Most vaccines work by taking something from a harmful germ and putting it inside the patient's body without any parts of the germ that cause harm. Then the patient's own cells learn to identify the germ using its other parts. Then, if a living germ ever attacks the body, the patient's cells fight it off without having to learn what it is first. Some vaccines do this by putting a dead, weak germ inside the patient's body. Other vaccines do this by putting lots of proteins that the germ makes inside the body but not any germs themselves. mRNA vaccines work by putting mRNA inside the body instead of protein. Then the body uses the mRNA to make protein itself.