40-60% of people already have somewhat immunity response against coronavirus through previous exposure to common cold.
Figured out by a team researching with T-cell response to coronavirus.
Link to paper
https://www.cell.com/cell/pdf/S0092-8674(20)30610-3.pdf
Last line in summary : "Importantly, we detected SARS-CoV-2−reactive CD4+ T cells in ~40-60% of unexposed individuals, suggesting cross-reactive T cell recognition between circulating ‘common cold’ coronaviruses and SARS-CoV-2."