By RACHEL LEESON | Ragon Institute Communications Image: NiseriN/iStock/Getty Images Plus This article is part of Harvard Medical School’s of medicine, biomedical research, medical education, and policy related to the SARS-CoV-2 pandemic and the disease COVID-19. Does an infection with SARS-CoV-2 render people protected from subsequent infections with the virus? And if so, how do we know which individuals have built up their immune defenses? These have been two of the most important, and confounding, questions about COVID-19, and the answers hold valuable clues about the interplay between the human immune system and the virus responsible for the ongoing global pandemic.