I have read that the SARS vaccine didn’t get past animal trials because of ADE problems ( https://en.wikipedia.org/wiki/Antibody-dependent_enhancement).
I have also read that if Covid-19 is caught a second time it can be much more deadly due to what is likely an ADE reaction.
These factors in addition to the long asymptomatic and long transmissivity periods is what is really scary about Covid-19.
UPDATE: This interview with a virologist is very educational in connection to these issues:
