amazing paper. 40-60% of population that have NOT had SARS-CoV-2 already have activated T Cells to the virus! likely due to cross-reactivity w/ other "common cold coronaviruses". may explain large % having easy time clearing virus and/or mild/no symptoms. https://www.cell.com/action/showPdf?pii=S0092-8674%2820%2930610-3

"... CD4+ T cell responses were detected in 40-60% of unexposed individuals. This may be reflective of some degree of crossreactive, preexisting immunity to SARSCoV-2 in some, but not all, individuals..."

"Whether this immunity is relevant in influencing clinical
outcomes is unknown—and cannot be known without T cell measurements before and after SARSCoV-2 infection of individuals—..."

"...but it is tempting to speculate that the crossreactive CD4+ T cells may be of value in protective immunity, based on SARS mouse models "