That glimmer of hope has turned the conversation to the daunting challenge of when and how to reopen without setting off another cataclysmic wave of contagion. To do so, Italian health officials and some politicians have focused on an idea that might once have been relegated to the realm of dystopian novels and science fiction films.

Having the right antibodies to the virus in one’s blood — a potential marker of immunity — may soon determine who gets to work and who does not, who is locked down and who is free.