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exercises a protective influence which diminishes in its effect after
the lapse of some years, it is of moment to ascertain whether that
influence can be restored by a repetition of the vaccine operation.
Moreover, if it should be found that re-vaccinated persons are
more favourably situated with reference to an attack of Small-Pox
than unvaccinated persons or than persons vaccinated only in
infancy, this would obviously have a direct bearing on the disputed
question whether vaccination has a protective influence.
Unfortunately, it is not possible to obtain any statistics
shewing the amount of re-vaccination in this country generally.
It is certain that it varies greatly in different towns, and the
amount is probably not anywhere large, in proportion to the
number of the population who have passed the age of childhood.
The proportion of re-vaccinated persons to the population almost
certainly increases in any town immediately after it has been
visited by an epidemic of Small-Pox. A panic then arises which
leads many people to resort to vaccination.
In speaking of re-vaccination it is necessary to distinguish
between cases in which the operation has been performed without
result and cases of successful re-vaccination. It is only when the
vaccine virus has induced vaccinia that a person can properly
be called re-vaccinated. The term is, however, often applied
where the attempt to re-vaccinate has failed. In that case the
subject of the operation has acquired no more protection by the
process than if re-vaccination had never been attempted. No
doubt the want of success shews, if the operation has been
thoroughly performed, that the person is at the time insusceptible
to the virus, and, it may be, to the virus of Small-Pox also. But
this condition of insusceptibility is not necessarily permanent,
and it is impossible to predicate how long it may last. Moreover,
experience shows that where re-vaccination has led to no
result, a repetition of the process after a lapse of a few days only
may produce the normal features of successful re-vaccination. A
single unsuccessful attempt at re-vaccination cannot therefore be