those entering the country.

Representative Jim Jordan, Republican of Ohio, and Dr. Fauci reprised their heated clash from the last time Dr. Fauci testified before the panel in July.

“You’re ranting again,” Dr. Fauci said at one point.

“I’m not ranting,” Mr. Jordan replied.

“Yes, you are,” Dr. Fauci insisted.

Representative Maxine Waters, Democrat of California, whose sister died of Covid-19, told Dr. Fauci, “I love you,” and told Mr. Jordan, “Shut your mouth.”

Noah Weiland, Rebecca Robbins and Sharon LaFraniere contributed reporting.