Vaccination rely on the immune system to work, so in situations where the immune system breaks down, or is overloaded, vaccinations stop being reliable. Like wi

Vaccination rely on the immune system to work, so in situations where the immune system breaks down, or is overloaded, vaccinations stop being reliable. Like with auto-immune disorders, or after heavy chemotherapy (as in the OP). I also know of a case where a child got an illness it was vaccinated against due to a generally weak immune system (induced by a heart disease and several surgeries).