A group of mice was vaccinated against Type II pneumoccci and subsequently tested for immunity against different numbers of the live bacteria. The immunity tests were conducted within two zones of dosage. In the first zone where the doses were kept within reasonable limits (10–6 to 10–3 cc. of culture), the number of invading bacteria was without influence and the occurrence of infections was determined by the previous immunity response of the individual. In the second zone of dosage (where passive protection also fails), these relations were reversed, and invasion by overwhelming numbers of the bacteria invariably produced infection regardless of the previous immunity response of the individual.
These results present an extreme example of the importance of the immunity response of the individual as a factor always concerned in the effectiveness of vaccination.