﻿236 STRONG. 



well as the favorable results being detailed. By no method was it possible 

 by the employment of a single inoculation and a fixed dose, thoroughly 

 to protect all of the animals of a series against infection. In fact 

 a single vaccination of a number of animals of the series, even though 

 the dose was so large as to cause the death of some of them, would not 

 usually render thoroughly immune more than 75 to 80 per cent of 

 the remainder. (See experiments with the strain "Maassen Alt.") 



In spite of variations in the results of immunization sometimes 

 obtained in the different series by the same method of inoculation, 

 nevertheless, an examination of the experiments demonstrates conclusively 

 and beyond any doubt the great value of vaccination against plague 

 infection and its evident superiority to the other methods of immu- 

 nization. Xo doubt of this fact is left after a study of the combined 

 table of inoculations on page 238 where a comparison of the value of 

 the different methods may be made at a glance. 



In relation to the time which the immunity takes to develop and 

 the time which it persists after vaccination, it may be stated that in 

 three instances attempts were made to infect monkeys numbered 1581, 

 1593 and 1626 which had been vaccinated only six days previously. All 

 of these animals remained alive and well. Xine monkeys, numbered 281:2 

 to 2850 (see Series 55, p. 234), which had been vaccinated with the strain 

 "Pest Avirulent" or "Maassen Alt" between nine and ten months pre- 

 viously were collected and their immunity tested by the inoculation of 

 § oese of the strain "Pest Virulent;" five of the animals succumbed and 

 four (41 per cent) remained alive. In connection with these experiments 

 it must be emphasized that the animals during the entire time between 

 the vaccination and the infection (nine to ten months) had been in close 

 captivity and were not in particularly good physical condition at the time 

 they were infected. Moreover, it is obvious that in testing their immunity 

 a much greater amount of plague bacilli (f oese of the virulent strain) 

 was employed than either man or animals would ever receive from a 

 natural plague infection. 



Prophylactic inoculations of natural aggressin appear to be next in 

 value to vaccination as a means of immunization against pest. Inocula- 

 tions with artificial plague aggressin did not, in my experiments, prove 

 to be nearly as efficacious as those with natural aggressin. A much 

 higher immunity was obtained with the latter prophylactic. However, 

 as I have already pointed out elsewhere, 40 there was apparently no 

 difference in the quality of the immunity obtained with the natural 



^This Journal (1906), 1, 512. 



