OTOH, that could explain some of the mild or asymptomatic cases. In spite of the Chinese Frankenstein Monster being novel, it could still have some antigens similar to the mild “cold” viruses that run around the country every winter, meaning some folks could already be partially immune. Thus, the antibody prevalence.
I have been wondering about that. A recent bout of the common cold *could* provide some protection against Covid-19. Or it could contribute to the serious cases.
With influenza, infection with one strain of virus can provide partial protection against another similar strain. This is why, even if the flu shot does not prevent you from getting the flu, your illness will be less severe than if you had not been vaccinated.
But with some other diseases, like dengue, a recent infection with one strain can make an infection with a second strain of dengue extremely serious and even fatal, due to the way the antibodies, the virus, and the immune system interact. This is why vaccine development for dengue has been such a challenge. (Protect yourself from mosquitoes if you travel overseas.)