A vaccine prevents a disease before a person gets it by presenting some or all of the germs that cause the disease into your immune system. These germs have been killed or weakened so that they do not make you sick. Typically, a vaccine is in the form of an injection and at a low dose. After getting the vaccine, your body produces antibodies, like it would if you were actually exposed to the disease. You develop immunity to the disease so that if you are ever exposed to the disease, the antibodies become active to fight it. It is estimated that it may take 12 to 18 months to develop a vaccine for COVID-19.
Watch this video on how vaccines work.