As U.S. health authorities expand use of the leading Covid-19 vaccines, researchers investigating heart-related risks linked to the shots are exploring several emerging theories, including one centered on the spike protein made in response to vaccination.

Researchers aren’t certain why the messenger RNA vaccines, one from Pfizer Inc. and partner BioNTech SE and the other from Moderna Inc., are likely causing the inflammatory heart conditions myocarditis and pericarditis in a small number of cases.