The timeline of the antibody development and the potential possibility of some immunity following exposure to Covid-19 are based on current scientific understandings of Covid-19. If there is a reaction (seropositive), it may mean that it is likely the person has developed antibodies because they were previously infected or are currently infected. If there is no reaction (seronegative), it may mean they have not contracted the virus or it is too early in the infection for the antibodies to have developed. It can take 1-2 weeks from the first onset of Covid-19 symptoms for antibodies to develop in the body to fight the virus and antibodies could develop about 2-3 weeks after infection. For many other similar viruses, antibodies are protective for years and longer, but we do not yet have enough data to know for Covid-19. With respect to immunity, the interpretation of serological antibody tests relies on a clear understanding of the immune response to SARS-CoV-2, which currently remains undefined.

To quote Dr. Francis Collins, director of U.S. National Institutes of Health, “Antibodies are blood proteins produced by the immune system to fight foreign invaders like viruses, and may help ward off future attacks by the same invaders. Antibody tests if well-validated, indicate a person has previously been infected with Covid-19 and is now potentially immune.”

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http://documents.worldbank.org/curated/en/145161586536712080/pdf/Purpose-and-Options-for-Testing-for-SARS-Cov2-the-COVID-19-Virus-Considerations-for-World-Bank-Task-Teams-Managing-COVID-19-Fast-Track-Facility-Operations.pdf
https://www.nature.com/articles/s41591-020-0897-1