An overactive immune response to the mRNA Covid vaccines may be the culprit in rare cases of heart inflammation seen in some young men after they receive the shot, a small study published Friday in the journal Science Immunology suggests.

The study was based on 23 patients ages 13 to 21 who developed myocarditis after their second dose of either the Pfizer-BioNTech or Moderna vaccine. An analysis of blood samples from nine of these patients — all of whom had gotten Pfizer — found elevated cytokine levels.

Cytokines help coordinate the body’s immune response to pathogens, including bacteria, viruses and other invading microbes, as well as vaccines. But at too high levels, cytokines can trigger a rapid expansion of white blood cells that are capable of causing inflammation.