I am not sure that these antibody tests mean what people generally think they mean. For example, there seem to be multiple events where people get sick even after it has been established that they have antibodies.

Furthermore, people in environments with a lot of virus (i.e., cruise ships, hospitals, or just northern italian towns where the virus has run amok, tend to get sick and die at much higher rate than those antibody tests would suggest.

There may be a mechanism for multiple infection which makes multiple exposure more dangerous even if you have antibodies.