Antibody tests, they drew blood samples. Same kind of tests my internist runs on me when I suspect a reactivated or new viral infection, IgG and IgM, one says you have had the infection in the past and have antibodies to it, the other measures current level of antibodies and indicates an active infection. All participants were asymptomatic, they'd had it already - my guess is most may not have even suspected they'd been infected.
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