320 
Smallpox and Vaccination 
vaccination was taken for the different age periods of all those in infected houses. 
Unfortunately the vaccinal scars are simply divided into two classes according as 
they exhibit good or imperfect characters. It will be seen that under ten years 
those well vaccinated have a certain measure of protection probably for the reason 
TABLE V. 
Correlation between Exposed and Attacked, Vaccinated Well and Badly, 
Warrington, 1893. 
Well Vaccinated 
Badly Vaceiuated 
Age Periods 
Not Attacked 
Attacked 
Not Attacked 
Attacked 
Correlation 
0—10 
466 
18 
124 
9 
+ ■21 
10—20 
346 
114 
136 
32 
-13 
20—30 
144 
135 
74 
66 
-•03 
30—40 
95 
36 
46 
56 
+ •43 
40—50 
98 
19 
61 
15 
+ •09 
50— 
4.5 
7 
68 
14 
+ •14 
already indicated, from ten to thirty years there is practically no difference 
between the classes. Between the ages of thirty and forty the advantage however 
is very definitely in favour of those with good marks, and after the age of forty 
this is still present, though not nearly so marked. These figures so far as they 
go confirm the explanation just given. 
Before proceeding further to discuss the character of the immunity conferred 
by vaccination it is necessary to have as far as possible an accurate knowledge of 
the susceptibility to smallpox exhibited at different ages among persons who have 
no acquired protection at all either from vaccination or previous attack of the 
disease. With regard to the age susceptibility to smallpox in an unvaccinated 
community not a great deal can be said. In pre vaccination days statistics were 
rarely kept with any accuracy, and though records of death in age periods exist 
relating to considerable numbers of persons and considerable periods of time, yet 
these afford a quite inadequate basis on which to compute the susceptibility at 
different ages. The most complete tables calculated from the number of deaths 
are those published by Duvillard with the authority of the French Academy of 
Sciences in 1806. He deduces the result that persons at all ages are about 
equally susceptible to smallpox, but that the maximum susceptibility is between 
the ages of ten and eleven years, though the excess is small. His conclusions 
lead him, however, into the difficulty of requiring that a much larger number of 
persons at a higher age should be attacked b}^ smallpox than is actually observed. 
He explains this on the ground that the great majority of these attacks are so mild 
as to escape notice. Now of this there was no evidence at the time at which 
Duvillard wrote, nor is there any at the present date, wlien the diagnosis of 
modified smallpox has been brought to as great a pitch of perfection as any 
clinical diagnosis can. Consequently his conclusions afford an example of the 
untrustworthiness of calculating the susceptibility at different ages from the 
number of deaths alone without any knowledge of the actual case mortalities. 
For the determination of the susceptibility at different ages among persons who 
