STATISTICAL STUDIES IN IMMUNITY. 
SMALLPOX AND VACCINATION. 
By JOHN BROWNLEE, M.A., M.D., Glasgow. 
In writing this paper on smallpox and vaccination I have no intention of 
entering into any controversy upon the subject. The feet that vaccination 
protects against smallpox is assumed, and consideration is alone given here to 
the relationship between vaccination, revaccination, and smallpox, in so far as 
the facts throw light upon the growth, decline, or establishment of immunity in 
persons who have passed through one or other of these infections. I would rather 
have chosen for this investigation some other disease, but as an inquiry of this 
kind is essentially statistical there was no other series of figures available. The 
disadvantage of choosing vaccination and smallpox arises from the fact that the 
former is to a considerable extent different from the latter, and though there 
seems good reason for believing that vaccination is a disease caused by the parasite 
of smallpox so modified as to have lost one stage in its life history, yet the fact 
that it protects chiefly against the second stage of smallpox causes the immunity 
relationship to be of a more complex character than is desirable. 
Before proceeding, however, to consider the I'elationship of smallpox to vacci- 
nation one fact demands special preliminary notice : that is the change which 
has taken place in the age at which vaccinated persons are attacked by smallpox. 
All over the country from the earliest period of last century for which statistics 
are available there has been a steady increase in the mean age at which such 
persons have been attacked by smallpox. 
This is quite distinct from the rise in the mean age at death, shown by the 
combined mortality statistics of the vaccinated and unvaccinated which is com- 
monly and correctly looked upon as largely due to the pi'otective influence of 
vaccination upon the younger members of the community. What I refer to is 
a process which applies specially to the vaccinated, and which does not seem to 
admit of any complete explanation from the point of view of alteration in the 
character of the population. In London, for instance, between the years 1836 
Biometrika iv 40 
