The U.S. FDA on Tuesday recommended an immediate halt of the use of Johnson & Johnson's COVID-19 vaccine, citing cases of a rare blood clot disorder that six women developed within two weeks of receiving the shot.

Why it matters: The recommendation was issued "out of an abundance of caution" as health officials review data and alternative treatments, but it could risk slowing down the U.S. vaccine rollout.

The big picture: Nearly 7 million doses of the shot have been administered in the U.S., and another 9 million have been shipped out to the states.

• The six women who developed the blood clots were between the ages of 18 and 48, according to the FDA.
• The FDA green-lighted the vaccine for emergency use on Feb. 27. The U.S. will stop administering the vaccine at federal sites and recommends that states do the same.

What they're saying: "Right now, these adverse events appear to be extremely rare," the CDC wrote in a statement with the FDA.

• "Treatment of this specific type of blood clot is different from the treatment that might typically be administered."
• "Usually, an anticoagulant drug called heparin is used to treat blood clots. In this setting, administration of heparin may be dangerous, and alternative treatments need to be given."

What to watch: The agencies will provide a press briefing on Tuesday at 10 a.m., and will convene a meeting on Wednesday to "review these cases and assess their potential significance."