328 
Smallpox and Vaccination 
periods with those who are unvaccinated at the same age period it is found 
that the correlation is practically constant from birth onwards, and that the 
smaller correlation coefficients obtained when all ages are grouped together is 
brought about by the fact that a large proportion of the total cases are under one 
year, and that among these there is so special a mortality as to vitiate seriously 
the value of tlie correlation coefficients of the statistics when all ages are 
combined. 
As to the actual susceptibility of persons vaccinated after exposure to small- 
pox there are no actual figures in existence. Some, however, can be inferred in 
the Gloucester epidemic ; out of 653 persons vaccinated after exposure only 8-5 
developed the disease as compared with 60-t out of 832 among the unvaccinated 
who were exposed to infection and were not vaccinated. Out of the 85, 23 were 
vaccinated on the day of onset of the disease or on the day after, at a time when, 
therefore, the process could have no preventive action. The distribution of the 
remaining 02 persons is as follows: The number of those vaccinated from 0-3 
days before the onset of attack was 19, from 3-6 days also 19, from 6-9 days 18, 
while for 9 days and upwards only 6. Now the great majority of persons 
exposed to infection are vaccinated within four days of the exposure, so that it 
becomes evident that if the operation be performed up to the ninth day before 
that on which onset of the attack could be expected there is a very high proba- 
bility that no further developments will take place. This is a question on which 
statistics cannot be compiled, because, though a person may be exposed to infec- 
tion for three to four days there is nothing to indicate on which day he may be 
infected, if at all, unless he subsequently develops the disease, so that the above 
figures are probably as satisfactory as any can be. The similar relation which 
exists between persons revaccinated more than six days before the onset of the 
TABLE XIV. 
Correlation between Recoveries and DeatJts of those Vaccinated in Infancy 
only, and those Revaccinated 6 — 14 days prior to onset of illness. 
Age Periods London, 1902 
30—40 -46 
40—50 -35 
illness shows a correlation of a similar kind, but not so great as in the former 
class, which is to be expected as before remarked, as the level of the protection 
from which the improvement is made is originally much higher. The smallness 
of the figures does not permit it to be given for more than two age groups. 
The effect which an attack of smallpox exercises in establishing immunity is 
much more difficult to ascertain than that produced by vaccination. In general 
the great bulk of persons who are vaccinated have been vaccinated in infancy. 
With regard, however, to those who have had smallpox there are no data as to 
the ages at which they were attacked. In Loudon between 1836 and 1851 out 
of 2654 unvaccinated persons suffering from smallpox treated in hospital only 
