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12
Thus over 50 years the attack-rate has varied over an approximate range 1.5-3.0
and a decline in the rate could not be regarded as significant of a major change of
experience, unless it proceeded at least below 1.0 per 1,000. What has been the wartime
experience? A sharp fall in the rate occurred in 1939. This was probably due
not to immunisation, but to reduced facility of transmission of the disease owing to
the evacuation of large numbers of children from the metropolis and the consequent
"thinning out" of the remainder. There was a further fall in 1940, when the full
effect of the initial evacuation was manifest, followed by a rise in 1941. Thereafter
the attack-rate has steadily fallen and since 1944 has been less than 0.5, well
below any previous trough, pointing to a new influence and leading to the suggestion
that the immunisation campaign is yielding tangible results. This inference is
supported by the subjoined diagram, on which the continuous lines indicate the trend
of the notification rates in the three age groups 0-4, 5-14, 15+, and the broken line
indicates the percentage not artificially immunised. Two features evoke comment—
(1) since 1941 the attack-rates among children have declined more
rapidly than among adults who have not been affected by the campaign for
immunisation ; and
(ii) there is suggestive parallelism between the "unimmunised"
percentage and the juvenile attack-rates. (The word "unimmunised"
is only broadly correct because though these include all children who have
not received inoculations some of them will have recovered from actual
attacks, i.e., active immunisation).