You need both doses to give you the best cover and protection from COVID-19. The first injection prompts your body to begin producing an immune response, whereas the second dose helps your body to boost its immune response to the virus. You need both doses for best protection. It is also important to give your body enough time to build this immune response, so protection is not immediate. It can take a week or two for your body to produce that immune response, and so you should still follow the advice around social distancing and hand hygiene etc.