This summer, computational biologist Luke Hutchison volunteered for a trial of Moderna’s COVID-19 vaccine. But after the second injection, his arm swelled up to the size of a “goose egg,” Hutchison says. He can’t be sure he got the vaccine and not a placebo, but within a few hours, Hutchison, who was healthy and 43, was beset by bone and muscle aches and a 38.9°C fever. “I started shaking. I had cold and hot rushes,” he says. “I was sitting by the phone all night long thinking: ‘Should I call 911?'” Hutchison’s symptoms resolved after 12 hours. But, he says, “Nobody prepared me for the severity of this.” He says the public should be better prepared than he was, because a subset of people may face intense, if transient, side effects, called reactogenicity, from Moderna’s vaccine. Some health experts agree.