WASHINGTON (AP) — 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 it was investigating clots in six women in the days after vaccination, in combination with reduced platelet counts.

More than 6.8 million doses of the J&J vaccine have been administered in the U.S. 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.