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48
inoculation of cow-pox (vaccination) has precisely the
same protective influence in respect of small-pox that a
previous attack of small-pox has: that, taking any number
of. persons who have just had an attack of small-pox, and
an equal number of persons who have just been successfully
vaccinated, the chances of escape from subsequent
attacks of small-pox are equal for each group.
It will be seen by collating the above statements of facts
that it follows, that while a single successful vaccination
is protective in a very high degree against small-pox, it
cannot be expected to give perfect immunity in all cases;
and that the immunity which it does afford tends in a
greater or less degree to lessen as years progress; but
that, nevertheless, of the comparatively small number of
vaccinated persons who contract small-pox, by far the
larger proportion have it in the form of a very slight and
(so far as they are personally concerned) unimportant
ailment.
It has been proved, however, by the most ample experience
that vaccination may be made almost absolutely
protective, if it be repeated at the time, or from time to
time, when its protective power is undergoing diminution.
By repeated vaccination the nurses at the Small-pox
Hospital have, ever since the foundation of the hospital,
been preserved from small-pox ; and by the same method,
even during the present epidemic, the progress of the
disease has been at once arrested when it has found an
occasional entrance into any of the general hospitals of the
metropolis. It is believed now by those who have given
most attention to the subject, that a second vaccination