WASHINGTON (AP) — The U.S. on Tuesday recommended a “pause” in using the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of rare but potentially dangerous blood clots, a development that could jeopardize the rollout of vaccines around the world.

The Centers for Disease Control and Prevention and the Food and Drug Administration announced that they were investigating unusual clots that occurred 6 to 13 days after vaccination. The FDA commissioner said she expected the pause to last a matter of days.

The clots occurred in veins that drain blood from the brain and occurred together with low platelets, the fragments in blood that normally form clots. All six cases were in women between the ages of 18 and 48. One person died, and all of the cases remain under investigation.

More than 6.8 million doses of the J&J vaccine have been given in the U.S., the vast majority with no or mild side effects.

Any slowdown in the dissemination of the shots could have broad implications for the global vaccination effort. The J&J vaccine held particular promise for less affluent countries because its single-dose regimen and relatively simple storage requirements would make it easier to use in the developing world.

The FDA said the cases under investigation appear similar to unusual clots that European authorities say are possibly linked to another COVID-19 vaccine not yet cleared in the U.S., from AstraZeneca. European regulators have stressed that the AstraZeneca risk appears to be lower than the possibility of developing clots from birth control pills.

Federally run mass vaccination sites will pause the use of the J&J shot, and states and other providers are expected to follow. The other two authorized vaccines, from Moderna and Pfizer, make up the vast share of COVID-19 shots administered in the U.S. and are not affected by the pause.

“I’d like to stress these events appear to be extremely rare. However COVID-19 vaccine safety is a top priority,” acting FDA Commissioner Janet Woodcock said at a news conference.

A CDC committee will meet Wednesday to discuss the cases, and the FDA has also launched an investigation into the cause of the clots and low platelet counts.

Authorities have not seen similar clots after use of the Pfizer or Moderna vaccines, the CDC’s Dr. Anne Schuchat said.

FDA officials emphasized that Tuesday’s action was not a mandate. Doctors and patients could still use J&J’s vaccine if they decide its benefits outweigh its risks for individual cases, said Dr. Peter Marks.

The agencies recommend that people who were given the J&J vaccine should contact their doctor is they experience severe headache, abdominal pain, leg pain, or shortness of breath within three weeks.

J&J said in a statement that it was aware of the reports of blood clots, but that no link to its vaccine had been established. The company also said it would delay the rollout of its vaccine in Europe as a precaution.

U.S. health authorities cautioned doctors against using a typical clot treatment, the blood-thinner heparin. “In this setting, administration of heparin may be dangerous and alternative treatments need to be given,” the FDA and CDC said.

European authorities investigating the AstraZeneca cases have concluded clots appear to be similar to a very rare abnormal immune response that sometimes strikes people treated with heparin, leading to a temporary clotting disorder.

While it's not clear yet if the reports among J&J recipients are related, doctors would treat these kinds of unusual clots like they treat people who have the heparin reaction — with different kinds of blood thinners and sometimes an antibody infusion, said Dr. Geoffrey Barnes, a clot expert at the University of Michigan.

As authorities investigate whether the clots really are related to the J&J vaccine, Barnes stressed that Americans should get vaccinated as soon as possible using the other two available vaccines, from Pfizer and Moderna.

“If you have a chance to get vaccinated with those, we strongly encourage it. The risks of COVID are real and they’re high,” Barnes said.

Jeff Zients, the White House COVID-19 response coordinator, said 28 million doses of the Pfizer and Moderna vaccines will be available for states this week, more than enough to keep up the nation's pace of 3 million shots a day despite the J&J pause.

Asked if the government was overreacting to six cases out of more than 6 million vaccinations, Schuchat said recommendations will come quickly.

Because these unusual clots require special treatment, “it was of the utmost importance to us to get the word out,” she said. “That said, the pandemic is quite severe and cases are increasing in lots of places and vaccination’s critical.”

States and cities swiftly moved to implement the pause. New York state Health Commissioner Dr. Howard Zucker said people with Tuesday appointments for J&J vaccines at state-run mass vaccination clinics will instead get the Pfizer vaccine.

The city of Dallas had planned to begin an in-home vaccination program using the J&J vaccine for homebound or elderly people. The city said it will pause the program until more guidance is released.

The J&J vaccine received emergency use authorization from the FDA in late February with great fanfare. Yet the shot only makes up a small fraction of the doses administered in the U.S. J&J has been plagued by production delays and manufacturing errors at the Baltimore plant of a contractor.

Last week, the drugmaker took over the facility to scale up production in hopes of meeting its commitment to the U.S. government of providing about 100 million doses by the end of May.

Only about 9 million of the company's doses have been delivered to states and are awaiting administration, according to CDC data.

Until now concern about the unusual blood clots has centered on the vaccine from AstraZeneca, which has not yet received authorization in the U.S. Last week, European regulators said they found a possible link between the shots and a very rare type of blood clot that occurs together with low blood platelets, one that seems to occur more in younger people.

The European Medicines Agency stressed that the benefits of receiving the vaccine outweigh the risks for most people. But several countries have imposed limits on who can receive the vaccine; Britain recommended that people under 30 be offered alternatives.

But the J&J and AstraZeneca vaccines are made with the same technology. Leading COVID-19 vaccines train the body to recognize the spike protein that coats the outer surface of the coronavirus. But the J&J and AstraZeneca vaccines use a cold virus, called an adenovirus, to carry the spike gene into the body. J&J uses a human adenovirus to create its vaccine while AstraZeneca uses a chimpanzee version.

U.S. stock markets initially dropped on the J&J news, but some indices were up slightly by late morning. Johnson & Johnson shares were down nearly 3 percent, an unusually big drop for the drug giant, with more shares changing hands in the first two hours than on an average day.

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Associated Press writers Emily Wagster Pettus, Karen Matthews, Jill Bleed and Linda A. Johnson contributed to this report.