As states grapple with the question of when it will be safe to reopen businesses and relax social distancing, there's increasing urgency to better understand who's immune to Covid-19. Does having the virus and recovering mean you can't get it again, or at least that you can’t be reinfected for some time? No one yet has good answers to these questions. Kristen V. Brown looked into what we do, and don't, know about the science of coronavirus immunity.

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