I recall reading something - probably here - about T cells perhaps conferring some resistance to Covid-19 in people who had encountered previous Corona viruses. I can’t find that specific article, now; but this seems related:
T-cell immunity seems to be important in respiratory virus infections in general, including SARS. This paper is from 2018:
The CD8 T Cell Response to Respiratory Virus Infections
This excerpt from the text speaks to the cross-protection of different strains:
...robust memory CD8 T cell responses efficiently reduced lung viral titers in the absence of neutralizing antibodies following RSV, IAV, or SARS secondary infections. An important property of CD8 T cells is that they often recognize conserved viral proteins, allowing for cross-protection between different virus strains.
So, it's possible that people who have been previously exposed to a similar coronavirus might have a much better immune response to COVID-19.