We have already discussed the recurrent epidemic as it was studied in Detroit. The salient characteristics were a rapid and fairly symmetrical evolution, a shorter duration than in 1918, a lower morbidity with a higher mortality rate, and finally, a smaller total number of deaths than in 1918.
The 1920 recurrence was widespread. It was not confined entirely to the large cities. Semi-official reports from small towns and villages show very much the same conditions as were observed in the larger cities. On the whole, however, most communities, both large and small, suffered less severely than in the first spread. The few exceptions to this were distributed over the continent without uniformity.
The first among the large cities to show an increase in death rate from the epidemic was Kansas City, in which the mortality first climbed in the week ending January 17th. The following week there was an increased rate in Chicago, New York and Milwaukee, and one week later, Boston, Detroit, San Francisco and Philadelphia were affected. New Orleans was one of the last large cities affected, not showing a definite increase until the week ending February 14th. In contrast to the 1918 pandemic, the influenza of 1920 showed no clearcut direction of spread, and was as in the years following 1889 due, without doubt, to firing up of the pandemic virus as it had been left scattered in many endemic foci throughout the earth. There probably were instances of spread from the larger centers to outlying districts, but there was no continuous spread over large areas. The accompanying table shows clearly that the disease this year commenced in the center of the continent, a fact which would seem to disconnect it entirely from the late epidemics of 1919, occurring in Europe:
| Annual death rates from all causes by week in certain large cities of the United States from week ending January 3d to week ending February 21st. | ||||||||
|---|---|---|---|---|---|---|---|---|
| City. | Week Ending | |||||||
| January. | February. | |||||||
| 3 | 10 | 17 | 24 | 31 | 7 | 14 | 21 | |
| Kansas City | 16.3 | 15.8 | 19.3 | 32.7 | 39.5 | 61.5 | 44.0 | 29.1 |
| Chicago | 14.4 | 13.8 | 15.1 | 23.5 | 41.3 | 39.1 | 24.6 | 17.7 |
| New York | 14.0 | 15.3 | 14.6 | 19.5 | 28.0 | 35.0 | 35.1 | 24.8 |
| Milwaukee | 11.6 | 12.5 | 9.0 | 15.6 | 29.4 | 34.5 | 27.1 | 16.9 |
| Detroit | 13.7 | 13.0 | 14.2 | 15.5 | 33.9 | 60.9 | 42.9 | 21.6 |
| Boston | 16.8 | 16.9 | 14.1 | 16.8 | 20.3 | 27.1 | 33.7 | 32.1 |
| San Francisco | 16.5 | 15.4 | 19.6 | 19.2 | 22.9 | 25.2 | 31.8 | 28.8 |
| Philadelphia | 15.6 | 16.7 | 16.2 | 16.8 | 18.3 | 22.1 | 34.3 | 37.2 |
| New Orleans | 18.8 | 19.6 | 22.6 | 18.8 | 20.9 | 20.1 | 25.0 | 32.3 |
The relative severity of the two epidemics in certain of the large cities has been compared by H. F. Vaughan, and he has found as is seen by the table that Kansas City and Detroit, two of the early cities affected, had the highest mortality in 1920. Philadelphia in 1918 lost nearly three times as many people as Detroit did in 1920. Detroit was higher than Chicago in 1920, but lower in 1918.
| Per cent. of population killed by influenza. | |||
|---|---|---|---|
| 1920. | 1918–1919. | ||
| First seven weeks. | First seven weeks. | Twenty-one weeks. | |
| Detroit | 0.20 | 0.17 | 0.28 |
| Chicago | 0.12 | 0.34 | 0.41 |
| Kansas City | 0.24 | 0.30 | 0.63 |
| Philadelphia | 0.10 | 0.76 | 0.82 |
| New Orleans | 0.05 | 0.55 | 0.77 |
The following table taken from the “Final Influenza Bulletin,” by E. R. Kelley, Commissioner of Health in Massachusetts, shows distinctly the difference that must be always borne in mind between curves of influenza incidence and death curves. In his table the mortality climbed first on the week ending January 13th, as in the table above, but the increase in influenza cases began at least one week earlier. It is characteristic of influenza epidemics that the rise of mortality curves follows that of morbidity by about a week:
| Influenza and pneumonia cases in Massachusetts in the first three months of 1920. | ||||
|---|---|---|---|---|
| Influenza. | Lobar pneumonia. | |||
| Cases. | Deaths. | Cases. | Deaths. | |
| Week ending January 3d | 41 | 0 | 109 | 9 |
| Week ending January 10th | 46 | 2 | 142 | 50 |
| Week ending January 17th | 58 | 0 | 145 | 52 |
| Week ending January 24th | 489 | 4 | 201 | 56 |
| Week ending January 31st | 4,495 | 48 | 313 | 96 |
| Week ending February 7th | 9,627 | 272 | 382 | 212 |
| Week ending February 14th | 10,747 | 133 | 583 | 140 |
| Week ending February 21st | 5,601 | 181 | 510 | 147 |
| Week ending February 28th | 2,375 | 147 | 313 | 114 |
| Week ending March 6th | 1,144 | 54 | 206 | 34 |
| Week ending March 13th | 490 | 31 | 130 | 54 |
| Week ending March 20th | 254 | 20 | 105 | 44 |
| Week ending March 27th | 147 | 14 | 102 | 94 |
| Week ending April 3d | 218 | 6 | 97 | 12 |
In Massachusetts in the first three months of 1920 there were reported 35,633 cases of influenza and 3,158 of lobar pneumonia, with 906 deaths from the former disease and 1102 from the latter. The case rate per 100,000 from influenza was 883.4; from lobar pneumonia, 78.3; the death rate from influenza, 22.4; from lobar pneumonia, 27.4; and the fatality per cent. from the former disease was 2.5, and from the latter, 34.9.
The epidemics in Detroit and Boston both showed a symmetrical evolution and a single wave. This appears to have been the more frequent type of recurrence in this country. There are examples of the secondary curve. At the Great Lakes Naval Training Station the epidemic began during the week ending January 17th. On January 12th there were fifty-one cases. The peak was reached on the third day with the admission of 182 new cases during twenty-four hours. Although the peak came early the decline was less rapid and there were four secondary peaks, but the outbreak terminated on the twenty-fourth day. On the whole the epidemic was less severe than in 1918. Pneumonia was a complication in about ten per cent. of the reported cases of influenza at the training station.