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This table shows that, per 1,000 population, in the case of
diphtheria, there has been for practically every year some cxcess
of incidence rising by 1027 to 2½ times the normal rate of infection.
When calculated, however, not per 1,000 population, but per 1,000
susceptible population, it will be seen that in the case of both
diseases the relative incidence is not nearly so high.
The accompanying graph 1 shows more clearly the trend of
relative incidence for both diseases. Excluding the year 1922, in
which there were relatively a large number of cases due to infection
in an overcrowded school, for the early years of the development
of the Estate, infection was very slight. A rise in the relative
incidence appeared in 1926 which was continued to 1927, when the
diseases were 50% and 30% in excess of the normal rate. The
following year saw a dccline in the rate which, however, was still
above normal. By 1929 the rate of infection for both diseases
was, calculated on the susceptible population, the same as that
obtaining in the country as a whole. In 1930 the relative incidence
of both infections rose slightly.
The incidence of infection amongst the susceptible population:
exhibited locally, shows the same trend when compared with the
London figures ; though, the incidence in London being higher
than the average for the country, the actual figures are lower.
From 1922 to 1930 the relative incidence amongst the under 15
population was, for scarlet fever, 0.22, 0.18, 0.26 0.23, 0.89,1.11,
0.95, 1.05 and 0.95, and for diphtheria, 2.29, 0.38, 0.33, 0.15,
0.37, 0.83, 0.94, 0.69 and 0.91.