How do mRNA vaccines work inside the body?

In an mRNA vaccine, a small strand of messenger RNA is delivered in a lipid particle so it can enter cells. The cells briefly use that instruction to make a protein from the target pathogen, such as the spike protein from SARS-CoV-2. The immune system then recognizes the protein as foreign and builds antibodies and memory cells. The mRNA does not enter the cell nucleus and does not alter DNA. After a short time, the body breaks down the mRNA naturally. The first widely used mRNA vaccines were authorized in late 2020 during the COVID-19 pandemic.