Messenger-RNA (mRNA) vaccines against the coronavirus that causes COVID-19 provoke a swift and strong response by the immune system’s T cells—the heavy armor of the immune system—according to a study from researchers in the Perelman School of Medicine at the University of Pennsylvania. Although recent studies of vaccines tend to focus on the antibody response, the T-cell response is also an important and potentially more durable source of protection—yet little has been reported so far on the T-cell response to COVID-19 vaccines.