Share on Pinterest
Tom Williams/CQ-Roll Call, Inc via Getty Images
  • There have been six cases of a serious, potential side effect documented in people given the Johnson & Johnson vaccine in the United States.
  • The potential side effect, a type of blood clot, is extremely rare, as more than 6 million people have had this vaccine so far.
  • The CDC will convene a meeting of the Advisory Committee on Immunization Practices on April 14.
  • Experts say they’re still trying to investigate whether the vaccine actually caused these rare side effects.

Both the Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) have recommended pausing use of the Johnson & Johnson COVID-19 vaccine due to concerns over six reported cases of a “rare” and “severe” type of blood clot.

“Today FDA and @CDCgov issued a statement regarding the Johnson & Johnson #COVID19 vaccine. We are recommending a pause in the use of this vaccine out of an abundance of caution,” the FDA posted to Twitter today.

According to the FDA, the CDC will convene a meeting of the Advisory Committee on Immunization Practices on April 14 to “further review these cases and assess their potential significance. FDA will review that analysis as it also investigates these cases.”

In a joint statement from Dr. Anne Schuchat, principal deputy director of the CDC, and Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, Marks said a type of blood clot called cerebral venous sinus thrombosis was observed in combination with low levels of blood platelets, called thrombocytopenia.

All six cases happened to women between the ages of 18 and 48. Symptoms occurred 6 to 13 days after vaccination.

Marks emphasized that treatment of this specific type of blood clot is different from what might usually be administered.

He said that typically, an anticoagulant drug called heparin would be used to treat blood clots, but in this setting, heparin may be dangerous, and alternative treatments must be given.

According to Dr. Eric Cioe-Peña, director of global health at Northwell Health in New York, these six reported cases represent a very small number of people, as more than 6 million doses have been given.

“This move is to allow an independent data review board to review those cases and evaluate whether the inappropriate clotting could be caused by the vaccine,” he told Healthline.

Cioe-Peña emphasized that “there is no causal link at this time.”

He added that this pause is appropriate given the importance of public confidence in these vaccines under an emergency use authorization (EUA).

“As of now, at least in our health system, patients scheduled for J&J vaccine will be offered Pfizer or Moderna instead, and this will not affect our currently scheduled appointments,” Cioe-Peña said.

Cioe-Peña advised that if you recently received the J&J vaccine, remember that the data right now shows it’s overwhelmingly safe and effective.

“If you develop signs or symptoms of a blood clot within 3 weeks of receiving the vaccine, please contact your doctor or seek medical care,” Cioe-Peña said.

In a press call, Dr. Peter Marks, the director of the Center for Biologics Evaluation and Research at the FDA, said that if people have a severe headache, leg pain, or abdominal pain about a week after vaccination, they should seek care at an emergency room. However, milder symptoms including flu-like symptoms that appear in the days immediatley after the vaccine are not associated with severe side effects.

“It’s always possible (I would not say expected) that very rare side effects like this one will be discovered in postlaunch monitoring,” said Elaine M. Youngman, PhD, assistant professor in the department of biology at Villanova University.

“This particular side effect seems to occur in something like one in a million vaccinated individuals, so it would have been difficult to pick up in even a large clinical trial of 30,000 people,” she said.

Comparatively, the risk of developing a blood clot while using hormonal birth control is about 3 to 15 cases for every 10,000 people, according to some research.

Youngman also said that the fact this side effect was identified — and the quick response that we’re seeing from regulatory agencies — “is really a sign that the monitoring system is working.”

According to Youngman, “we can make a pretty solid guess.”

She explained that a rare side effect that’s clinically “very similar” was found in Europe in response to the AstraZeneca COVID-19 vaccine, which like the J&J vaccine uses an adenoviral vector.

“Given that similarity, it’s likely that these rare cases of blood clotting combined with a low platelet count are caused by a rare immune response to the vaccine,” she said.

“In the case of the [AstraZeneca] vaccine, it’s thought that what happens is that, in very rare cases, a vaccinated person has an inappropriate immune response that generates antibodies to a protein called platelet factor 4,” she continued.

According to Youngman, those antibodies then cause the clotting symptoms, and it will be easy to test whether these clotting cases in the United States have the same cause.

“I’m sure those tests are underway,” she said, adding that the common blood thinner heparin has been known for years to cause similar problems in very rare cases.

“So, the good news is that physicians should be able to use what they know from treating those cases to care for patients who experience symptoms,” Youngman said.

After reports of a very small number of people experienced a rare type of blood clot after getting the Johnson & Johnson COVID-19 vaccine, both the FDA and CDC have paused use of this vaccine pending further investigation.

Experts say this particular side effect seems to occur in about 1 in 1 million vaccinated people, making it difficult to pick up even in a large clinical trial of 30,000 people.

They also emphasize that no causal link has been identified, and the quick response by the FDA and CDC indicate the monitoring system is working.