The current stance of U.S. authorities was described by two government officials who spoke on the condition of anonymity to discuss internal deliberations. They said the position could be affected if there were a sudden flood of reports of blood-clot cases, which appears unlikely, or if other surprises emerged connected to the vaccine.

The fate of the Johnson & Johnson vaccine is scheduled to be discussed publicly in a pivotal meeting Friday of an influential advisory group to the Centers for Disease Control and Prevention.

In separate interviews this week, the heads of the CDC and FDA declined to say whether federal authorities are leaning toward recommending lifting the pause. Acting FDA Commissioner Janet Woodcock said Thursday she didn’t want to “preshadow” the deliberations of the CDC advisory committee. CDC Director Rochelle Walensky said Wednesday, “I don’t want to get ahead” of the panel.

Both, however, offered encouraging news about the incidence of blood clots. Walensky has said the government has received only a “handful” of additional cases. In an interview, she added there are “more that are being adjudicated” and that a final number would be presented Friday. But, she noted, “we are not being inundated with things that we are concerned about. We didn’t have hundreds and thousands of people coming in and saying, ‘Oh wow, I had one of those.’ ”

Woodcock agreed officials have not seen a “huge avalanche” of clot cases. “That’s a great relief,” she said.

The rarity of cases has persuaded many federal officials that the complication can be addressed by adding a warning that describes the groups at higher risk for the adverse event, and by working to ensure that doctors know how to spot and treat the problem. Most notably, physicians are advised to avoid using heparin, a common treatment for blood clots, because it can make the vaccine-related condition worse.

If the CDC advisory committee votes Friday that the vaccine pause should be lifted, the CDC and FDA could recommend the resumption of the shots within hours or days. That outcome would be good news for many state officials eager to begin using the one-dose vaccine again. But if the advisory panel has a different view — and recommends, for example, that the vaccine not be used for certain age groups or not be used at all for now — it is not clear what happens next.

It’s also possible that the committee will have a general discussion about the issues and leave it to FDA and the CDC to decide whether to lift the pause.

The FDA and CDC share responsibilities on vaccines. The FDA makes initial decisions on whether to authorize or approve a vaccine; both agencies collect data on safety. The CDC advisory panel weighs who should get a vaccine, a recommendation that must be approved by Walensky.

“I recognize that the eyes of the country and across the world are on this decision, and the gravity of the decision,” Walensky said in the interview, adding that the CDC and FDA are working closely together. “I want to hear what ACIP has to say, and then all of us are motivated to move quickly thereafter.”

The six cases of blood clots previously identified by officials occurred in women between the ages of 18 and 48. They developed symptoms, most often headaches, six to 13 days after vaccination. One vaccine recipient, a Virginia woman, died in March.

Another, 18-year-old Emma Burkey of Las Vegas, began having seizures several days after receiving the vaccine. She initially was treated in the Las Vegas area, then airlifted to a hospital in Loma Linda, Calif., according to a family spokesman, Bret Johnson. She has had three surgeries to remove blood clots in her brain and is slowly improving, he said. Doctors are “cautiously optimistic” because she is off a ventilator and can blink her eyes and stick out her tongue, he said.

The CDC is working on an analysis requested by the advisory panel that looks at the risks and benefits of the Johnson & Johnson vaccine in the context of the two other authorized shots, one from Pfizer and partner BioNTech and another by Moderna, that are made using a different scientific method, Walensky said.

“In the absence of J&J, was it that you were going to get another vaccine or was it that you were going to get nothing at all? That was some of the risk-benefit analysis that we’ve been doing over the last week,” Walensky said.

During last week’s advisory panel meeting, Doran Fink, an FDA vaccine expert, said the agency believed that the risk of the blood clots could be addressed by including new warning statements in fact sheets that accompany the Johnson & Johnson vaccine, and by coordinated efforts by the FDA, CDC and others to alert health-care providers and vaccine recipients to the potential risk and symptoms.

Some argue that the FDA should have the final say on vaccine-safety questions because of its extensive experience on drug-safety issues.

“The FDA’s expert staff are the right people to collect and analyze the data and figure out how to use the vaccine safely,” said former agency commissioner Scott Gottlieb in a recent opinion column for the Wall Street Journal. Putting the issue before the CDC’s advisory panel confused the process, he said.

But Jason L. Schwartz, an assistant professor of health policy and management at the Yale School of Public Health, disagreed. He said that the CDC and FDA have long been partners on vaccines, and when safety issues emerge, the CDC’s advisory panel typically makes recommendations before the FDA makes changes to the vaccine’s label.

Others noted that the immunization advisory panel’s lengthy public meetings, full of detailed scientific presentations, increase transparency and help shore up public trust in vaccines.

The rare, severe clots that emerged in recent weeks alarmed officials because they were accompanied by low levels of blood cells involved in clotting, a seemingly paradoxical combination virtually unheard of among healthy, young people.

“These are not just run-of-the mill clots,” Walensky said.

Supplies of the Johnson & Johnson vaccine will be limited for the foreseeable future. An estimated 9.2 million doses of the vaccine are available at administration sites, CDC officials said last week. Those doses were not made in the Emergent BioSolutions plant in Baltimore that was the subject of an FDA inspection report issued Wednesday that detailed unsanitary conditions and other problems.

Emergent and Johnson & Johnson did not provide an estimate Wednesday for when issues at the plant, currently shut down, would be corrected, although Emergent said it was working on them.

Before the clotting problem emerged, the Johnson & Johnson single-shot vaccine received a warm welcome from state and local health officials who said its ease of use made it especially suitable for vulnerable communities, such as homebound people or homeless populations who might be unwilling or unable to return for a second shot, which is needed for the Pfizer-BioNTech and Moderna coronavirus vaccines.

Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia, said last week’s advisory panel decision to leave the pause in place probably did more harm than good with about 5,000 people dying every week in the United States from covid-19, the illness caused by the virus.

“By putting a scarlet letter on this vaccine without doing a good job of explaining the risks, you have to ask, did you do harm, and I think the answer is, I think you did. Because we did this, there are people now who will not get a vaccine,” he said.

Offit said he hopes the advisory committee on Friday urges a resumption of use of the vaccine while detailing its risks and benefits. He advised the panel to steer clear of age or gender restrictions.

“There are no risk-free choices. You are more likely to get killed driving to a vaccination site,” Offit said.

Helen Keipp Talbot, an associate professor of medicine at Vanderbilt University and a voting member of the advisory panel and the panel’s vaccine safety subcommittee, said the subcommittee has been reviewing case data this week.

“We’re trying to gather as much information as possible, so we can make informed decisions,” she said, adding that committee members realize “there’s a certain level of anxiety” if a decision is postponed again. “We may have been overly cautious in some people’s eyes.”

This week, the safety group is looking for any prior reports or new ones, “so we can say, what is the risk if you get covid-19, what is the risk if you get vaccines, and which risk is greater and whether it may be different for different age groups,” she said. For an older person, the risk of dying from covid may be far greater than the risk of getting a clot, “but that may not be true in a younger person.”