The U.S. Centers for Disease Control and Prevention (CDC) said on May 12, 2021, it had found more cases of potentially life-threatening blood clotting among people who received the Johnson & Johnson COVID-19 vaccine and sees a “plausible causal association.”

The CDC said in a presentation the agency has now identified 28 cases of thrombosis with thrombocytopenia syndrome (TTS) among the more than 8.7 million people who had received the J&J vaccine. TTS involves blood clots accompanied by a low level of platelets—the cells in the blood that help it to clot.

So far, 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.

The Advisory Committee on Immunization Practices or ACIP, which advises the U.S. CDC, recommended on April 23 that the U.S. lift a 10-day pause on the J&J vaccinations imposed to review safety data on the clotting issue. The panel will review the new data later on Wednesday.

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

Both vaccines are based on technology that uses adenoviruses, which can cause the common cold, as vectors. The modified viruses are used to carry DNA instructions into the body to make specific coronavirus proteins, priming the immune system to make antibodies that fight off the actual coronavirus.

Scientists are working to find the potential mechanism that would explain the blood clots. A leading hypothesis appears to be that the vaccines are triggering a rare immune response that could be related to these viral vectors.

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, with rates among women aged 30-39 at 12.4 cases per million and those aged 40-49 at 9.4 cases per million.

Only six of the clotting events identified were in men.

Symptoms typically occur several days after vaccination to up to 2 weeks.