The U.S. regulators recommend Tuesday a "pause" in the administration of the single-dose Johnson & Johnson COVID-19 vaccine to investigate potentially dangerous blood clots.

In a joint statement, the Centers for Disease Control (CDC) and Prevention and the Food and Drug Administration (FDA) 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.

The CDC's Advisory Committee on Immunization Practices will meet Wednesday to discuss the cases and the FDA has also launched an investigation of the cases.

"Until that process is complete, we are recommending a pause in the use of this vaccine out of an abundance of caution," Dr. Anne Schuchat, Principal Deputy Director of the CDC and Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research said in a joint statement.

The J&J shot was previously reported to prevent severe cases of the disease and new variants after large worldwide clinical trials.