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instructive. In connexion with this point it is necessary to bear
in mind that experience has led to the conclusion that whatever
be the protective effect of vaccination it is not absolutely
permanent; the most convinced advocates of the practice admit
that after the lapse of nine or ten years from the date of the
operation, its protective effect against an attack of Small-Pox
rapidly diminishes, and that it is only during this period that its
power in that respect is very great, though it is maintained that,
so far as regards its power to modify the character of the disease
and render it less fatal, its effect remains in full force for a longer
period and never altogether ceases. The experience upon which
this view is founded is derived almost exclusively from the case of
infantile vaccination. It has been supposed by some that the
transitory character of the protection results from changes
connected with the growth from infancy to adult years. Whether
this be so or not, we have no means of determining.
No doubt when Jenner drew the attention of the public to
the value of vaccination, he believed that a single successful
inoculation of vaccine matter secured absolute immunity for the
future from an attack of Small-Pox. It is certain that in this he
was mistaken. It may well be doubted whether the anticipation
was a reasonable one. No such immunity is secured by an attack
of Small-Pox, though there are few who would maintain the
proposition that it is without protective influence against another
attack. A priori there would seem to be no sound ground for
expecting that vaccinia would afford more potent protection than
Small-Pox itself. The extent of the protection afforded (assuming
that there is some protective influence) could only be determined
by experience. It soon became apparent that Jenner had, in the
first instance, over-rated the effect of vaccination. That he should
thus have over-estimated it is not to be wondered at, when the
tendency to be unduly sanguine, which besets the discoverer of
any new prophylactic, and, indeed, every discoverer, is borne
in mind.
The fact has been already noted that in the eighteenth