By measuring antibodies against the SARS-CoV-2 spike protein in participants in a Phase III trial of the Moderna COVID-19 vaccine, researchers found that the higher the antibody level, the greater the vaccine protection against COVID-19. The results help define “correlates of protection”—or molecular biomarkers to measure how much immunity is needed to fight infection—and may guide approval decisions for mRNA COVID-19 vaccines and other COVID-19 vaccines.