evidence: COVID vaccines train your immune system to fight off COVID-19. Most work by giving your body a set of instructions (mRNA) to make a harmless piece of the virus for your immune system to recognize. Others work by causing an immune reaction to COVID’s spike protein. You might have some side effects like a sore arm, muscle aches or fatigue. [...] COVID vaccines, like all vaccines, work by training your immune system to fight off harmful germs (pathogens) that attempt to invade your body. But what does that mean? First, we have to understand how your immune system fights off viruses, bacteria and other pathogens. [...] mRNA vaccines don’t use a part of the virus to train your immune system. Instead, they give your body instructions that it uses to manufacture harmless proteins that look like parts of the virus (in the case of COVID, the spike protein). Moderna and Pfizer-BioNTech vaccines are mRNA vaccines. Your body uses the mRNA instructions to make the spike protein for your immune system to recognize.