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36
These figures show a much larger proportion of the children of school age
to have been "primary cases," i.e. to have caught their infection outside their
homes (211 out of 257), than was the case amongst children below school
age (274 out of 489).
The actual number of measles cases recorded as attending schools were:—
primary cases 257, secondary cases 56 (313 in all).
By a careful examination of the records it is possible, by studying the
dates of onset, etc., of the cases in school outbreaks, to form a rough
estimate of the number of cases who did in fact become infected at school.
The number of such cases in the series under investigation was 209, so that
the great majority of primary cases in school children are to be regarded as
having caught the disease at school.
But the causing of 209 cases out of 800 by no means represents the
amount of infection spread by schools. The secondary cases in the same
homes as these 209 cases amounted to 170, so that the number of cases
directly traceable to school infection was 379, or nearly half the total reported
cases. And this is not a full statement of the effect of schools in spreading
measles, for there are undoubtedly a large number of cases in other houses
caught from the school cases. Our experience shows that when there are no
school outbreaks there is very little measles, and that a measles outbreak in a
neighbourhood (speaking of such an urban district as this) is practically always
ushered in by a school outbreak. Occasionally outbreaks in day nurseries or
similar places where children are aggregated have the same effect.
During the six months over which this numerical inquiry extends measles
was comparatively quiescent, and there were only 12 marked school outbreaks
affecting the borough, one of them numbering 33 cases, and one—16 cases—in
a day nursery. In the corresponding months of 1917 there were 37 such
school outbreaks, and two in day nurseries.

Thus nearly two-thirds of the total number of cases reported were of
definitely known origin.