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40
The figures in brackets are the numbers dealt with in any
group. As can be seen from the table, the figures are too small
when split up in this large number of groups to give satisfactory
results; and to ignore the ages of the children and to total the
ligures gives erroneous results owing to the extreme difference in
the percentage of reactors at different ages. Furthermore, these
tests were performed in 1931 ; the results would have been more
valuable if they had been known for the years in which the general
susceptibility of the population was assumed to be high. The
excessive number of cases occurring for a period would have altered
the immunity of the population both by reason of the immunity
produced in those who had suffered an attack and also by reason
of the effect of these patients in immunizing their contacts. An
alteration in the percentage of susceptibles brought about in this
way, might be sufficient to reduce the proportion of immunes to
below that figure which favours an outbreak, even while leaving
the population as a whole more susceptible than the average population.
Should this be the case, the population might still be considered
to be vulnerable and liable to an epidemic, and, whilst the
extent of immunizing carried out might show no marked effcct in
reducing the number of cases each year -though having a definite
effect on the individual children immunized—it should at least help
towards keeping the immunity index of the population sufficiently
high to prevent the occurrence of a violent outbreak.

The percentage of those susceptible after receiving three
doses is higher than the average. It is seen that very young
children react negatively to the test, as do also the older
children. A large proportion however of those of ages of