Injections of Johnson & Johnson’s vaccine was suddenly halted across the United States due to the emergence of a rare blood clotting.

Iran Press/America: Last week, British regulators and the European Union’s medical agency said they had established a possible link between AstraZeneca’s Covid-19 vaccine and very rare, though sometimes fatal, blood clots.

On Tuesday, Johnson & Johnson said it would pause the rollout of its vaccine in Europe and the United States over similar concerns.

Regulators have asked vaccine recipients and doctors to be on the lookout for certain symptoms, including severe and persistent headaches and tiny blood spots under the skin. Doctors’ groups have circulated guidance about how to treat the disorder.

The troubles with two major vaccines are casting a cloud over the European Union’s vaccine rollout just as it has finally begun to gain momentum after months of short supplies and logistical problems.

According to a YouGov poll published last month, 61 percent of the French, 55 percent of Germans and 52 percent of Spaniards consider the AstraZeneca vaccine “unsafe.” That is in stark contrast to the findings of a similar poll from February, when more people in those countries, with the exception of France, believed that the shot was more safe than unsafe.

Almost everywhere across the European Union, it seems, many are eager for alternatives, as the new types of vaccines that include the Moderna and Pfizer, which utilize science known as “mRNA,” have not been associated with similar side effects.

Although all E.U. countries have been offered a chunk of each vaccine approved in the bloc so far — AstraZeneca, Johnson & Johnson, Moderna and Pfizer — many opted to forgo parts of their share of more expensive or cumbersome vaccines like Pfizer and Moderna early on, instead favoring the AstraZeneca jab.

“In Britain or Eastern Europe, a big part of the campaigns are based on AstraZeneca,” said Yves Van Laethem, a top epidemiologist who is Belgium’s Covid task force spokesman.

Wealthier bloc members like Denmark, France, Germany and the Netherlands can better compensate for the loss of confidence in AstraZeneca, because they acquired extra doses of other vaccines — especially Pfizer — through a secondary market after poorer E.U. nations gave theirs up.

Those countries — including Bulgaria, Croatia, Latvia and Slovakia — are likely to be less able to quickly offer alternatives.

Injections of Johnson & Johnson's single-dose coronavirus vaccine came to a sudden halt in much of the country on Tuesday after federal health agencies called for a pause in the vaccine’s use after discovering a rare blood clotting disorder in six recipients.

All six were women between the ages of 18 and 48 and all developed the illness within one to three weeks after being vaccinated. One woman died and a second woman in Nebraska has been hospitalized in critical condition.

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

“We are recommending a pause in the use of this vaccine out of an abundance of caution,” Dr. Peter Marks, the director of the Food and Drug Administration’s Center for Biologics Evaluation and Research, and Dr. Anne Schuchat, principal deputy director of the C.D.C., said in a joint statement. “Right now, these adverse events appear to be extremely rare.”

While officials framed the move as a recommendation to health practitioners, the impact was immediate. By Tuesday evening, all 50 states, Puerto Rico and Washington D.C. announced a pause in administration of the vaccine. Ohio’s governor, Mike DeWine, announced that eight mass vaccination sites across his state would shut down for the week.

The federal government temporarily halted administration of the shots by the U.S. military, federally-run sites and CVS and Walgreens, two pharmacy giants that participate in a federal vaccination program, officials said.

In a statement, Johnson & Johnson said it supported “open communication” with health care professionals and the public and had been working closely with medical experts and health authorities. The company said that it was pausing vaccinations in its clinical trials. It is now testing the effectiveness of two doses, as opposed to the single dose that was authorized in February.

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 modify the authorization, possibly limiting it to certain population groups. An emergency meeting of the C.D.C.’s outside vaccine advisory committee has been scheduled for Wednesday.

What does this mean for people who have recently received the Johnson & Johnson vaccine?

Fewer than one in a million J&J vaccinations are now under investigation. If there is indeed a risk of blood clots from the vaccine — which has yet to be determined — that risk is extremely low. The risk of getting Covid-19 in the United States is far higher.

The F.D.A. recommends that people who have received the Johnson & Johnson vaccine within the past three weeks should contact their doctors if they experience severe headaches, abdominal pain, leg pain or shortness of breath.

What is the problem that led to the pause?

Six women in the United States who received the Johnson & Johnson vaccine developed a rare disorder involving blood clots within about two weeks of vaccination. In the condition, called cerebral venous sinus thrombosis, clots form in veins that drain blood from the brain.

Researchers studying a very similar disorder in AstraZeneca recipients in Europe say it appears to be caused by an intense reaction to the vaccine by the immune system, which generates antibodies that activate platelets, a blood component that helps form normal clots to repair wounds. In addition to clots, abnormal bleeding occurs.

Dr. Peter Marks of the F.D.A. said it was the unusual combination of clotting and bleeding that set up a red flag for regulators as a possible safety signal.

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