People are going nuts thinking you cannot be immune to the coronavirus because antibodies only last a few weeks. The case for everything the body is not going to waste energy making antibodies it doesn't need. So after they cure you the body realises there not using them any more stops making it but your immune system has a special cell called memory cell B. This cell remembers how to make antibodies so the next time you get the virus this cell see the virus remembers it produces the antibodies immediately. So course you won't have antibodies after a few weeks your body stops making them but that doesn't mean your body doesn't know how to make them. So it doesn't really matter if antibodies go away you still have immunity.