U.S. health officials paused use and distribution of Johnson & Johnson COVID-19 vaccines on Tuesday to investigate reports of potentially dangerous blood clots.
In a joint statement, the Centers for Disease Control and Prevention and the Food and Drug Administration said they were investigating unusual clots that occurred 6 to 13 days after vaccination. The clots occurred in veins that drain blood from the brain and occurred together with low platelets. All six cases were in women between the ages of 18 and 48; there was one death.
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Here are key takeaways about the vaccine investigation:
How long will J&J shots be on hold?
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,” FDA Acting Commissioner Janet Woodcock said at a news conference. “We expect it to be a matter of days for this pause.”
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.
CDC’s Dr. Anne Schuchat said authorities have not seen similar clots after use of the Pfizer or Moderna vaccines, and that people should continue to get vaccinated with those shots.
The agencies are recommending that people who were given the J&J vaccine who are experiencing severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after receiving the shot contact their health care provider.
J&J said in a statement it was aware of the reports of blood clots, but that no link to its vaccine had been established. The company also said it is delaying the rollout of its vaccine in Europe as a precaution.
What is the blood clot disorder?
The reports appear similar to a rare, unusual type of clotting disorder that European authorities say is possibly linked to another COVID-19 vaccine not yet cleared in the U.S., from AstraZeneca.
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. 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.
How common are side effects?
More than 6.8 million doses of the J&J vaccine have been administered in the U.S., the vast majority with no or mild side effects.
As authorities investigate whether the clots really are related to the J&J vaccine, Barnes stressed that it’s important Americans 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.
Will this impact vaccination efforts in the U.S.?
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.