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2nd.—By observing the effect of Vaccination of particular groups
of persons exposed to the contagion of Small Pox under more or less
similar conditions.
3rd.—By observing the comparative frequency with which Small Pox
attacks the vaccinated and unvaccinated in large communities, the degree of
the severity of the attack in each of these classes, and the resulting death
rate.
4th—By observing the effect of Vaccination upon the age at which
Small Pox is most prevalent amongst those who have been vaccinated in
infancy and amongst those who have not. May I here parenthetically
explain what I mean by "vaccinated"? I mean efficiently vaccinated.
It is well known that Vaccination in infancy gives its most
effective protection only for a certain number of years. It is
necessary to bear this in mind, since there are many fallacies based on
statements made to the effect that in adult life more vaccinated than
unvaccinated persons suffer from Small Pox. This statement is one of
those falsehoods masquerading in the guise of truth, which, as I shall
show later on, has but little justification, since a child vaccinated in
infancy, although in fact vaccinated, does not continue to benefit to any
large extent from the Vaccination after from 7 to 10 years; and as
regards liability to attack from Small Pox gradually approximates to
the condition of those who have not been vaccinated.
1.—The method of testing the efficacy of Vaccination by the
subsequent inoculation of Small Pox is now out of the question, since
inoculation is forbidden by law; but it was frequently adopted by
Jenner, who, in 1801, wrote "that upwards of 6,000 persons had been
inoculated with the virus of cow pox, and that the far greater part of
them has since been inoculated with that of Small Pox, and exposed
to its infection in every rational way that could be devised, but without
effect." This was, at any rate, an experiment on a sufficiently
large scale, and, even if the problem was not capable of being finally
solved in this manner, was sufficient to convince his contemporaries
of the efficacy of Vaccination.
2.—The second method of attempting to estimate the utility or
the reverse of Vaccination is to study its effect upon limited communities
living under approximately the same conditions. There cannot
be any better illustration than the experience of the staffs of the
various Small Pox Hospitals. In Highgate Small Pox Hospital,
from 1836 to 1896—a period of no less than 60 years—one case
of Small Pox, and one only, occurred among the Hospital Staff,
and he was the gardener, least exposed to infection, but not revaccinated.
During the years 1883—1896 there were 137 nurses and