The Covid-19 vaccines were powerfully protective, preventing millions of deaths. But in a small number of people, the shots may have led to a constellation of side effects that includes fatigue, exercise intolerance, brain fog, tinnitus and dizziness, together referred to as “post-vaccination syndrome,” according to a small new study.

Some people with this syndrome appear to show distinct biological changes, the research found — among them differences in immune cells, reawakening of a dormant virus called Epstein-Barr, and the persistence of a coronavirus protein in their blood.

The study was posted online Wednesday and has not yet been published in a scientific journal. “I want to emphasize that this is still a work in progress,” said Akiko Iwasaki, an immunologist at Yale University who led the work.

“It’s not like this study determined what’s making people sick,” she said, “but it’s the first kind of glimpse at what may be going on within these people.”

Independent experts noted that the findings were not conclusive on their own. Yet the results, from a scientific team known for rigorous work, suggest that post-vaccination syndrome deserves further scrutiny, they said.

“One of the most important things is that we get some attention to really shine a light on this and try to understand exactly what it is,” said John Wherry,director of the Institute for Immunology at the University of Pennsylvania. (Dr. Wherry has previously collaborated with Dr. Iwasaki’s team, but did not participate in this work.)