If it didn't confer immunity we couldn't do an ELISA test for that specific antibody:
Epitope Diagnostics Inc., a certified medical device manufacturer, specializes in niche immunoassays, offering ELISA kits, rapid tests, and antibodies.
www.epitopediagnostics.com
It's believed that SARS-CoV 2 will confer the same or similar, approximately 3 year, immunity that was seen with SARS-CoV 1 and MERs. There is a small portion of the population, 5-10%, who previously tested positive, tested negative, and then tested positive again. This can happen for several reasons some of which are:
(1) Faulty specimen collection,
(2) Faulty tests,
(3) Person not fully recovered shows a dip in viral shedding. The path of a viral infection is not linear it can increase and decrease with our bodies immune system so at times we could still have this illness but be subclinical,
(4) Other rare issues
You wouldn't have so many vaccines in the pipeline right now and they wouldn't be using convalescent sera if we didn't build antibodies. This is an good article about convalescent plasma:
www.jci.org
Tyler thanks for being a calm voice of reason and rational thought.