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38
Herd Immunity.
On page 104 of 1930 Annual Report, appears the following
paragraph :—
"It is suggested that, in general, the gradual increase in
relative immunity to the ordinary infectious diseases acquired
with increasing age, is due to subliminal infections which are mostly
received by the children, in their homes, from healthy carriers.
In this district, owing to the relatively smaller proportion of
carriers compared with the number of susceptibles, the children
do not receive these small doses to the same extent as formerly,
and so far from acquiring, by repeated small infections, their
immunity—in certain cases they might even lose that amount of
resistance they had previously acquired. This loss of resistance
would render them in a position to succumb to infection at some
period after their original date of taking up residence here."
To determine the accuracy of this suggestion, three lines of
enquiry have been followed :—

The columns show the percentage of Schick-positive reactors
of children of the various ages according as to whether there are
3, 4, 5, 6 or over immunes in the house. An immune, for these
purposes, is accepted as a person of over 15 years of age, a person
who has had diphtheria (unless a Schiek-reaction shows that he is
still positive) and those who are proved to be Schiek-negativeThis
is admittedly, inaccurate, as it ignores the fact that many of
over 15 years of age and also some who have had diphtheria
not be immune, and that many of under 15 will be insusceptiable
This latter objection is less weighty than might be supposed
usually all children of a family will come together for testing. The
figures in brackets show the total number of persons considered.