Molecular model of the spike protein of the SARS-CoV-2 virus, showing the shield of glycan molecules (light blue) that help hide parts of the protein from recognition by the body's antibodies. SARS-CoV-2 is the virus that causes covid-19, and it uses the spike proteins on its outer surface to enter and infest human cells. As the major external protein on the virus, the spike is susceptible to antibody recognition, and is the target of all covid vaccines. By adding non-antigenic glycan groups to the protein surface, derived from host molecules as the virus replicates inside cells, the spike can hide from the immune system.