Federal health officials said Tuesday they are recommending an immediate pause in the use of the single-dose Johnson & Johnson COVID-19 vaccine. The announcement comes after six recipients, all women aged 18-48 years, developed a rare disorder involving blood clots within about 2 weeks of vaccination. Of those individuals, one woman died and another has been hospitalized in critical condition. “We are recommending a pause in the use of this vaccine out of an abundance of caution,” said Peter Marks, MD, director of FDA’s Center for Biologics Evaluation and Research, and Anne Schuchat, MD, principal deputy director of CDC, in a joint statement. “Right now, these adverse events appear to be extremely rare.”

CDC data show that nearly 7 million people in the United States have received the J&J vaccine, and about 9 million more doses have been shipped to states. The news was framed as a recommendation for health providers in the states, but the federal government is expected to pause administration of the J&J vaccine at all federally run vaccination sites. FDA and CDC scientists will investigate possible connections between the vaccine and the blood clotting disorder and determine whether FDA should continue to authorize use of the vaccine for all adults or limit its use.

CDC’s Advisory Committee on Immunization Practices will hold an emergency meeting to discuss the issue on Wednesday. Officials expressed concern that physicians may not be trained to be on the alert for the rare disorder if vaccine recipients develop symptoms. FDA and CDC said that “treatment of this specific type of blood clot is different from the treatment that might typically be administered” for blood clots. “Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given,” the statement said. In a news release, Johnson & Johnson said: “We are aware that thromboembolic events including those with thrombocytopenia have been reported with COVID-19 vaccines. At present, no clear causal relationship has been established between these rare events and the Janssen COVID-19 vaccine.” Janssen is the Johnson & Johnson’s division that developed the vaccine.