The U.S. is recommending a "pause" in the use of the single-dose Johnson & Johnson COVID-19 vaccine after six recipients developed rare and possibly dangerous blood clots.
In a joint statement on Tuesday, the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) said it was investigating six reported cases of a "rare and severe" type of blood clot in six women who received the J&J vaccine.
The vast majority of the U.S. vaccine supply comes from Pfizer/BioNTech and Moderna—both two-shot vaccines. But more than 6.85 million shots of the J&J vaccine have been administered in the U.S., according to CDC data.
The Biden administration has said any pause in the use of J&J shots "will not have a significant impact on our vaccination plan" because of the availability of other vaccines.
Officials said the type of blood clot, identified as cerebral venous sinus thrombosis (CVST), was seen in the six women between 6 and 13 days after vaccination, in combination with reduced platelet counts.
All six of the cases were women between the ages of 18 and 48, officials said.
The New York Times reported that one woman died and a second woman in Nebraska has been hospitalized in a critical condition.
Dr. Anne Schuchat, Principal Deputy Director of the CDC and Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, released a joint statement on Tuesday morning.
The statement said that "right now, these adverse events appear to be extremely rare."
"Treatment of this specific type of blood clot is different from the treatment that might typically be administered."
Usually, an anticoagulant drug called heparin is used. "In this setting, administration of heparin may be dangerous, and alternative treatments need to be given," they said.
The CDC will convene a meeting of its Advisory Committee on Immunization Practices on Wednesday, the statement said, in order "to further review these cases and assess their potential significance."
The FDA will review that analysis as it also investigate the cases.
"Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution," the statement added.
"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 due to the unique treatment required with this type of blood clot."
It added that people who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks of vaccination should contact their health care provider.
Responding to the calls for a pause, J&J said "no clear causal relationship has been established between these rare events" and its vaccine, which is made by its Janssen subsidiary.
In a statement, J&J said "The safety and well-being of the people who use our products is our number one priority. We share all adverse event reports about individuals receiving our COVID-19 vaccine, along with our assessment of these reports, with health authorities in compliance with regulatory standards."
The company said it was aware that "thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines."
It added: "At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine. We continue to work closely with experts and regulators to assess the data and support the open communication of this information to healthcare professionals and the public."
The recommendation comes after similar curbs were placed on the AstraZeneca COVID-19 vaccine over evidence it may be linked to rare blood clots. British authorities last week recommended the shot not be given to adults under 30 where possible.
The CDC and FDA will provide additional information and answer questions at a news conference at 10 a.m. on Wednesday.
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Update 4/13/21, 9:53 a.m. ET: The White House's response has been reflected in this article.