Definition

A vaccine is a shot or medicine that teaches your body to fight a germ before you ever catch it, using a weakened or harmless piece of the germ so the immune system can practice, much like a fire drill in which nobody gets hurt but everyone learns what to do when the real thing happens. More formally, a vaccine is a biological preparation containing antigens from a pathogen, such as a weakened or killed microbe, a piece of its protein, or instructions for cells to make that protein, that stimulates the adaptive immune system to produce antibodies and memory cells without causing disease, so that later exposure to the real pathogen is met with a response fast enough to prevent or greatly reduce illness, and when enough of a population is vaccinated, herd immunity protects those who cannot be. At the biochemical level, vaccines induce protective immunity through several formats, live-attenuated, inactivated, subunit or conjugate, toxoid, viral vector, and mRNA vaccines that deliver lipid-encapsulated messenger RNA translated by host cells into the antigen, with adjuvants boosting the innate response, efficacy measured in randomized trials, duration depending on memory cell persistence, and herd immunity requiring vaccination above roughly $1 - 1/R_0$ of the population.