Scientific American Clear messaging and transparency are vital, say some experts on risk assessment and decision-making Advertisement Unforeseen safety issues routinely emerge after any new medicine or vaccine goes from testing in tens of thousands of volunteers to actual public use on tens of millions. So it was no major surprise when an extremely small percentage of people developed a strange blood clotting problem after receiving either the Johnson & Johnson (J&J) COVID-19 vaccine or the AstraZeneca shot, which is widely used outside of the U.S. Rare but dangerous side effects from vaccines can present a tricky dilemma for public health authorities. In this case, the life-threatening blood clots, accompanied by an oddly low count of clot-promoting platelets, appear to strike about two individuals per million people vaccinated with J&J’s shot and about one per 100,000 receiving AstraZeneca’s. Both are minuscule risks, compared with COVID-19 itself, which, by one estimate, kills roughly two people out of 1,000 infected (though fatality rates vary greatly by age, location and other factors). On one hand, it is crucial to be transparent with the public—and to alert health care providers to the problem and advise them on how best to identify and treat it. On the other hand, there is a chance of sowing unwarranted doubts about these vaccines and perhaps others as well, inflaming already worrisome levels of vaccine hesitancy.