The U.S. Food and Drug Administration (FDA) on Tuesday asked states to temporarily pause using the one-dose coronavirus vaccine developed by Johnson & Johnson (JNJ  ) as the federal health agency investigates a possible link between the vaccine and very rare blood clots.

The FDA issued a joint statement with the U.S. Centers for Disease Control and Prevention (CDC) stating that there have been six cases of a rare blood clot called cerebral venous sinus thrombosis (CVST) seen in combination with low levels of blood platelets occuring in women between the ages of 18 and 48, with symptoms developing 6 to 13 days after vaccination. One of the cases was fatal and another patient is in critical condition.

"Right now, these adverse events appear to be extremely rare," the two agencies stated. "COVID-19 vaccine safety is a top priority for the federal government, and we take all reports of health problems following COVID-19 vaccination very seriously."

In a separate statement, Johnson & Johnson said that it is working closely with the FDA and CDC to review the cases. The pharamucial giant is also reviewing cases with European health authorities and has delayed its vaccine rollout in Europe and paused all clinical trials at this time.

Dr. Anthony Fauci, the nation's top infectious disease expert, stated during a White House press briefing on Tuesday that roughly 6.9 million have been dosed with the J&J vaccine in the United States, making the possible CVST side effective very rare.

"We are totally aware that this is a rare event. We want to get this worked out as quickly as we possibly can and that's why you see the word 'pause,' in other words, you want to hold off for a bit, and it very well may go back to that, maybe with some conditions or maybe not," Fauci told White House reporters. "We want to leave that [decision] up to the FDA and the CDC to investigate this carefully."

Fauci said that the pause is expected to last no more than a few weeks, but the investigation could be concluded as soon as a few days. The CDC is meeting the with Advisory Committee on Immunization Practices on Wednesday to further review the cases, try to understand what the patients may have in common, and why the blood clots are occuring.

Fauci added that health officials haven't documented any severe adverse side effects from either the Pfizer (PFE  )-BioNTech (BNTX  ) or Moderna (MRNA  ) vaccines, which rely on a different technology called messenger RNA or mRNA.

Last week, Europe's medicines regulators discovered a possible link between the COVID vaccine developed by AstraZeneca (AZN  ) in partnership with Oxford University and very rare blood clots. Both the J&J and AstraZeneca vaccines are vector-based, meaning they use a modified virus to carry a piece of the coronavirus to ignite an immune system response against it.