In this Thursday, April 8, 2021 file photo, the Johnson & Johnson COVID-19 vaccine sits on a table at a pop up vaccinations site the Albanian Islamic Cultural Center, in the Staten Island borough of New York. 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.

WASHINGTON — 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.

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.

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.

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.

CDC's Advisory Committee on Immunization Practices will meet Wednesday to discuss the cases and the FDA has also launched an investigation into the cause of the clots and low platelet counts.

"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.

They are recommending that people who were given the J&J vaccine who are experiencing severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after receiving the shot contact their health care provider.

U.S. health authorities cautioned doctors against using a typical clot treatment, the blood-thinner heparin. "In this setting, administration of heparin may be dangerous and alternative treatments need to be given," the FDA and CDC said.