In Sweden, the highest death-rate before vaccination (1774-1800) was 7·23 per 1,000 inhabitants, the lowest 0·31; under permissive vaccination (1801-1815) the highest 2·57 per 1,000 inhabitants, the lowest 0·12; under compulsory vaccination (1816-85) the highest 0·94 per 1,000 inhabitants, the lowest 0·0005. It has been stated that these results, which might be extended by quotations from the statistics of other countries, have been obtained not by vaccination, but by improved sanitation, including in this term not only improved housing and better water and food supply but also increased means of isolating the infectious sick. Improved housing may by diminishing overcrowding aid in diminishing the spread of this disease. Whether in view of the immense increase in the proportion of the population which lives in towns, it can be said that this has occurred is doubtful. Hospital isolation undoubtedly prevents the spread of infection when promptly effected. But a large share of the improvement in small-pox mortality occurred before either hospital or home-isolation of small-pox patients was generally enforced. There is no reason for supposing that impure water or food, or nuisances about houses have any connection with the origin or spread of small-pox, any more than they have with the origin or spread of measles or whooping-cough; which still remain as prevalent as in the past. Further light can be thrown on the subject by an examination of the age-incidence of small-pox, and of its attack-rate and severity in vaccinated and unvaccinated respectively.

The age incidence of deaths from small-pox has, since 1847, when returns classified according to age became available, undergone a remarkable alteration. Prior to 1870 the small-pox deaths in infants nearly always formed 20 per cent. or more of the total mortality from this disease, between 1870 and 1890 they did not greatly exceed 10 per cent. of the total, while since 1890 they have again begun to form an increasing proportion of the total small-pox mortality. At ages 1-5 the change is even more remarkable. Before 1870 deaths at these ages nearly always exceeded 30 per cent. of the total; since 1870 they have varied between 5 and 14 per cent. of the total; and since 1890 they have, like the proportion of deaths under one, again increased. At the higher ages the proportion of deaths has correspondingly increased, so that the curves of age incidence have become curiously inverted.

The lowered birth-rate can only account for a small portion of this transference of the chief mortality due to small-pox from childhood to adult life.

Furthermore it must not be supposed that the only change which has occurred is that the deaths which formerly occurred in childhood now occur in adult life. The death-rate at all ages has greatly declined. The only explanation which in my judgment satisfactorily explains this remarkable change in age-incidence of small-pox mortality is the fact that vaccination protects children from small-pox and that the protection diminishes, though it never entirely disappears, with advancing years. This conclusion is confirmed by the evidence obtained as to the proportion of vaccinated and unvaccinated attacked, and as to the severity of the attacks occurring when a community is invaded by small-pox.

Attack-rate among Vaccinated.—If the protective effect of vaccination, like that of a preceding attack of small-pox, wears off, it will not be expected that no attacks of small-pox will occur among the vaccinated. For evidence of immunity from attacks we must examine the records as to revaccinated persons exposed to infection. During the six years 1890-95, out of a staff in the London small-pox hospitals varying from 64 to 320, the percentage attacked by small-pox was nil, except in 1892 when it was 1·4, and in 1893 when it was 1·9.

Taking the experience of towns in which during recent years epidemics of small-pox have occurred, the following attack-rates have occurred. By attack-rate is meant the percentage number of attacks occurring among persons living in infected houses. By fatality is meant the number dying out of 100 attacked.

ATTACK RATE UNDER 10 YEARS OF AGE.ATTACK RATE OVER 10 YEARS OF AGE.
VACCINATED.UNVACCINATED.VACCINATED.UNVACCINATED.
Dewsbury10·250·827·753·4
Leicester2·535·322·247·6
Gloucester8·846·332·250·0

Severity (Fatality) among Vaccinated.—The experience of the same three towns comes out as follows:—

FATALITY RATE UNDER 10 YEARS OF AGE.FATALITY RATE OVER 10 YEARS OF AGE.
VACCINATED.UNVACCINATED.VACCINATED.UNVACCINATED.
Dewsbury2·232·12·618·7
Leicester0·014·01·07·8
Gloucester3·841·010·039·7

In view of such results as the above it is not surprising that the Royal Commission, in their majority report, summed up the advantages of vaccination as follows: