There are two main types of COVID-19 vaccines: ones that use live viruses and ones called mRNA vaccines. The Pfizer and Moderna vaccines are mRNA vaccines, which use a genetic molecule called RNA that causes your own cells to make a viral protein, leading your immune system to make antibodies. On the other hand, the Johnson & Johnson/AstraZeneca vaccine uses a live virus. But, the virus used in these type of vaccines is not the virus that causes COVID-19 (SARS-CoV-2). This is a common myth. If you do happen to get sick with COVID-19 soon after receiving the vaccine, it’s likely you were already infected with it at the time.