How vaccines work

The basic idea

The immune system learns from exposure. When a germ infects the body, immune cells recognize parts of it, build a targeted response, and keep a memory of that encounter, so a later infection with the same germ is met faster and more effectively.

A vaccine creates that learning without the full disease. It presents the immune system with a harmless form, fragment or instruction derived from the germ, so the body builds its response and memory while the person stays safe from the illness itself.

What different vaccines contain

Vaccines reach the same goal in different ways. Some use a weakened or inactivated form of the germ. Some use only a piece, such as a protein or a sugar coating. Newer types deliver a temporary genetic instruction that the body's own cells use to make one harmless fragment for the immune system to practice on. These approaches are designed not to cause the disease they protect against. Vaccines that use a live, weakened germ are the exception to watch: they produce strong immunity but carry a small, well-studied risk in rare cases, which is why some older live vaccines have been replaced by safer versions in national schedules.

Protection beyond one person

When enough people in a community are immune, a germ struggles to find people it can infect, and transmission slows or stops. This community protection matters most for people who cannot be vaccinated themselves, such as some newborns and people with certain immune conditions. Vaccination programmes have eliminated or sharply reduced diseases that once killed or disabled large numbers of children.

Safety and side effects

Common effects after vaccination are mild and short-lived: a sore arm, tiredness or a brief fever, signs of the immune response doing its work. Serious reactions are rare, and vaccine safety is monitored continuously after introduction. Which vaccines are recommended, and when, depends on age, health conditions and country schedules set by national and international health authorities.