Such immune memory can fade with time, and sometimes quite rapidly, depending on the specific organism. (springcreekanimalhospital.com)
Some dogs never appear to mount an adequate antibody response to vaccination, but remain healthy, presumably because of persistence of immune memory cells and development of cell - mediated and mucosal immune responses, or alternatively, lack of exposure to infectious virus. (canine-epilepsy-guardian-angels.com)
Measles is known to cause transient immunosuppression, but close inspection of the mortality data suggests that it disables immune memory for 2 to 3 years. (science.sciencemag.org)