John Brownlee 
315 
to the factors before mentioned. Unfortunately there is a complete absence of 
statistics referring to this matter, so that an examination of the relative iutiuences 
of these different factors is impossible. The effect of revacciuation on the age 
incidence of persons suffering from smallpox can be more readily estimated. 
When performed in the absence of an epidemic, or when slight outbreaks only 
of this disease are present, there is not a sufficient amount done to influence 
statistics much one way or another. In Sheffield, for instance, when the great 
epidemic of 1887 occurred, about five to seven per cent, of the population at 
each age period above fifteen years were found to be revaccitiated. When, how- 
ever, as in this epidemic in Sheffield and in the late epidemics in Glasgow 
revacciuation is performed on a large scale, it is found that a much larger projjor- 
tion of persons between ten and twenty years of age avail themselves of this 
means of protection than at higher ages. The effects of this on later epidemics 
are obviously, (1), if the second epidemic occurs within a few years of the first to 
raise the mean age ; and (2), if the later epidemic occur about fifteen years there- 
after to lower the mean age of attack. Revacciuation, however, as a rule only 
begins to have an important modifying effect on the epidemic concerned when it 
is naturally approaching its end. 
The change in the age incidence of smallpox just discussed is one which not 
only applies to the vaccinated as a class, but equally to the different groups 
comprehended in this class. Persons suffering from smallpox are for the purpose 
of classification commonly divided into groups as they present on their arms one, 
two, three, and four or more vaccinal cicatrices. In each of these groups the 
mean age has steadily risen. Now with regard to the group of those who pi'esent 
one cicatrix this is to be expected. Since 1870 persons vaccinated in this manner 
have been becoming progressively fewer and fewer, and consequently in London 
the bulk of those presenting one vaccinal cicatrix must consist of persons at the 
higher ages. Exactly the same change in the mean age, however, is found to be 
among the members of the group presenting one cicatrix in Glasgow, although no 
such change in the method of vaccination has taken place there. With regard to 
the group, however, of persons with four or more scars, year by year since 1870 a 
larger and larger number of such have come into existence at lower ages and 
consequently it would be expected that the mean age among this group would be 
at any rate stationary if not lower. This is not found to be the case. The 
annexed table exhibits the change in the mean age which has taken place in 
London since 1870. The rise noted in the mean age of persons with four marks 
TABLE II. 
Mean Age of Cases and Deaths in the Epidemics in London, 1870 — 1902. 
One Scar Two Scars Three Scars Four Scars 
Cases Deaths Cases Deaths Cases Deaths Cases Deaths 
1870-83 21-8 26-2 21-0 25-1 18-1 24-0 IT'O 24-8 
1892-95 26-8 35-3 29-5 43-9 26-5 31-7 23-2 35-7 
1902 34-2 39-6 34-0 41-3 31-0 377 27-2 34-5 
40-2 
