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Noah Weiland, Sharon LaFraniere and Carl Zimmer

WASHINGTON — Federal health agencies on Tuesday called for an immediate pause in use of Johnson & Johnson’s single-dose coronavirus vaccine after six recipients in the United States developed a rare disorder involving blood clots within about two weeks of vaccination.

All six recipients were women between the ages of 18 and 48. One woman died and a second woman in Nebraska has been hospitalised in critical condition, officials briefed on the cases said.

Nearly seven million people in the United States have received Johnson & Johnson shots so far, and roughly nine million more doses have been shipped out to the states, according to data from the Centers for Disease Control and Prevention.

“Today FDA and @CDCgov issued a statement regarding the Johnson & Johnson #COVID19 vaccine. We are recommending a pause in the use of this vaccine out of an abundance of caution,” the Food and Drug Administration announced on Twitter at 7 a.m. A news conference is scheduled for 10 a.m.

While the move was framed as a recommendation to health practitioners in the states, the federal government is expected to pause administration of the vaccine at all federally run vaccination sites. Federal officials expect that state health officials will take that as a strong signal to do the same.

Federal officials are concerned that doctors may not be trained to look for the rare disorder if recipients of the vaccine develop symptoms of it. The F.D.A. said Tuesday morning that “treatment of this specific type of blood clot is different from the treatment that might typically be administered.”

Scientists with the F.D.A. and C.D.C. will jointly examine possible links between the vaccine and the disorder and determine whether the F.D.A. should continue to authorize use of the vaccine for all adults or limit the authorization. An emergency meeting of the C.D.C.’s outside advisory committee has been scheduled for Wednesday, officials said.

The move could substantially complicate the nation’s vaccination efforts at a time when many states are confronting a surge in new cases and seeking to address vaccine hesitancy. Regulators in Europe and elsewhere are concerned about a similar issue with another coronavirus vaccine, developed by AstraZeneca and Oxford University researchers. That concern has driven up some resistance to all vaccines, even though the AstraZeneca version has not been authorized for emergency use in the United States.The vast majority of the nation’s vaccine supply comes from two other manufacturers, Pfizer-BioNTech and Moderna, which together deliver more than 23 million doses a week of their two-shot vaccines. There have been no significant safety concerns about either of those vaccines.

But while shipments of the Johnson & Johnson vaccine have been much more limited, the Biden administration had still been counting on using hundreds of thousands of doses every week. In addition to requiring only a single dose, the vaccine is easier to ship and store than the other two, which must be stored at extremely low temperatures.

It is unclear to what degree the pause in the use of the Johnson & Johnson vaccine will set back the Biden administration’s plans to deliver enough vaccine to be able to inoculate all adults in the United States by the end of May.

In the United States alone, 300,000 to 600,000 people a year develop blood clots, according to C.D.C. data. But the particular blood clotting disorder that the vaccine recipients developed, known as cerebral venous sinus thrombosis, is extremely rare. All of the women developed the condition between six and 16 days of vaccination, and government experts are concerned that an immune system response triggered by the vaccine was the cause.

The decision is a fresh blow both to Johnson & Johnson and to the administration’s plans. Late last month, the company discovered that workers at a Baltimore plant run by its subcontractor had accidentally contaminated a batch of vaccine, forcing the firm to throw out the equivalent of 13 million to 15 million doses. That plant was supposed to take over supply of the vaccine to the United States from Johnson & Johnson’s Dutch plants, which were certified by federal regulators earlier this year.

The Baltimore plant’s certification by the F.D.A. has now been delayed while inspectors investigate quality control issues, sharply reducing the supply of Johnson & Johnson vaccine. The sudden drop in available doses led to widespread complaints from governors and state health officials who had been expecting much bigger shipments of Johnson & Johnson’s vaccine this week than they got.

States have been using the vaccine in a broad range of settings, including at mass vaccination sites and on college campuses. The vaccine’s one-shot approach has proved popular, and officials have directed it to transient, rural and isolated communities where following up with a second dose is more complicated.

It is common for regulators to investigate “safety signals” in new vaccines and other medical products. Very often, the signals prove not to be of concern. But the concerns about Johnson & Johnson’s vaccine mirror concerns about AstraZeneca’s, which European regulators began investigating last month after some recipients developed blood clots.

Out of 34 million people who received the vaccine in Britain, the European Union and three other countries, 222 experienced blood clots that were linked with a low level of platelets. The majority of these cases occurred within the first 14 days following vaccination, mostly in women under 60 years of age.

On April 7, the European Medicines Agency, the main regulatory agency, concluded that the disorder was a very rare side effect of the vaccine. Researchers in Germany and Norway published studies on April 9 suggesting that in very rare cases, the AstraZeneca vaccine caused people to make antibodies that activated their own platelets.

Nevertheless, the regulators argued, the benefit of the vaccine — keeping people from being infected with the coronavirus or keeping those few who get Covid-19 out of the hospital — vastly outweighed that small risk. Countries in Europe and elsewhere continued to give the vaccine to older people, who face a high risk of severe disease and death from Covid-19, while restricting it in younger people.

Both AstraZeneca and Johnson & Johnson use the same platform for their vaccine, a virus known as an adenovirus. On Tuesday, the Australian government announced it would not purchase Johnson & Johnson vaccines. They cited Johnson & Johnson’s use of an adenovirus. But there is no obvious reason adenovirus-based vaccines in particular would cause rare blood clots associated with low platelet levels.

AstraZeneca has not yet applied for an emergency use authorization in the United States.

The Moderna and Pfizer-BioNTech vaccines use a different technology to produce immunity.

The first sign of concern about Johnson & Johnson’s vaccine came on April 9, when the European Medicines Agency announced that it was investigating reports of four cases of blood clots in people who received the Johnson & Johnson vaccine in the United States. One case occurred in the clinical trial that took place before the vaccine was authorized. Three occurred in the vaccine rollout. One of them was fatal, the agency said.- The New York Times