WASHINGTON (NewsNation Now) — The Food and Drug Administration and Centers for Disease Control and Prevention on Tuesday recommended pausing the use of Johnson & Johnson’s single-dose COVID-19 vaccine after six cases of blood clots were reported.

In a joint statement, the FDA and CDC said it was investigating unusual clots that occurred six to 13 days after vaccination, in combination with reduced platelet counts.

According to the FDA, more than 6.8 million doses of J&J have been administered.

“I’d like to stress these events appear to be extremely rare. However COVID-19 vaccine safety is a top priority,” FDA Acting Commissioner Janet Woodcock said at a news conference Tuesday. “We expect it to be a matter of days for this pause.”

“It really allows both the FDA and the CDC to further investigate these cases to try and understand,” Dr. Anthony Fauci, the nation’s top infectious diseases expert, said at a White House press briefing Tuesday.

The agencies 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.

Fauci echoed the agencies Tuesday saying the chances of the blood clots occurring are less than one in a million but Americans should pay attention if they received the vaccine.

“Don’t get an anxiety reaction,” Fauci said as he urged Americans to monitor themselves for possible symptoms.

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.

J&J said in a statement it was aware of the reports of blood clots, but that no link to its vaccine had been established. The company also said it is delaying the rollout of its vaccine in Europe as a precaution.

The delay is another blow to vaccination drives in European Union member nations, which have been plagued by supply shortages, logistical problems and concerns over unusual blood clots in a small number of people who received the AstraZeneca vaccine.

European authorities investigating the AstraZeneca cases have concluded clots appear to be similar to a very rare abnormal immune response that sometimes strikes people treated with heparin, leading to a temporary clotting disorder.

While it’s not clear yet if the reports among J&J recipients are related, doctors would treat these kinds of unusual clots like they treat people who have the heparin reaction — with different kinds of blood thinners and sometimes an antibody infusion, said Dr. Geoffrey Barnes, a clot expert at the University of Michigan.

The one-shot J&J version was considered an important addition in bringing the vaccine into harder-to-reach populations since it does not require cold storage or a second appointment.

As authorities investigate whether the clots really are related to the J&J vaccine, Barnes stressed that it’s important Americans get vaccinated as soon as possible using the other two available vaccines, from Pfizer and Moderna.

CDC’s Dr. Anne Schuchat said authorities have not seen similar clots after the use of the Pfizer or Moderna vaccines, and that people should continue to get vaccinated with those shots.

“There have been no red flag signals from those,” Fauci said Tuesday.

Jeff Zients, the White House COVID-19 response coordinator, said the J&J pause will not have a “significant impact” on the country’s vaccination plan, adding that J&J vaccines make up less than 5 percent of the recorded shots in arms in the U.S. to date.

“Based on actions taken by the President earlier this year, the United States has secured enough Pfizer and Moderna doses for 300 million Americans,” Zients said in a statement.

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,” said Schuchat and the FDA’s Dr. Peter Marks in a joint statement.

The Associated Press and Reuters contributed to this report.