Up until just hours before the pause was recommended by American officials, regulators had planned for a revision to the F.D.A.’s emergency use authorization similar to the one formalized Friday, with warnings about the blood clots.
But top health officials decided in a meeting on April 12 that the government should call for a pause while federal authorities and the C.D.C.’s expert panel investigated a possible link between the clots and the vaccine. They feared a number of cases of the disorder had not been identified and wanted to allow more time for those who had just received the vaccine to reach the point at which the rare clotting typically appears.
“As we did this intensive scientific evaluation over recent days, I think we became more and more confident about the decision that was made today,” Dr. Janet Woodcock, the acting F.D.A. commissioner, said on Friday.
In the C.D.C. panel analysis, women between 30 and 39 appear to be at greatest risk, with 11.8 cases per million doses given. Among women 18 to 49, there have been seven cases per million doses. The condition, which the C.D.C. is calling thrombosis with thrombocytopenic syndrome, causes severe blood clots and also a tendency to bleed at the same time, because of abnormally low levels of platelets, a blood component involved in clotting.
The disorder is “rare but clinically serious,” Dr. Tom Shimabukuro, the deputy director of the C.D.C.’s immunization safety office, said at the meeting.
Additional potential cases, including some in men, are being reviewed. There was also a case in a 25-year-old man who participated in a clinical trial of the vaccine.
The patients’ symptoms closely resemble a rare syndrome that can be caused by heparin, a widely used blood thinner, Dr. Michael Streiff, a hematologist at Johns Hopkins University, told the panel. Heparin, typically used to treat blood clots, should not be given to these patients, he said.