Until a few hours before the break was recommended by American officials, regulators had planned a revision of the FDA’s emergency approval, 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 CDC panel of experts investigated a possible link between the blood clots and the vaccine. They feared 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 done 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, acting FDA commissioner, on Friday.

In the CDC panel analysis, women between 30 and 39 appear to be at greatest risk with 11.8 cases per million doses. In women between 18 and 49 there were seven cases per million doses. The condition the CDC calls thrombosis with thrombocytopenic syndrome causes severe blood clots and a tendency to bleed due to abnormally low levels of platelets, a component of blood that is involved in clotting.

The disorder is “rare but clinically severe,” said Dr. Tom Shimabukuro, Assistant Director of the CDC Vaccination Safety Office, 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 closely resemble 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.