Last year, Michel Goldman, a Belgian immunologist and one of Europe's best-known champions of medical research, walked into a clinic near his house, rolled up his sleeve, and had a booster shot delivered to his arm. Michel was having night sweats, and he could feel swollen lymph nodes in his neck. It was cancer of the immune systemlymphoma. Michel understood this meant he'd soon be immunocompromised by chemotherapy. He had just a narrow window of opportunity in which his body would respond in full to COVID vaccination. Having received two doses of Pfizer the prior spring, Michel quickly went to get his third. Within a few days, though, Michel was somehow feeling even worse. His night sweats got much more intense, and he found himself"quite out of character"taking afternoon naps. Most worryingly, his lymph nodes were even more swollen than before. Scans ... showed a brand-new barrage of cancer lesions. It looked like someone had set off fireworks inside Michel's body. More than that, the lesions were now prominent on both sides of the body, with new clusters blooming in Michel's right armpit in particular, and along the right side of his neck. When Michel's hematologist saw the scan, she told him to report directly to the nearest hospital pharmacy. He'd have to start on steroid pills right away. Such a swift progression for lymphoma in just three weeks was highly unusual. As he followed these instructions, Michel felt a gnawing worry that his COVID booster shot had somehow made him sicker.