> This is saying that 40-60% of existing immune responses (learned over many years of previous coronaviruses) are also protective against the SARS-Cov-2 virus w

> This is saying that 40-60% of existing immune responses (learned over many years of previous coronaviruses) are also protective against the SARS-Cov-2 virus which causes Covid-19 disease. Existing cross-reactive adaptive immune cells or antibody != protection from this new virus. It only shows that these viruses are similar so there is some cross-reactivity from immune cells and antibodies designed to detect some viral epitopes (not terribly surprising). The question of whether these CD4+ t-cells that cross react are protective in humans would still need to be determined. (ie, what you’re saying is possible but not at all a foregone conclusion). The possibility of ADE mentioned in the discussion, for example, would actually mean that some cross-reactivity (in this case of antibody) could actually cause very severe disease in those individuals. This, by the way, would also be a good explanation for why more naive individuals to all human coronaviruses (ie children) do not generally get as severe of disease.