NEW YORK (Reuters) -The U.S. Centers for Disease Control and Prevention said on Wednesday it had identified 28 cases of serious, potentially life-threatening cases of blood clotting, among the more than 8.7 million people who had received the Johnson & Johnson COVID-19 vaccine.

The CDC said in a presentation that current evidence “suggests a plausible causal association” with the J&J vaccine and the cases of thrombosis with thrombocytopenia syndrome (TTS). TTS involves blood clots accompanied by a low level of platelets – the cells in the blood that help it to clot.

Three of the 28 have died. Previously, as of April 25, the CDC had reported 17 cases of clotting among nearly 8 million people given vaccines.

It said the events appear similar to what is being observed following administration of the AstraZeneca COVID-19 vaccine in Europe.

The syndrome does not appear to be associated with either of the COVID-19 vaccines produce by Pfizer Inc and BioNTech SA or Moderna Inc.

Most of the cases were among women aged 18 to 49, the CDC said. Rates among women aged 30-39 and 40-49 were particularly high, according to the presentation, at 12.4 cases per million and 9.4 cases per million, respectively.

Only six of the clotting events identified were in men.

(Reporting by Michael Erman; editing by John Stonestreet and Bernadette Baum)