200 



ACTIVE IMMUNIZATION 



between the twenty-second and twenty-fifth day (2000 to 4000). 

 Normal opsonins, according to Leishman, are not demonstrable, 

 while the stimulins (immune opsonins, i. e., elements occuring in 

 heated serum, which favor phagocytosis) are increased after eleven 

 days. 



The Duration of the Protection. The duration of the protection 

 afforded by the vaccination Wright estimates at from two to three 

 years, while Kuhn speaks of a single year. 



Results. In the human being it is, of course, out of the question 

 to study the protective value of the vaccination, as is possible in 

 the animal experiment. All that we can do is to compare the rate 

 of morbidity from typhoid fever, in a large body of vaccinated indi- 

 viduals who have been more or less exposed to infection, with what is 

 occurring in a similar body of men who have not been protected, 

 and who have been exposed to a similar extent. We can further 

 compare the rate of mortality among the non-vaccinated with that 

 of those who have been vaccinated, but who have nevertheless 

 developed the disease. Studies of this kind were carried on in 

 the English army at the time of the Boer War, in the Germen 

 army in Southwest Africa, and later in the United States concen- 

 tration camp on the Mexican border (1911). 



Some of the data obtained in the English army are given in the 

 accompanying table, from which it is clear that the vaccinated 

 individual enjoyed a much greater security, both as regards the 

 probability of infection, and the outcome, in the event that the 

 disease nevertheless developed: 



From this table it is also clear that the protection is not absolute. 

 The results obtained in our own army are even more striking. 



