Scientists still don’t have a clear explanation yet for why the vaccines cause the condition, according to Dr. Paul Burton, Moderna’s chief medical officer. He expects the virus’s spike protein, once produced in the cell after vaccination, may generate a reaction in the body that can cause inflammation in the heart.

“We don’t understand yet and there’s no good mechanism to explain it,” he said.

And that’s the exact reason why, under normal circumstances, it takes years until you can bring a new vaccine on the market. Even more so, if said vaccine is based on a new technology that’s never been widely tested on humans before.