WASHINGTON, D.C. — Federal health officials said early Tuesday they were urging a pause in the use of Johnson & Johnson’s COVID-19 vaccine after reports of six serious blood clots, and officials in Los Angeles and around the country quickly complied.

The joint announcement from the Centers for Disease Control and Prevention and the Food and Drug Administration is a blow to the U.S. vaccination campaign, which has counted on public faith in the rapidly developed inoculations and growing supplies from multiple manufacturers to protect Americans from the coronavirus and bring an end to the pandemic.

Officials said the pause may last only a few days, depending on what they learn in their review of medical data, and that it’s possible they will adjust their guidance on the best candidates to receive the Johnson & Johnson vaccine — for example, by age group. The CDC has scheduled an emergency meeting for Wednesday, and officials promised to move quickly.

“Out of an abundance in caution, we’re recommending a pause in the use of the Johnson & Johnson COVID-19 vaccine,” said Janet Woodcock, the acting FDA commissioner, during a briefing. “We’re recommending this pause while we work together to fully understand these events and also so we can get information out to healthcare providers and vaccine recipients.”

She emphasized that reports of serious blood clots are “extremely rare,” but “COVID-19 vaccine safety is a top priority for the federal government, and we take all reports of adverse events after vaccination very seriously.”

The fallout rippled across the country as state and local governments scrambled to respond to the announcement. Along with Washington state, health officials in New York, Virginia, Maryland and Washington, D.C., said they would pause administering Johnson & Johnson shots. Some appointments are being canceled and rescheduled; others will be fulfilled with vaccines produced by other companies. Federal vaccination sites will also stop providing Johnson & Johnson shots.

New York City Councilman Mark Levine, who chairs its health committee, tweeted that officials now faced “the biggest messaging challenge yet.” He added, “We have to do everything possible to avoid a collapse in confidence in vaccination overall.”

Jeff Zients, who leads the White House COVID-19 task force, downplayed the effect of the announcement by saying Johnson & Johnson vaccines made up less than 5% of shots administered so far. Officials said they have no concerns about blood clots from the Pfizer and Moderna vaccines, which use a different approach to protect against the coronavirus.

Zeints said the two companies were producing enough to meet President Joe Biden’s goal of administering 200 million shots by his 100th day in office, which is April 30.

“This announcement will not have a significant impact on our vaccination plan,” he said in a statement.

Anne Schuchat, principal deputy director of the CDC, said people should continue to get inoculated with the Pfizer or Moderna vaccines when an appointment is available, although she admitted that the process “may be a bit bumpy” as some appointments are rescheduled.

The officials in Tuesday’s briefing emphasized that reports of serious blood clots are extremely rare. They said people who received a Johnson & Johnson vaccine in recent weeks should contact their healthcare provider if they experience severe headaches, abdominal pain, leg pain or shortness of breath.

Unlike the Pfizer and Moderna vaccines, the Johnson & Johnson version requires only one shot and does not need to be stored at extremely cold temperatures, making officials optimistic that it could be administered and transported more easily. Roughly 7 million Americans have already received a dose of the Johnson & Johnson vaccine.

The New Jersey-based company, however, has been battling perceptions that its vaccine is less effective than others, as well as the revelation that millions of doses produced by a Baltimore factory had to be thrown out because of quality problems. The latest announcement will likely create an even greater cloud at a time when health officials have been struggling to persuade hesitant Americans to get the first shot available to them.

Johnson & Johnson said it was aware of the reports of blood clots and was working with officials on the matter.

“At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine,” the company said in a statement, referring to the division that developed the vaccine. “We continue to work closely with experts and regulators to assess the data and support the open communication of this information to healthcare professionals and the public.”

Federal health officials said six women between the ages of 18 and 48 developed a “rare and severe” blood clot called cerebral venous sinus thrombosis six to 13 days after receiving a Johnson & Johnson vaccine. One woman died and another is in critical condition.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research said this type of blood clot is more difficult to address because the usual treatment — an anticoagulant drug called heparin — can actually be dangerous in these circumstances.

Gigi Gronvall, a senior scholar of immunology at the Johns Hopkins Center for Health Security, described the blood clots as “literally a one-in-a-million adverse event” given how many Johnson & Johnson shots had been administered so far. She said the FDA was “a very conservative organization” and people should have confidence that officials were carefully scrutinizing the vaccines.

“There will be more of an effort to help people understand all of the complexities involved here,” she said. “But it’s an important job, and we’ll just have to do it.”

Scattered cases of such clots have also arisen in Europe among recipients of the AstraZeneca COVID-19 vaccine, leading several countries to restrict use of that shot to older people.

In Britain, residents younger than 30 will be given the choice of other vaccines as a precautionary measure. The European Union’s drugs regulator, however, has ruled that the benefits of the AstraZeneca vaccine, as a generally safe and highly effective guard against COVID-19, outweigh the risks. It has not recommended any age restrictions for recipients.

AstraZeneca says it intends to apply for emergency-use authorization of its vaccine in the U.S.