A vaccine helps a person build up immunity against a germ to protect themselves against an infectious disease. It works by injecting something that looks very much like a germ that stimulates the immune system to produce an immune response. This is what protects a person from a future infection if they ever come across the real infection. That is the way that it works for most diseases and the way that it works for COVID-19. When we are vaccinated, we are not just protecting ourselves, but also those around us. Each person vaccinated is one person that the virus cannot use to spread within the population.