An Intensive Study of the Spread of Influenza in Small Groups of Closely Associated Individuals.
We have been discussing the disease under consideration chiefly from the viewpoint of the statistician. The statistician, possessing a wealth of information of a general nature, and from all parts of the world, and armed with fascinating complex mathematical instruments, is able to dissect the information at his disposal, and to reconstruct therefrom both facts and apparent facts of absorbing interest. To him we are indebted for the bulk of our knowledge of the distribution and spread of the influenza through metropolises, through countries, and through continents.
Like the aviator flying over enemy territory he acquires a breadth of vision and a general perspective which is to a great extent denied to those remaining on the ground. But also like the aviator, from the very fact of his high position he loses the ability to recognize detail. The small subdivisions in the enemy lines are slightly blurred and he can distinguish the front line trenches in which most of the action is occurring no more clearly than the reserve and support trenches. An Army depending entirely upon its aeroplane reconaissance would find itself helpless in combating the enemy. The aeroplane is useful, yes, it could not be dispensed with, but never an opportunity is lost for scouting parties to explore the enemy front lines; it is these latter who bring back information as to the local strength and distribution of the enemy, as to what particular divisions are in action, as to the strength of the entrenchments, the enemy distribution within the trenches, and finally what is most important, information as to the weak points in the enemy’s lines, places at which we may concentrate our attack with the hope of driving the enemy from its stronghold. Occasionally a raiding party will return with a prisoner. He will be examined thoroughly and may yield some valuable information. All such prisoners are not dressed alike. We recognize that some belong to one regiment and some to another. After we have extracted what information we can from the prisoner he is incarcerated, if we may extend our metaphor, in a test tube, and there he remains for future reference. We do not believe that these individual private soldiers are the cause of the war, but we do know that they are doing their share of the killing—that they are doing most of the killing.
It is characteristic of human enthusiasm and hopefulness that each raiding party prays that it may bring back with it a general officer, a field marshall, the one who is chiefly responsible for the enemy offensive. In our own little battle with our invisible host we have long since discovered that field marshalls here as elsewhere are difficult to discover by raiding parties. But the raids are and should be made just the same. They almost invariably bring back some new item of information, and it is the experience of many wars that even though the commanding general be never captured, repeated small or large attacks following preliminary reconaissance, if diligently and valiantly prosecuted under good leadership may win the war.
In studying the life and habits of the influenza virus and its army of secondary invaders, and the results thereof in small groups composed of individuals as the unit, instead of large groups with communities as a unit, we will be able to discover a certain number of additional facts, some of which may have considerable value.
In the study by the author of six selected districts in Boston a special study was made of the occurrence and manner of spread of the influenza in the household or family as a unit. The 10,000 individuals canvassed were distributed through 2,117 families. Of these two thousand odd families, 45.44 per cent. were visited with one or more cases of the disease in the 1918–19 epidemic, and 27.25 per cent. in the winter of 1920. Of these, 14.31 per cent. had cases in both epidemics. In either one or both of the two epidemics under consideration, 58.38 per cent. of all families had influenza (see Table III).
| TABLE III. | ||||
|---|---|---|---|---|
| Per cent. of families invaded by influenza. | ||||
| Boston District No. | 1918–19. | 1920. | 1918–19 and 1920. | Total. |
| 1 | 49.59 | 32.79 | 20.05 | 62.33 |
| 2 | 36.04 | 17.36 | 7.25 | 46.15 |
| 3 | 45.89 | 26.43 | 14.71 | 57.61 |
| 4 | 48.48 | 32.20 | 14.39 | 66.29 |
| 5 | 52.48 | 34.11 | 19.53 | 67.06 |
| 6 | 43.16 | 24.21 | 11.23 | 56.14 |
| All Districts | 45.44 | 27.25 | 14.31 | 58.38 |
| Explanatory note: | 45.44 per cent. of all families were invaded in 1918. |
| 27.25 per cent. of all families were invaded in 1920. | |
| 14.31 per cent. of all families were invaded in both epidemics. | |
| 58.38 per cent. of all families were invaded in one or the other or both. | |
| 41.62 per cent. of all families remained free from influenza throughout both epidemics. |
In this discussion of family incidence, as in our work on the incidence among individuals, the question naturally arises as to the reliability of our information and the accuracy of our results. We have shown the close correspondence between our own results and those of Frost, done on a vastly larger number of individuals. The information for families was obtained from the same sources and from the same individuals. The thoroughness with which the inspectors did their work is indicated by the fact that in addition to the 2,117 families on which we base our results, only the records of 194 families have been discarded for various reasons. In discarding the family records we also discarded the individual records and such are, therefore, for individuals above our total of 10,000. One hundred and fifty-four of these were for families whose homes were in the districts surveyed, but who were not at home at the time of the first survey. These were omitted during the second survey, irrespective of whether individuals were at home. In this group are also included a few in which children were at home, but were unable to give reliable information. Fifteen of the 194 families gave insufficient information, and 25 refused to co-operate. The small number in this last group speaks well for the efficiency and methods of the inspectors. All families accepted for tabulation co-operated to the best of their ability, and we believe that the records are as accurate as this type of work may be made.
Dr. Niven, in the work referred to by Carnwath, made an inquiry covering 1,021 houses, with a population of 4,721. Five hundred and three households or almost exactly one-half, were invaded in either the summer 1918, or the autumn-winter 1918 epidemic. This proportion of families is quite similar to our own, but it must be pointed out that Niven was not studying the same two epidemics that we are discussing. Two hundred and sixty-six of his total households, or 26.05 per cent. were invaded in the autumn epidemic.