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. PFE 10.86% and partner BioNTech SE BNTX ‑20.92% and the other from Moderna Inc., MRNA ‑16.56% are likely causing the inflammatory heart conditions myocarditis and pericarditis in a small number of cases.

Some theories center on the type of spike protein that a person makes in response to the mRNA vaccines. The mRNA itself or other components of the vaccines, researchers say, could also be setting off certain inflammatory responses in some people.