• White House chief medical advisor Dr. Anthony Fauci said the FDA's recommended pause will give U.S. health regulators the time they need to thoroughly investigate how six women developed a rare blood clotting disorder that left one dead.
• Fauci said officials at the FDA and the CDC want to see if there are "any clues" and "find some common denominators among the women who were involved."

White House chief medical advisor Dr. Anthony Fauci said Tuesday that the Food and Drug Administration's recommended pause on the use of Johnson & Johnson's Covid-19 vaccine will give U.S. health regulators the time they need to thoroughly investigate how six women developed a rare blood clotting disorder that left one dead.

Fauci said officials at the FDA and the Centers for Disease Control and Prevention want to see if there are "any clues" and "find some common denominators among the women who were involved" that could be enabling the blood clotting disorder known as cerebral venous sinus thrombosis, or CVST.

He also said the recommended pause will make physicians aware of the issue. Doctors typically treat that type of blood clot with heparin, a blood thinner, but Fauci said that could be dangerous in this case and noted health officials recommended a different treatment.

"If someone comes in with this really rather rare syndrome of thrombotic thrombocytopenia where you get thromboses, the most common way to treat that would be with heparin," Fauci said during a White House news briefing. "That would be a mistake in this situation because that could be dangerous and make the situation worse." 

Out of the 120 million people in the U.S. who have had at least one dose, roughly 6.9 million have received J&J's vaccine, Fauci said, noting that there haven't been any "red flag signals" from the Pfizer or Moderna Covid-19 vaccines that rely on MRNA technology in their shots. Out of the 6.9 million people who got the J&J jab, six developed blood clots, he said.

"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," Fauci said. "We want to leave that up to the FDA and the CDC to investigate this carefully. I don't think it was pulling the trigger too quickly."

Fauci said the pause could last from a matter of days to a few weeks. The CDC will also convene a meeting of the Advisory Committee on Immunization Practices on Wednesday to further review the cases.

During an earlier call Tuesday, Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said officials learned the blood clotting generally occurs about a week after vaccination, but not more than three weeks after, with a median time frame of nine days.

"We know that for these vaccines, that for the first several days after vaccination, there are flu-like symptoms that can include headache," he said, adding that those are likely common side effects. "If someone presented to an emergency room with very severe headache or blood clots" doctors should ask whether they recently got a vaccine before putting together a plan of care. If the patient also has low platelets, doctors should consider that it may be related to the vaccine, he said.

Marks told reporters that researchers haven't yet found a specific cause for the blood clotting in J&J vaccine recipients, but they believed it was similar to other adeno viral vector vaccines. "That is an immune response that occurs very, very rarely after some people receive the vaccine and that immune response leads to activation of the platelets and the extremely rare blood clots," he said.

Correction: About 120 million people in the U.S. have received one dose of a vaccine. An earlier version mischaracterized the figure.