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influence is correct, yet it is by no means satisfactory, or free from considerable risks of error and
the preponderating influence of school dissemination is certainly not yet beyond doubt. The usual
course of a scarlatinal outbreak in a school is not a class outbreak, but for the first half a dozen
cases to occur in probably five different classes, a mode of onset which is distinctly suggestive of other
influence than school infection as the chief factor. The importance of the subject, however, would
justify much time and trouble to discover the mystery of its diffusion, which really still eludes
explanation. Even the extent of its prevalence is unknown. It is given roughly by returns of
notified cases, but if we could compare it with diphtheria there may be very many more missed
cases than is usually supposed. All estimates of prevalence necessarily omit a considerable number of
children protected by mild and unrecognised attacks.
Dr. Stevenson has attempted to calculate the proportions of children known to be immunised
by previous attack. The method employed is first to calculate the attack and death rates for each
year of life up to the age of fourteen. The differences between these rates yield protection rates,
showing the proportion of children per 100,000 living in each year of life, and in each calendar year
Diagram diphtheria.
Diagram XIII.
who acquire immunity by surviving an attack. It is then assumed that this immunity is permanent
in all cases, and that the subsequent general death rate of " protected " children equals that of
children unprotected from scarlet fever. On this latter assumption the proportion of children protected
by attack during any year of life will be maintained as regards attack in that year by their survivors at
all subsequent ages. To ascertain, therefore, the proportion of children aged 5-6, say at the end of 1905,
who were protected by previous attack, it is only necessary to add together the protection rates of
age 0-1 in 1900, 1-2 in 1901, 2-3 in 1902, 3-4 in 1903, 4-5 in 1904 and 5-6 in 1905. By this means
the proportion of London boys, protected by previous attack at the end of 1905, has been shown
to vary from 2 per cent., at the age of 3, to 14½ per cent. at the age of 13, beyond which the data
at present available do not suffice to carry the calculation, so that at present in London about 2 per
cent. of the boys are immunised at the age of 3 by previous attack, and about 15 per cent. by the
age for leaving school. This is approximately verified by finding that 17 per cent. of the boys
examined in the secondary schools knew of a previous attack of scarlet fever—a rate which might
have almost been calculated by this method.
14958 F