Rare cases of clotting that have been seen with two COVID-19 vaccines have put the spotlight on an uncommon reaction that occurs when the body unleashes its immune firepower against blood platelets.

Health officials are exploring whether and how the immune reaction may occur in people who have received vaccines made by AstraZeneca PLC and Johnson & Johnson. Concerns have risen so high that the U.S. Centers for Disease Control and Prevention and Food and Drug Administration jointly recommended a pause in the use of the J&J vaccine on Tuesday.

The syndrome is highly unusual in that it involves increased clotting along with low levels of platelets, the blood component mainly responsible for coagulation, and has only been seen at low rates in vaccine recipients. The pressure is on for governments that want to hasten immunizing millions of people over the next few months to understand the risk and avoid panic.