Morbidity by age.—Before discussing the incidence of influenza in the various age periods we should explain that the charts for 1920 are based on the ages given by the individuals, and those for 1918 upon these ages, corrected by the subtraction of 15 months from the age as given. In our study of cases recurring during both epidemics the age used in the calculations is that of 1920. It is for this reason that in all of our age charts we have a first age period from zero to 15 months. Infants of less than 15 months at the time of our survey were born subsequent to the peak of the 1918–19 epidemic, and are not included in computations for that time.
The general similarity of the age incidence in the six districts studied (Charts XIX and XX) is evident. As a rule two peaks can be discerned, one falling somewhere between 15 months and 9 years, and the other between 20 and 39 years. There are individual variations in the different districts, and in Districts IV, V and VI there is a tendency toward a peak in the period 55 to 64. This, however, disappears when the total 10,000 is tabulated, when the two peaks, 15 months to 9 years, and 20 to 39, show out clearly for the year 1918 (Chart XVII).
Frost found for the same epidemic that the attack rate was highest in the age group 5 to 9, declining with almost unbroken regularity in each successive higher age group, with the exception of the groups 25 to 34, in which the attack rates were higher than in the age groups 15 to 24, but not as high as that of 5 to 9.
Both series of observations agree in finding relatively high incidence in early childhood and in early adult life.
For 1920 (Chart XVII) we find that these peaks, although present, have become decidedly less prominent, and that there is a relatively higher incidence in individuals past the age of 40 (Charts XXI and XXII). There is some tendency toward straightening out of the curve; age appears to have played a less important part, and those higher ages which were relatively insusceptible in 1918 have become more susceptible in 1920. We cannot generalize in the statement that all ages which were lightly attacked in the first epidemic were more severely attacked in the 1920 spread, because the ages from 10 to 19 are found to be relatively lower during both epidemics.
Other observations have been made regarding the age incidence particularly during the 1918 pandemic. Jordan’s figures for the October epidemic show a higher incidence among school children of ages 4–13 than among those of higher school age, 14–18. The teachers in these schools had a lower attack rate than the pupils. The pupils in both school groups were from the same section of the city and to a large extent from the same families and were presumably exposed in similar degree.
Lynch and Cumming found that of 49,140 children in public institutions the influenza rate was 412 per 1,000, while among 703,006 adults in similar institutions the rate was 263 per 1,000. These figures include children in a large number of institutions scattered throughout the United States, and would indicate that in childhood the susceptibility is much greater than in adults.
Many writers agree that nursing infants show a relative insusceptibility. However Abt records a case of an expectant mother who, within two weeks of term, developed influenza, and during the course of her illness gave birth to a baby boy, who at birth was found to be suffering from bronchitis and bronchopneumonia, but who lived for three days, finally dying of bronchopneumonia. Abt concludes from a review of all of the facts that the newly born infant had influenza and that the baby had become infected before birth.
CHART XXII.
