Are we sure about that?
Just today we learned that two relatives have just tested positive for the antibodies. They only recall having some symptoms in December. I suspect that my son-in-law may also have had the virus. He was ill when he returned from Germany in January. He'll get tested for antibodies as soon as possible.
These three cases are not asymptomatic. However, these people had no idea that they might be suffering from COVID-19.
Have any of them had a cold recently?
In December, there were still fewer than 100 cases, all in China. Mathematically and biologically, it is not plausible that Covid-19 had already infected billions by December. Pathogens just don’t work that way.
The problem with antibody tests is that antibodies very often cross-react. Covid-19 is a killer cold virus that bears many structural similarities to ordinary corona cold viruses. Because of these similarities, antibodies that a person has from a cold could very well react to Covid-19 in an antibody test. So I would take any results of an antibody test with a grain of salt.
What the cross-reactivity means for Covid-19 infection is an interesting question. Pre-existing antibodies against an ordinary corona cold could protect a person from Covid-19, making their disease less severe—or they could make the symptoms far worse. It is an interesting research question.