Federal health authorities said Wednesday they have identified a total of 28 serious, potentially life-threatening cases of blood clots among nearly 9 million people who received the Johnson & Johnson vaccine in the United States.

This latest count from the Centers for Disease Control and Prevention (CDC) was done on May 7. Previously, on April 25, 17 cases of thrombosis had been identified in almost 8 million people who received this vaccine.

All identified cases date from before the Food and Drug Administration (FDA) put Johnson & Johnson’s vaccine on April 13 while it investigated some fatal cases of thrombosis. The vaccine was used again in the US as of April 23.

In all cases, the patients received the Johnson & Johnson vaccine before the April 13 hiatus.

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