144 Proceedings of Royal Society of Edinburgh. [sess. 
figures at their disposal — which were, however, some three times les& 
than the 924 cases which I have been able to collect — arrived at 
the conclusion that “ Inoculation does not appear to confer any great 
degree of protection within the first few days after the inoculation 
has been performed.” * Nevertheless, we have seen that in the case- 
of Undhera this degree of protection was sufficient to at once differ- 
entiate the group of inoculated from their unprotected relatives. 
The last point to be decided is — How long does the pro- 
tection last ? To this, time alone can furnish a complete 
answer. If, five years after Jenner made his immortal discovery, 
he had been asked — How long does the protection conferred 
by vaccination last? he would probably have given it as his 
opinion that it was lifelong. Time has proved it otherwise, how- 
ever, and one would be rash to predict what it may evolve in the 
case of plague inoculation. The Indian Plague Commission are of 
opinion that “Inoculation confers a protection which certainly 
lasts for some considerable number of weeks. It is possible that 
the protection lasts for a number of months. The maximum 
duration of protection can only be determined by further obser- 
vation.” f Mr Haffkine, in his “Discourse on Preventive Inocula- 
tion,” delivered at the Royal Society of London on 8th June 
1899, J says: “As to the duration of the effect of the plague 
inoculation, the statement which can be made for the present is 
that it lasts at least for the length of one epidemic, which, on 
the average, extends from over four to six months of the year.” 
Lately, statistics have been received in the Plague Research 
Laboratory from the Collector of Dharwar, which seem to show 
that the effect has not worn off after eighteen months or two 
years ; but the matter is one which cannot be definitely settled 
for the present. 
The final conclusion then appears to be, that in this discovery 
of Haffkine’s we have within our reach a means of controlling 
an epidemic of plague, and converting it into a manifestation of 
sporadic cases only, easily controlled and exciting little alarm. 
* Report of the Indian Plague Commission, vol. v. p. 262. 
f Ibid. X Lancet, 24th June 1899. 
(. Issued separately May 1, 1902.) 
