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* Report for the year 1937 of the School Medical Officer (Annual Report of the Council, 1937, Vol. III, IPart II).
disease before admission to the nursery subsequently developed the disease. The experience
of the unit for contacts of infectious diseases admitted to the Council's children's
homes in 1937 was that only 10 per cent, of the contacts subsequently developed the
disease*.
Primary and secondary cases
In order to achieve uniformity in the survey the decision as to whether a case of infectious
disease was primary or secondary was made quite arbitrarily. For each infectious disease
minimum and maximum serial intervals were estimated, these being the limits of time
after a primary case during which secondary cases might be expected from a knowledge
of the limits of the incubation period. Any case occurring within the period between
the minimum and maximum serial intervals after another case was regarded as a secondary
case ; all other cases were called primary cases. Possible errors in attribution arising from
this method are fully realised.
The likelihood of spread of infection within the nursery
It will be seen from Table (v) that the likelihood of spread of infection was remarkably
similar in the two nurseries. At St. Margaret's secondary cases followed 31 out of 105
primary cases (30 per cent.). At the Downs primary introductions produced secondary
cases in 40 out of 124 cases (32 per cent.). At both nurseries Sonne dysentery cases gave
rise to secondary spread in nearly two-thirds of the primary introductions, and in
individual nurseries over the period the common cold, tonsillitis and gastro-enteritis
showed a degree of dispersibility somewhat less than that of dysentery when measured
in this way. As would be expected measles and chicken-pox on the few occasions on
which they were introduced were seldom limited to one case.
Degree of spread as judged by secondary attack rates
It is customary to calculate secondary attack rates by using the number of secondary
cases as the numerator and the number exposed to risk as the denominator. As the latter
figure varied very considerably at different periods during the survey this method of
calculation could not be used. The index of dispersal of infection used was obtained
merely by dividing the number of secondary cases by the total number of primary cases.
The results are seen in Table (vi) from which it is noted that the two diseases giving rise
to the widest spread in the nurseries were measles and Sonne dysentery.