John Brownlee 
329 
690 were under the age of ten years; between 1870 and 1883 out of 2169 like 
persons 1197 were under the age of ten years. In both of these sets of statistics 
the distribution of the numbers fi-om birth till ten years is fairly uniform. So 
that it may be taken for granted that towards the end of last century among 
persons over twenty years of age who had had smallpox previously one half was 
the greatest proportion of these who had the attack during the first ten years of 
life. In the accompanying table I have collected the details of fifty-one cases 
TABLE XV. 
Table of Second Attack of Smallpox. 
Age at Second Attack. 
1 
o 
so 
1 
"o 
CO 
1 
1 
1 
1 
1 
! 
1 
1 
1 
Totals 
ick. 
o 
so 
IQ 
■-0 
J- 
Att 
0— 5 
4 
5 
2 
4 
4 
1 
2 
2 
1 
1 
2 
1 
1 
30 
5—10 
4 
1 
2 
1 
1 
9 
m 
10—15 
1 
1 
2 
1 
1 
6 
Fir 
15—20 
20—25 
1 
1 
1 
3 
at 
25—30 
1 
1 
2 
bc 
40—^5 
i 
1 
Totals 
4 
9 
3 
7 
6 
3 
5 
6 
1 
2 
3 
1 
1 
51 
where persons have had a second attack of smallpox between the years 1887 and 
1902, and of these fifty-one it is seen at once that thirty suffered from their first 
attack of smallpox below the age of five years, nine between the age of five and 
ten years, leaving a total of only twelve who had their first attack of smallpox 
above this age. This would seem to show that the protection afforded by an 
attack of smallpox in childhood is much less complete than that afforded by an 
attack after ten years. A study of the figures in the table seems to show that 
this is not due simply to the natural lessening in numbers which occurs in the 
population at each age period from childhood onwards. A certain amount of 
support is lent by this to the suggestion advanced earlier in this paper that 
vaccination performed in early adult life afforded a longer period of protection 
than when performed in infancy, and in both instances it may be surmised that 
as the natural susceptibility to smallpox increases from birth onwards till about 
twelve years there may be a greater loss of acquired immunity during the same 
period of life than is likely to occur when the natural protection against smallpox 
has become more definitely established. With regard to the protection which an 
attack of smallpox provides against death even less can be said. Of these fifty- 
one cases seven died. Five of these deaths, however, occurred in one epidemic 
out of twenty-three cases, and, although one of these five died from apoplexy, 
yet even four deaths out of twenty-three cases represents a mortality so much in 
Biometrika iv 42 
