Vaccines train your immune system using a harmless form of the virus, SARS-CoV-2, which causes COVID-19. Vaccines stimulate an immune response without causing illness. Each type of vaccine for COVID-19 works differently to introduce antigens, which are unique features of the SARS-CoV-2 virus, to your body. The antigen triggers a specific immune response and this response builds immune memory, so your body can fight off SARS-CoV-2 in future.