There are several different types of COVID-19 vaccines. Among these are mRNA vaccines, which deliver genetic instructions that prompt cells to produce the viral spike protein on SARS-CoV-2. Viral-vector vaccines use a harmless modified virus to carry the same instructions to cells. Protein-subunit vaccines supply pieces of the spike protein directly, while inactivated vaccines use whole viruses that have been rendered noninfectious. Each type teaches the immune system to identify and respond to SARS-CoV-2 before illness can occur.