Until a few hours before the break was recommended by American officials, the regulators had a revision of the emergency permit of the F.D.A. planned, similar to the one formalized on Friday, with warnings about the blood clots.
However, in a meeting on April 12, senior health officials decided that the government should take a break while federal agencies and the C.D.C. investigated a possible link between the blood clots and the vaccine. They feared that a number of cases of the disorder had not been identified and wanted to give those who had just received the vaccine more time to get to the point where infrequent clotting typically occurs.
"As we have been doing this intense scientific evaluation over the past few days, I think we have become more and more confident about the decision that was made today," said Dr. Janet Woodcock, the incumbent F.D.A. Commissioner said on Friday.
In the C.D.C. Panel analysis, women between 30 and 39 seem to have the greatest risk at 11.8 cases per million doses. In women between 18 and 49 there were seven cases per million doses. The state that the C.D.C. calls thrombosis with thrombocytopenic syndrome, which causes severe blood clots and is also prone to bleeding due to abnormally low levels of platelets, a component of the blood that is involved in clotting.
The disorder is "rare but clinically severe," said Dr. Tom Shimabukuro, Assistant Director of the C.D.C. Vaccination Security Bureau, at the meeting.
Other potential cases, including some in men, are currently being investigated. There was also a case in a 25-year-old man who was in a clinical trial of the vaccine.
Patients' symptoms are very similar to a rare syndrome that can be caused by heparin, a widely used blood thinner, said Dr. Michael Streiff, a hematologist at Johns Hopkins University, joined the panel. Heparin, which is normally used to treat blood clots, shouldn't be given to these patients, he said.