Booster shots, partly as there is progressive cell death.
Disease outbreaks for pathogens that are not vaccinated against.
I think you are confusing passive immunity with active immunity. The injection of antibodies or serum is not the same as a vaccination program conferring active immunity. Injecting antibodies as is the case with antivenom or rhogam ( anti rhesus factor) will have a longevity equivalent to the functional use of the protein that makes up the antibody. Antibodies being protein degrade. You'll recall that an antibody is a tetramer consisting of four polypeptide chains.
