US regulators are recommending a halt in administration of the Janssen COVID-19 vaccine from Johnson & Johnson after several unexplained cases of blood clots in women who received the shot.

The US FDA and CDC issued a joint statement on Tuesday that they are reviewing data involving six reported US cases of a rare and severe type of blood clot in individuals after receiving the vaccine. The adverse events appear to be “extremely rare” but the agencies recommend a halt in the administration as they investigate.

As of April 12, more than 6.8 million doses of the vaccine have been administered.

According to the FDA/CDC, a type of blood clot called cerebral venous sinus thrombosis (CVST) was seen in combination with low levels of blood platelets (thrombocytopenia). All six cases occurred among women between the ages of 18 and 48, and symptoms occurred 6 to 13 days after vaccination.

The agencies say 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 to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given.

CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review the cases and assess their potential significance, followed by an FDA review of that analysis as it’s own investigation continues.

“Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution. This is important, in part, to ensure that the healthcare 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,” the agencies said in their statement.

The FDA/CDC say that “right now, these adverse events appear to be extremely rare” but recommends people who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their healthcare provider. The agencies are requesting healthcare providers to report adverse events to the Vaccine Adverse Event Reporting System at