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FDA & CDC “pause” J&J Covid-19 vaccine to launch inquiry into rare blood clot cases

Blood clots found in six women given Johnson & Johnson’s Covid-19 vaccine have led federal health officials to recommend a pause on dosing with that shot. A CDC advisory committee on vaccine practices is meeting Wednesday to review the cases, which appear similar to cases observed in Europe with AstraZeneca’s vaccine.


Federal health officials are asking clinicians to “pause” dosing of Johnson & Johnson’s Covid-19 vaccine following reports of blood clots in six people out of the more than 6.8 million people vaccinated with the shot to date.

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While the apparent side effect is rare, the FDA and the Centers for Disease Control and Prevention are asking clinicians to take precautions because heparin, the blood-thinning drug typically used to treat blood clots, can actually harm the patient in these cases, potentially fatally. Blood clots found in vaccinated patients must be addressed with different treatments, the agencies said.

The reported brain blood clots were in women between the ages of 18 and 48. Those clots were identified six to 13 days after vaccination. Of the six cases, one was fatal and one patient remains in critical condition, said Peter Marks, director of the FDA’s Center for Biologics Evaluation. Those U.S. cases mirror the demographic in Europe that regulators there said appear to be at a higher risk of a blood clotting side effect potentially associated with AstraZeneca’s Covid vaccine.

So far, the number of U.S. cases is too few to determine that any particular subgroup of individuals is at risk, the agencies said. But Marks noted that these blood clots happened along with low platelet levels in the blood. While either of those problems can occur independently, health officials were concerned that they happened together.

“It’s their occurrence together that makes a pattern, and that pattern is very, very similar to what was seen in Europe with another vaccine,” he said during a news briefing Tuesday. “So I think we have to take the time to make sure we understand this complication and we address it properly.”

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The CDC’s Advisory Committee on Immunization Practices is scheduled to meet Wednesday to analyze the cases. That analysis will be reviewed by the FDA as part of its own inquiry. Janet Woodcock, acting director of the FDA, said that she expects the review will take “a matter of days.” In the meantime, the FDA is revising the fact sheet for the J&J vaccine to include this adverse event information.

The J&J vaccine and the AstraZeneca vaccine are viral vector viruses, which means that they employ an engineered virus. Both vaccines use adenovirus, the virus that causes the common cold. While the AstraZeneca vaccine uses a version of the virus that infects chimpanzees, J&J’s shot uses one that infects humans. Both are different than the messenger RNA vaccines from Moderna and partners Pfizer and BioNTech, which are delivered via lipid nanoparticles.

Adenovirus is widely used in vaccines, as well as some genetic medicines, to deliver a payload into cells. Though the blood clot side effect in the U.S. cases is similar to what was reported in Europe, Marks stopped short of saying the side effects observed with the vaccine made by J&J subsidiary Janssen is a problem with the adenovirus class.

“I hesitate to call it a class effect,” he said. “But I think it is plainly obvious to us already that what we’re seeing with the Janssen vaccine looks very similar to what was being seen with the AstraZeneca vaccine.”

Woodcock said that one theory is that the blood clots are a rare immune response that occurs in some people after vaccination, sparking activation of platelets or other parts of the blood coagulation system. This rare immune response is also a working theory for the blood clots reported in Europe. Last week, an inquiry by the European Medicines Agency, conducted with the help of an independent group of experts, concluded that blood clots should be listed as a “very rare side effect.” The agency is doing more research and has directed AstraZeneca to do the same. Woodcock said that the FDA has been communicating with the EMA as well as other health regulators worldwide about potential side effects.

The pause that the FDA and CDC called for is a recommendation, not a mandate. Marks said that on an individual basis, a clinician and a patient can consider whether the benefits of the J&J vaccine outweigh its risks. He added that it’s important for patients and physicians to be aware of the signs of blood clots and that any adverse events potentially associated with vaccines should be documented. Other than the typical injection site reactions and flu-like symptoms, signs to look out for in the week to three weeks after injection include severe headache, abdominal pain, leg pain, and shortness of breath. Clinicians can log adverse events on the Vaccine Adverse Events Reporting System.

The U.S. pause on the J&J vaccine has implications overseas. J&J said it has been reviewing the U.S. blood clot cases with European regulators. Considering the FDA’s precautionary measures, the company said Tuesday that it has decided to delay the rollout of its vaccine in Europe.

In a prepared statement, Jeff Zeints, the White House Covid-19 response coordinator, said the U.S. vaccination plan remains on track. The government has secured enough doses of the Pfizer and Moderna vaccines to administer shots to 300 million Americans. In recent weeks, more than 25 million doses of those vaccines have been made available each week, which Zeints said is more than enough supply to continue the vaccination pace of 3 million shots per day. He added that the government is working with state and federal partners to ensure that anyone scheduled for a J&J vaccine can be rescheduled for one from Pfizer or Moderna.

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