By Britain Eakin

Law360 (April 13, 2021, 1:46 PM EDT) -- Federal health officials on Tuesday recommended a pause in the administration of Johnson & Johnson's single-dose COVID-19 vaccine pending a review of six U.S. cases of rare and serious blood clots among more than 6.8 million Americans who received the vaccine.

The blood clot condition is called cerebral venous sinus thrombosis and appeared in the six U.S. cases along with low platelets, which ordinarily help with clotting. All six U.S. cases occurred in women ages 18 to 48 within six to 13 days of getting the vaccine, with one death. In a joint statement from Dr. Anne Schuchat, principal deputy director of the U.S. Centers for Disease Control and Prevention, and the CDC and U.S. Food and Drug Administration, the blood clots "appear to be extremely rare."

It remains unknown whether the blood clots are a side effect of the J&J vaccine or are caused by something else. The CDC said an advisory committee will meet Wednesday to review the data and assess its potential significance. The FDA will also investigate the cases, the statement said. Anyone who develops headaches, abdominal or leg pain, or shortness of breath within three weeks of getting the vaccine should contact their health provider, Schuchat said.

"Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution," said the statement. "This is important, in part, to ensure that the health care provider community is aware of the potential for these adverse events and can plan for proper recognition and management due to the unique treatment required with this type of blood clot."

Schuchat said the blood clots can't be treated with the anticoagulant drug heparin, which is the usual treatment for blood clots.

"In this setting, administration of heparin may be dangerous, and alternative treatments need to be given," Schuchat said.

White House COVID-19 response coordinator Jeff Zients said in a statement the pause in the J&J vaccine won't significantly impact the U.S. vaccination plan. Zients said the J&J vaccine makes up only 5% of COVID-19 vaccines administered so far, and the U.S. has enough of the vaccines developed by Pfizer and its German partner BioNTech and the Moderna vaccine to vaccinate 300 million Americans and maintain the current pace of vaccination.

Johnson & Johnson said in a statement Tuesday the company has decided "to proactively delay the rollout" of its vaccine in Europe while European health officials investigate a small number of cases of blood clots among people who got the J&J vaccine there.

The blood clots prompting a pause in administration of the J&J vaccine in the U.S. appear similar to a rare clotting disorder that European health authorities think is possibly linked to AstraZeneca's COVID-19 vaccine, which has not yet been authorized in the U.S.

Both the J&J vaccine and the AstraZeneca vaccine use modified versions of adenoviruses. During a briefing with reporters Tuesday, the FDA director of the Center for Biologics Evaluation and Research, Peter Marks, was pressed about whether there's a "class effect" with this type of COVID-19 vaccine. 

Marks said he was hesitant to call it a class effect, but said "it's plainly obvious to us already that what we're seeing with the Janssen vaccines looks very similar to what was being seen with the AstraZeneca vaccine."

In an April 7 statement, AstraZeneca highlighted the rarity of the potential side effect, stressing that only a handful of cases of blood clots have been reported among the nearly 200 million people around the world who have gotten its vaccine.

--Additional reporting by Adam Lidgett. Editing by Alyssa Miller.

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