Extremely rare side effects like these are too uncommon to be detected in a phase 3 clinical trial, even if it has enrolled 40,000 participants. This is true for other drugs and vaccines outside of the context of Covid-19, and is one reason the Food and Drug Administration and the Centers for Disease Control and Prevention monitors a vaccine’s safety as it is rolled out to the public. Even if the risk is very low, experts need to figure out how low. This surveillance is also important for determining how to treat side effects when they do occur. In this case, the C.D.C. advisory committee determined that the blood clots studied should be treated in a specific way because of the platelet deficiencies observed in these cases. Blood clots are often treated with the blood thinner heparin. But since the clotting disorder resembles a rare condition caused by heparin, the experts are advising against using the drug, as it could make people’s conditions worse. By making this determination and distributing information about how the blood clots respond to common treatments quickly, the C.D.C. is giving medical workers the tools they need to treat anyone who might show any signs of these rare side effects from the Johnson & Johnson vaccine.