In a new Editorial, Peter Heeger, Christian Larsen, and Dorry Segev discuss recent evidence - including a recent Science Immunology study by Hector Rincon-Arevalo and colleagues - that points to a diminished immune response to COVID-19 vaccines among organ transplant recipients and others on immunosuppressive drug regimens. The authors note that this presents challenges at both the individual and population levels, since current vaccine protocols may not provide adequate protection to immunosuppressed patients - who could, in turn, become reservoirs for new and dangerous variants of the virus. As such, Heeger, Larsen, and Segev argue that developing vaccination strategies for transplant recipients should be a high priority in the next wave of research focused on fighting COVID-19. "The global scientific response to the SARS-CoV-2 pandemic has been a marvel," the researchers write. "But vulnerable populations remain, and there is no time to rest in the last mile. The full force of scientific effort is still needed to provide immune protection for those who remain at risk from this deadly virus and to control the reservoir to prevent fertile fields for mutations that put the entire population at risk."