John Brownlee 
319 
attacks at these ages. Persons with four marks are relatively well protected under 
five years, which is to a certain extent explained on the ground that tlie bulk of 
the patients at that age period belong to the class with very imperfect marks. 
Between twenty and forty years the advantage seems greatest, and after forty it 
has apparently entirely disappeared. Exactly the opposite would appear to be 
the case with persons with one mark. The cause of this in the early years, 
however, has just been explained, and the apparent decrease in later years is 
accounted for largely by the fact that a steady equalisation of susceptibility 
among the four groups takes place, and the number of persons with several 
cicatrices increases in proportion. It is to be noted that it is apparently between 
twenty and forty that persons with four scars have most advantage over persons 
with one. It may be added that the group of those with three marks resembles 
in general that with four, and the group with two that with one. This question 
of relative susceptibility at different ages is closely allied to that just discussed 
regarding the lowering of the mean age in those groups where vaccination is most 
perfectly performed. It certainly cannot be inferred from the latter that the 
more incomplete the vaccinal protection the more likely is the attack of smallpox 
to be delayed, provided such an attack is going to come. It appears much more 
likely that the decay of protective power is longer delayed, and that consequently 
while for the first twenty years or so any kind of fair mark indicates much the 
same protective power, there is a period in the middle ages of life when those 
who have good cicatrices are much better protected than those with inferior, and 
again after this period of relative protection has decayed all again become equally 
susceptible*. The only statistics which are available to check this conclusion are 
those of Warrington in the epidemic 1892-3, where a census as to the state of 
* How this produces a higher mean age among those badly vaccinated than among those well 
vaccinated can be seen if the figures of Table V. are examined, though it is perhaps not immediately 
apparent. Here the divisions do not lend themselves to any accurate calculation of mean ages. In the 
approximations however the mean age of all those found to be badly vaccinated is in much the same 
excess over that of all found to be well vaccinated, as the mean age of those in the former class who 
were attacked is in excess of the mean age of those in the latter. But this high mean age among the 
badly vaccinated attacked by smallpox is not necessarily a consequence of this grouping of the population 
in the houses invaded by smallpox. As high ages are more susceptible among the badly vaccinated the 
population of the houses invaded will necessarily contain more old persons as there must be some 
correlation of age among persons living in the same houses, young families being of course moi'e common 
among young parents. The high mean age referred to is independent of this, as will be seen if the 
percentage attacks in both are calculated. These show that in equal populations the mean age of those 
badly vaccinated attacked by smallpox would be in considerable excess of that of the well vaccinated. 
Proportions Attacked. 
Age Periods Well Vaccinated Badly Vaccinated 
0—10 3-7 6-8 
10—20 22-6 19-5 
20—30 48-3 47-1 
30—40 27-5 54-9 
40—50 16-2 19-7 
50— 13-4 19-5 
