April 13, 2021 11:56 AM EDT

The U.S. Food and Drug Administration and Centers for Disease Control and Prevention are recommending that vaccinations with the Johnson & Johnson/Janssen COVID-19 vaccine be temporarily halted while the agencies review reports of blood clots among vaccinated people.

On April 13, the two government agencies issued a joint statement announcing a recommended pause as federal regulators review six cases of blood clots reported six to 13 days after the people received the single-dose vaccine. “Until that [review] process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution,” the statement said.

Cities and states across the U.S. moved quickly to follow the federal guidance, although it’s not a mandate to stop using the vaccine. New York state urged people with vaccination appointments who were due to receive the J&J shot to keep those appointments, saying that the state would give them the Pfizer-BioNTech jab instead. Maryland, which just a day ago opened a mass vaccination site relying on J&J doses, advised providers to pause using the shot. Detroit’s health department reassured people that while the city decided to halt vaccinations with J&J-Janssen, it had enough doses of Moderna and Pfizer-BioNTech shots to swap in for people who already have appointments.

Federal health officials stressed that the clotting side effects are extremely rare among vaccinated people—in the U.S., nearly 7 million people have been vaccinated and only six have reported these adverse events.

The CDC’s Advisory Committee on Immunization Practices (ACIP), which makes the actual recommendation for how authorized vaccines should be used and in which age groups, will meet on April 14 to review the cases and issue additional guidance. The J&J-Janssen vaccine received FDA emergency use authorization in February.

What is the concern about the J&J vaccine?

The blood clots linked to the J&J-Janssen vaccine are similar to those reported among people getting vaccinated with the AstraZeneca shot, which is authorized in Europe and elsewhere around the world but not yet in the U.S. In two recent papers detailing the types of clots formed by a small number of people who got the AstraZeneca vaccine, scientists say the cause of the clots may be a rare and unusual immune reaction potentially triggered by the shot. The clots mirror those in a condition called heparin-induced thrombotic thrombocytopenia, in which the the commonly prescribed blood thinner heparin, rather than preventing clots, actually stimulates clot formation. In this scenario, people form vessel-clogging antibodies to the complex of heparin and platelets (which help blood to clot when you get cut or injured), which start to aggregate into blood-flow-blocking clumps. Experts believe a similar immune-based reaction and antibody formation is driving the clots in some vaccinated people.

During a briefing about the decision, Dr. Peter Marks, director of the FDA’s Center for Biological Evaluation and Research, said that a driving factor in the agency’s decision to recommend a pause is to give health care providers time to educate themselves about how best to recognize and treat the unusual clots. Based on the available research, the treatment for vaccinated people who develop clots as a side effect of the vaccine should not include the standard therapy for breaking up clots, heparin, since that could make the clotting worse. Instead, doctors should turn to other blood thinners, and some experts also recommend immune globulin therapy to break up the aggregated immune and platelet cells.

“The issue here is that for these types of blood clots, if one administers the standard treatments that we as doctors learned to give for blood clots, one can actually cause tremendous harm,” Marks said. “The outcome can be fatal.” In the April 13 press briefing, Marks said that of the six patients known to have experienced this side effect, one died and one was in critical care.

Why the clots form isn’t clear yet. There could be a number of factors involved, and investigators are already looking into the way the vaccines are made. Both AstraZeneca and J&J-Janssen’s vaccines rely on adenoviruses, or viruses that cause the common cold. These adenoviruses are deactivated so they can’t cause disease, and then can serve as vehicles for delivering COVID-19 virus genes that can trigger an immune response. “I hesitate to call it a class effect, but it’s plainly obvious to us already that what we are seeing with the Janssen vaccines looks very similar to what we are seeing with the AstraZeneca vaccine,” said Marks. “We can’t make a broad statement yet but obviously they are from the same general class of viral vectors.” AstraZeneca used a weakened adenovirus that usually infects chimpanzees, while J&J-Janssen used one that infects people. The other authorized vaccines in the U.S.—from Pfizer-BioNTech and Moderna—use a different, mRNA technology that more directly introduces genetic material from the COVID-19 virus into the human body.

I got the J&J vaccine. Should I be worried?

Dr. Anne Schuchat, principal deputy director of the CDC, said that people who have received the J&J-Janssen vaccine in recent weeks should look for any signs of the rare side effect, which include headaches, severe abdominal pain, leg pain or shortness of breath up to three weeks after getting the shot. If you do experience any of these symptoms, you should call a medical health professional.

How will this affect the mass-vaccination effort?

After reviewing the reports, the CDC’s ACIP may consider more limited use of the J&J-Janssen shot, as regulators in the U.K. did on April 7 by recommending that people under age 30 not receive it. Most of the clotting side effects are occurring in younger people, and more women than men appear to be affected.

Jeff Zients, White House COVID-19 Response Coordinator, said in a statement that the FDA and CDC recommendation “will not have a significant impact on our vaccination plan: Johnson & Johnson vaccine makes up less than 5% of the recorded shots in arms in the United States to date.” The company had been struggling to ramp up manufacturing since it received authorization, and faced another delay last week when 15 million doses had to be destroyed after a manufacturing facility that it had planned to use experienced cross contamination issues.

Because the J&J-Janssen vaccine is only a single dose, compared to the two doses, spaced about a month apart, required with the Pfizer-BioNTech and Moderna shots, it was welcomed by public health officials who saw it as a safe and efficient way to vaccinate harder to reach populations, not just in lower resource countries around the world but even in U.S. communities where scheduling and keeping two appointments is a challenge. Depending on how long the pause lasts, the latest development could hamper those efforts to reach underserved communities.

Contact us at letters@time.com.

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