How do vaccines work?

A vaccine shows your immune system a recognizable piece of a pathogen without the disease that normally comes with it. That piece might be a weakened or inactivated virus, a purified protein from its surface, or instructions your cells use to make that protein briefly, as mRNA vaccines do. Your immune system responds as it would to a real infection: it builds antibodies, activates T cells, and, critically, forms memory cells that persist. If the actual pathogen shows up later, that memory lets the response start in hours rather than the days an unprimed immune system needs, which is often the difference between a mild illness and a severe one.

Two things follow from this. First, the immunity is entirely your own, the vaccine supplies a training scenario, not borrowed protection, which is why side effects like a sore arm, fatigue, or a low fever are usually signs of the immune system working rather than of the vaccine causing illness. Second, protection is not always permanent or absolute: some pathogens mutate, some immune memory fades, and some vaccines reduce severity more than they prevent infection entirely, which is why boosters exist for certain diseases. High vaccination rates also protect people who cannot be vaccinated, infants, the immunocompromised, by leaving the pathogen fewer paths to spread, an effect that only works above certain coverage thresholds and weakens as coverage drops.