322 
Smallpox and Vaccination 
of children successfully vaccinated is published as a Government Return for every 
year since 1872, so that for epidemics occurring since 1890 for any definite place 
TABLE VII 
Table shotoing the rate per Million of Attack by Smallpox among the Unvaccinated 
calculated as described^ in the text tvith the number of Attacks in an epidemic 
of Smallpox in 1726 at Aynho in Northamptonshire for comparison. 
London, 
Leicester, 
Gloucester, 
Aynho. 
ye Periods 
1902 
1892-93 
1895-96 
i726 
0— 5 
255 
727 
13 
5—10 
315 
607 
909 
15 
10—15 
543 
700 
955 
33 
15—20 
584 
554 
774 
14 
20—25 
339 
396 
434 
16 
25—30 
225 
386 
446 
9 
30—35 
150 
77 
351 
32* 
a rough approximation can be made of the respective number of vaccinated and 
unvaccinated, it being always borne in mind that the number of unvaccinated 
thus calculated is a maximum, and that many persons included have probably 
been vaccinated at some period subsequently during the existence of smallpox 
scares. The occurrence of such vaccination obviously becomes more probable the 
older the patient is, so that if a maximum is found between ten and fifteen years 
it is probably, as thus measured, to be in defect of the reality. The relative 
insusceptibility of old pei'sons is also in statistics of epidemics an almost constant 
feature but the small numbers of unvaccinated persons attacked with smallpox 
from twenty years upwards makes the tracing of the development of insuscep- 
tibility in many cases less definite than in the case of Sheffield. This represents 
the main course of the increase and the decrease of susceptibility towards smallpox 
presented by the unvaccinated person. It is in general agreement with the few 
figures which exist regarding the age incidence of smallpox in the eighteenth 
century. This age incidence of course depends in a very large measure in any 
locality on the frequency with which epidemics occur. If epidemics break out 
every three or four years, as is known to have been the case in many places, then 
it is obvious that the most frequent age of attack will be that between two and 
three years, a characteristic well shown for example in the statistics of some German 
towns. The ages of two to three years indeed seem to constitute a period of 
greater relative susceptibility, as is already indicated in the figures for Sheffield, 
where the correlation coefficient between the age periods 0-1 and 1-5 years 
is greater than that between the periods of 1-5 and 5-10, though the 
numbers ai'e too small to make this absolute. If, however, epidemics occur at 
greater intervals the natural period of greatest susceptibility will tend to become 
more evident. A maximum between ten and fifteen years is shown markedly in 
the statistics of Aynho in Northamptonshire, given by Dr Jurin, and, though in 
his record there is no statement to indicate the intervals between the succeeding 
epidemics, it is probable that this represents an epidemic outbreak of smallpox 
among a comparatively virgin community. 
* This is number of cases above 30. 
