They believe that these high correlations may be the result of weaknesses in the population due to high incidences of organic diseases and tuberculosis in earlier years, or more probably that the correlation is an indirect one, due to the relation between each of the factors studied and one or more underlying conditions affecting both, such as age distribution of the population, race distribution, or social and economic conditions in the various cities studied. Or, finally, it may be that the high rate from tuberculosis and organic disease in 1916 was due to these latter factors, while the high incidence of influenza was due chiefly to proximity to the original focus of infection. None of these explanations are considered entirely satisfactory.
It is important to call attention to the fact that the American observers quoted have been studying the death rate from influenza as it is revealed in the increase of death rate from all causes, whereas Leichtenstern and Wutzdorff, and Greenwood, in his studies in the Royal Air Force have concerned themselves with morbidity. The comparison of morbidity and mortality cannot be easily made as we will show when discussing these two subjects, so we cannot conclude that the work of Pearl and of Winslow and Rogers is at variance with the other work quoted. The mortality curves form another characteristic of the local spread of influenza in a community.
It is characteristic of influenza that the curve of deaths does not fall as rapidly as does the curve for influenza cases. Thus in morbidity curves we may expect to find a symmetrical curve for a primary epidemic, but the mortality is rarely if ever symmetrical, the curve rising rapidly and falling very much more slowly.
Morbidity curves in 1920 recurrences.—The curves of influenza incidence in the recurrence of 1920 have varied in different localities, but in certain communities where the record has been carefully reported the epidemic appears to be characterized by a symmetrical evolution and usually a lower death rate as compared with 1918. The curve of incidence in the State of Massachusetts in January, February and March, 1920, is symmetrical, if anything falling away more rapidly than it ascends, and the duration is at least ten weeks. The crest of the influenza wave in Massachusetts was reached on February 4th, 5th and 6th. The peak is recorded as being in the week of February 7th.
During the 1920 epidemic the author made a house-to-house canvass in six representative districts in the city of Boston covering a population of 10,000 individuals. The curve of incidence of influenza corresponds closely with the curves for the city and the state as a whole. The peak was reached in the same week, the week ending February 7th, the curve was symmetrical, and the duration of the entire epidemic was about the same. The morbidity rate for 1920, according to our influenza census, was but half of that for 1918 for the same population. The recurrent epidemic as we will show later was decidedly milder (see Chart XVIII).
In Detroit the 1920 epidemic reached its peak for morbidity on the 9th day, and that for mortality on the 16th. In 1918 the morbidity peak was not attained until the 15th day and the death peak on the 22d. The recurrent outbreak had nearly run its course within three weeks. The following comparison between the influenza incidence in 1918 and 1920 in Detroit is taken from a report by H. F. Vaughan, Commissioner of Health for that city. In it is shown a comparison of the total figures on the twenty-seventh day of each of the two epidemics:
| A Comparison of the 1918 and 1920 Epidemics of Influenza in Detroit. Statistics Made to Include Through the Twenty-seventh Day of Each Epidemic. | ||||
|---|---|---|---|---|
| Influenza cases | Deaths from influenza and pneumonia | Normal influenza and pneumonia deaths for this season | Excess influenza and pneumonia deaths above normal | |
| 1920 (Jan.–Feb.) | 11,202 | 1,642 | 197 | 1,445 |
| 1918 (Oct.–Nov.) | 16,423 | 1,286 | 124 | 1,162 |
There had been fewer cases reported on the twenty-seventh day of the 1920 epidemic, but these had resulted in a greater number of deaths. On this day the recurrent epidemic had run its course, while the 1918 one was still in full swing. On the twenty-seventh day of 1918 there were 137 influenza cases reported and 49 deaths. On this day in 1920 there were but 24 cases and 34 deaths. Thus the second outbreak was of shorter duration, but was more deadly while it lasted.
Seven weeks of the 1920 epidemic in Detroit killed 0.20 per cent. of the population, two out of every one thousand people. A similar period at the beginning of the epidemic of 1918 witnessed the death of 0.17 per cent. of the population. This was a smaller number, but the epidemic at this time had not completed its course, and continued to be more or less prevalent for twenty-one weeks, resulting finally in the death of 0.28 per cent. of the population. The recurrent epidemic was more highly fatal, but, being of shorter duration, Detroit actually suffered less from it.