According to a new study published in the medical journal Nature, the researchers of University of Washington, declared that the SARS infected persons in 2003 may have antibodies to covid19. When they were examining old blood samples from an individual who had been infected with SARS, they discovered an antibody S309 in the blood of a person that effectively blocked SARS-CoV-2. When they attempted to isolate the antibody and then add the virus, SARS-CoV-2 wasn't able to enter cells and replicate.
However, this discovery is not fully reliable because it could be an odd case. We only examined one person that does means all infected persons who have the antibodies. To achieve a significant breakthrough, we really need more examples to prove it in the next steps.