According to this paper published recently in Nature Immunology, around 4 out of 5 people already have pre-existing immunity to SARS-COV-2.
> Of the SARS donors, 100% showed T cell responses to cross-reactive and/or specific ECs (HLA class I 86%, HLA-DR 100%; Fig. 5d,e), whereas 81% of PRE donors showed HLA class I (16%) and/or HLA-DR (77%) T cell responses to cross-reactive ECs (Fig. 5d).
> Of the SARS donors, 100% showed T cell responses to cross-reactive and/or specific ECs (HLA class I 86%, HLA-DR 100%; Fig. 5d,e), whereas 81% of PRE donors showed HLA class I (16%) and/or HLA-DR (77%) T cell responses to cross-reactive ECs (Fig. 5d).
https://www.nature.com/articles/s41590-020-00808-x.pdf