Bear with me on this. It has been discovered that antibodies to SARS-CoV provide some level of resistance to SARS-CoV-2 today, cf. this report from Live Science:
A person who had severe acute respiratory syndrome (SARS) 17 years ago could help scientists in the search for therapies to fight the new coronavirus, SARS-CoV-2, according to a new study from a biotech company.
The study researchers found that blood samples from this patient, who had SARS in 2003, contained an antibody that also appears to inhibit SARS-CoV-2.
Antibodies form part of the body’s immune response to pathogens. This particular antibody, which the researchers call S309, showed a strong ability to bind to and disable the “spike protein” on SARS-CoV-2 that allows the virus to enter cells, according to a statement from the University of Washington School of Medicine, which was involved in the research. Multiple authors on the study work for Vir Biotechnology, and the company is developing a therapeutic based on the study findings.
It has also been noted that there was asymptomatic spread of SARS 17 years ago as NCBI points out:
We conducted a study among healthcare workers (HCWs) exposed to patients with severe acute respiratory syndrome (SARS) before infection control measures were instituted. Of all exposed HCWs, 7.5% had asymptomatic SARS-positive cases. Asymptomatic SARS was associated with lower SARS antibody titers and higher use of masks when compared to pneumonic SARS.
We have a pretty good idea of which countries saw the greatest prevalence of SARS cases (China, Hong Kong, Taiwan, South Korea, et al.)
I would like to suggest that we should be prepared for the possibility that the reduction of severity in the outbreaks of COVID-19 in Hong Kong, Taiwan, and South Korea may be a result of more widespread distribution of SARS antibodies in those place than has been commonly recognized as well as the actions those places have taken in response to SARS-CoV-2. I don’t even know how you’d go about disaggregating those two factors.
We and the Europeans haven’t been willing to implement the measures that the countries mentioned above have taken to combat the virus but it’s not beyond the realm of possibility that even were we to do so we still wouldn’t get the same results.