Moderna is an mRNA vaccine. That means it sends a little genetic message into your cell to produce the spike protein. This is little spike on the edge of the coronavirus, which stops it getting into your body. The Moderna vaccine is very similar to Pfizer and has very similar side effects. These side effects include the second dose often being worse than the first dose, particularly in women. You may experience feeling slightly unwell, some flue like symptoms with a sore arm, and so on.
Moderna also has a very rare side effect, which is heart inflammation. But this tends to be a mild to moderate side effect, mostly in young men. This inflammation usually passes after a few days.
Dr Norman Swan, Physician and Journalist