The U.S. is recommending a “pause” in administration of the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of potentially dangerous blood clots.

In a joint statement Tuesday, the Centers for Disease Control and Prevention and the Food and Drug Administration said they were investigating unusual clots in six women that occurred 6 to 13 days after vaccination. The clots occurred in veins that drain blood from the brain and occurred together with low platelets. All six cases were in women between the ages of 18 and 48.

The reports appear similar to a rare, unusual type of clotting disorder that European authorities say is possibly linked to another COVID-19 vaccine not yet cleared in the U.S., from AstraZeneca.

More than 6.8 million doses of the J&J vaccine have been administered in the U.S., the vast majority with no or mild side effects.

U.S. federal distribution channels, including mass vaccination sites, will pause the use of the J&J shot, and states and other providers are expected to follow. The other two authorized vaccines, from Moderna and Pfizer, make up the vast share of COVID-19 shots administered in the U.S. and are not affected by the pause.

A federal advisory committee is scheduled to meet Wednesday to review the reactions and consider how to proceed, and officials said they expect more information in a few days.

A federal advisory committee is scheduled to meet Wednesday to review the reactions and consider how to proceed, and officials said they expect more information in a few days.