[nradov]

There is a huge variance in antibody response and duration between patients. It seems to correlate with symptom severity but there are other factors still not understood (possibly genetic). In some cases the innate immune system fights off the infection before antibodies are generated. In other cases the antibodies fade away quickly, but a high level of immunity is retained through memory T cells. Unfortunately there's no quick or easy way to do a specific T cell assay.

There is a hypothesis that prior exposure to other similar viruses provides a limited degree of immunity from SARS-CoV-2. It's plausible and fits some of the case patterns but we don't know yet if it's a significant effect.