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monkey showed an abortive reaction with 2 successive vaccinations which 
were subsequent to a variolation. 
(4) Variolation of the skin in 1 monkey protected against a subsequent 
variolation of the skin. 
(5) The time which elapsed between the first and second inoculation 
in these monkeys varied between 10 and 58 days. 
(6) Three of the monkeys which were shown to be susceptible to vac- 
cination after successful variolation were tested 37 days after the first 
inoculation. The 3 animals of the same series which were refractory to 
the vaccination after variola inoculata were tested 28 days after the pri- 
mary inoculation. The animal which did not seem to acquire an im- 
munity to inoculation (No. 153) was tested on the tenth and twenty-fifth 
days—that is, previous to the date in which complete: immunity was 
shown to exist in 3 monkeys and to the date in which 3 were shown not 
to be immune to a second inoculation. 
DISCUSSION. 
The results of our inoculations conform to the general law that vac- 
cination and variolation confer an immunity to subsequent infection with 
vaccine and variola virus upon the affected animal. The results of 
similar inoculations in man, which were performed in the early days of 
vaccination, seem to have yielded more constant results than we have ob- 
tained in monkeys. The immunity conferred by a vaccine lesion of the 
skin of the monkeys is complete against later inoculation with vaccine 
and variola virus. 
The conclusions are not so definite in primary variolation. In a cer- 
tain proportion of the animals a complete immunity to vaccination on the 
skin has been produced by a previous variola inoculata, but an equal 
number show only a diminished susceptibility to the vaccine virus. These 
observations agree with those made by Roger and Weil on Macacus mon- 
keys, in which substantially the same phenomenon was noted. 
In seeking for an explanation of this partial immunity conferred by 
variola inoculata against vaccination of the skin we might refer it simply 
to a dying out of the immunity, for we find the completely immune ani- 
mals were reinoculated on the twenty-eighth day of the experiment, while 
the animals showing partial immunity were tested on the thirty-seventh 
day; but this is contradicted by the single animal which was shown to 
react by a specific process to 3 successive inoculations, with an interval 
of 10 and 15 days, with variola and vaccine virus (experiment No. 153). 
The explanation that there is a qualitative difference in the reaction 
of this species of animal to the 2 viruses is not borne out by experimental 
inoculation. The inoculation of variola virus affords partial protection 
in a certain percentage of cases, and absolute protection in others, to sub- 
sequent vaccination. 
A third possibility lies in a hypothetical quantitative difference in the 
immune substance called forth by the 2 sorts of virus. 
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