Study: A majority of the population may have 'some degree' of preexisting immunity to COVID-19
Their paper warns that they don’t know if this immunity response actually works on Covid and that they are speculating:
“Whether this immunity is relevant in influencing clinical outcomes is unknown—and cannot be known without T cell measurements before and after SARS- CoV-2 infection of individuals—but it is tempting to speculate that the crossreactive CD4+ T cells may be of value in protective immunity,” they write.