John Brownlee 
323 
Still confining our consideration to the Sheffield figures we proceed to inves- 
tigate the susceptibility among the vaccinated. Among these, as we pass from 
age period to age period an increase of susceptibility occurs from the period of 
vaccination till between the ages of fifteen to twenty years. At this age the 
period of greatest susceptibility is reached, and thereafter immunity is gradually 
regained, increasing with succeeding years, but never actually reaching that 
existing immediately after the operation was performed. It is to be noticed that 
the growth of immunity with old age proceeds from the age of twenty upwards 
in practically the same manner as among the unvaccinated. The curve is much 
more continuous, as is to be expected from the larger numbers which are available 
for its construction. The correlation coefficients fall slightly short of the similar 
ones for the unvaccinated, which is to be expected, as the old age protection starts 
originally from a higher level, the acquired immunity not being wholly lost at the 
age chosen for comparison. It is a matter of great interest from the point of 
view of the theory of immunity that with this large mass of statistics suitable 
for the calculation of the effect which one form of immunity produces upon 
another, namely, the effect of the presence of acquired immunity upon the growth 
of natural immunity, it is found that the latter develops exactly as if the former 
were completely absent. This is a result which could not have been anticipated, 
and its meaning will be further discussed in another paper. 
The relative susceptibilities of the vaccinated and unvaccinated now falls to 
be examined, and here the Sheffield census is exceptionally valuable, as it allows 
the relative susceptibilities of vaccinated and unvaccinated to be determined and 
TABLE VIII. 
Table showing Correlation between Vaccinated and Unvaccinated Persons as regards 
Attack by Smallpox and escape from Attack when the numbei'S are considered 
of all persons (a) in the town itself (b) in infected Jiouses. 
(h) Those in the infected houses 
alone considered 
Sheffield, Dewsbury, Leicester, 
1887-88 1892* 1892-93 
■94 -90 I 
■94 -93 1 
■96 ^84 -93 
■99 -56 -77 
•72 ^73 ] 
74) .gg ■SO 
(a) 
All persons in 
the town considered 
Sheffield, 
Age Periods 
1887-88 
0— 1 
■84 
1— 5 
•63 
5—10 
•63 
10^15 
■67 
15—20 
•45 
20—25 
■34 
25—30 
■12 
30—35 
•17 
35—40 
•16 
40—45 
■11 
45—50 
■19 
50—55 
-■05 
55—60 
■05 
GO— 
■11 
■17 
■30 
•40 
•35 
•39 
•28 
•49 
-•IM 
•07 \ -■03 
■05 J 
* In this instance the number of revaccinated persons and of those who have had previous attacks 
of smallpox are excluded in the statistics, so that the correlation coefficients are as free from error due to 
the admittance of such persons as is possible. The whole numbers allected are however small. 
41—2 
