COVID-19 vaccines have saved millions of lives, and the world is gearing up for a new round of boosters. But like all vaccines, those targeting the coronavirus can cause side effects in some people, including rare cases of abnormal blood clotting and heart inflammation. Another apparent complication, a debilitating suite of symptoms that resembles Long Covid, has been more elusive, its link to vaccination unclear and its diagnostic features ill-defined.
“You see one or two patients and you wonder if it’s a coincidence,” says Anne Louise Oaklander, a neurologist and researcher at Harvard Medical School. “But by the time you’ve seen 10, 20,” she continues, trailing off. “Where there’s smoke, there’s fire.”
Cases seem very rare—far less common than Long Covid after infection. Symptoms can include persistent headaches, severe fatigue, and abnormal heart rate and blood pressure. They appear hours, days, or weeks after vaccination and are difficult to study.
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Researchers studying these complications also worry about undermining trust in COVID-19 vaccines. Harlan Krumholz, a cardiologist at Yale University, says concern that the antivaccine movement would seize on any research findings made him hesitant at first to dive in.
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“I’m persuaded that there’s something going on” with these side effects, Krumholz says. “It’s my obligation, if I truly am a scientist, to have an open mind and learn if there’s something that can be done.”
