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.

One new theory under examination: improper injections of the vaccine directly into a vein, which sends the vaccine to heart muscle.

To find answers, some doctors and scientists are running tests in lab dishes and examining heart-tissue samples from people who developed myocarditis or pericarditis after getting vaccinated.