A dynamic viral spike Efforts to protect human cells against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have focused on the trimeric spike (S) protein. Several structures have shown a stabilized ectodomain of the spike in its prefusion conformation. Cai et al. now provide insight into the structural changes in the S protein that result in the fusion of the viral and host cell membranes. They purified full-length S protein and determined cryo–electron microscopy structures of both the prefusion and postfusion conformations. These structures add to our understanding of S protein function and could inform vaccine design. Science , this issue p. 1586