evidence: Messenger RNA, or mRNA, vaccines work by instructing a small number of a person’s cells to make specific proteins. In the case of the approved mRNA vaccines for COVID-19, the cells make spike protein — one component of the virus that causes COVID-19. For mRNA vaccines to work, it’s not enough to just put mRNA molecules into a vial and then inject them into a person’s muscle. One innovation that made the current mRNA vaccines possible was the use of lipids to encircle the mRNA molecules. These fatty structures — called lipid nanoparticles — protect the mRNA from being broken down prematurely. They also help the mRNA cross the cell membrane and get into cells. The approved mRNA vaccines for COVID-19 each use a blend of four types of lipids.