Most vaccines were simply a form of the actual virus they provided immunity for. The vaccines for measles, mumps, rubella, oral polio, chicken pox, and shingles were weakened forms of the target viruses. The weakened forms reproduced poorly inside the body, allowing the immune system time to study and attack them. The body eventually manufactured antibodies, then committed the formula for those effective antibodies to memory—so that when the real virus appeared, the immune system could neutralize it quickly.
