They found that 28.5 per cent. of 3,905 men had had the disease in 1918, and that 22.6 per cent. were attacked in 1920. Of those who had the disease in 1918–19, 21.2 per cent. had a repeated attack in 1920, while of those who had not had a previous attack, 23.1 per cent. were attacked in 1920.
A similar study among 2,472 men at Camp Grant showed that 15.8 per cent. had had influenza in 1918–19, and 11.7 per cent. in 1920. Of those with previous influenza history 15.6 per cent. had a repeated attack, while of the remainder without previous history of influenza 10.9 per cent. were attacked in 1920. They conclude that no marked immunity to influenza exists 12 to 15 months after a previous attack, but that the results do not show that some degree of immunity may not obtain at an earlier period.
It is interesting while considering the subject of immunity to pay particular attention to those who did not develop the disease as well as to those who did. In our series 70 per cent. of all individuals escaped the disease in both epidemics. With some variation this figure will hold for all communities. Or, again, among those who had the disease in 1920, 75 per cent. had not had it in the preceding waves.
Hall states that in Copenhagen at the Bispebjaerg Hospital, among the 500 patients with influenza in the four weeks early in 1920, 91.8 per cent. had not had the influenza during the 1918–19 epidemic. H. F. Vaughan found in a review of 2,500 cases occurring in Detroit in January, 1920, that 84 per cent. had never had the influenza before. The true significance of these figures cannot be recognized, because we are not informed as to the per cent. of these populations attacked in 1918–19.
We observed such a universal distribution of influenza during the epidemic period that it is frequently assumed that all individuals are exposed to the disease, that the virus must enter the body of all or nearly all, and that it is due chiefly to a relative natural immunity that some do not fall victims. Is this the actual state, or is it true that the distribution of the virus is limited to about one-third of the population and that practically all of those who are actually exposed develop the disease? These are the two extremes; more probably the actual state is somewhere between.
This question cannot be definitely answered, and yet it is one of extreme importance, particularly with regard to prevention and combat of the disease. How universally is the influenza virus distributed during pandemics? What proportion of the population is actually exposed by invasion with the virus? What proportion of actually exposed individuals develops the disease? We will refer to this again when comparing influenza with other infectious diseases, but it is of particular interest now to review our individuals who were exposed by sleeping with cases of influenza. Fifty-five per cent. of all individuals sleeping in the same bed with cases of influenza in 1918 did not contract clinical influenza. Seventy per cent. of all individuals sleeping with influenza cases in 1920 did not contract the disease, in recognizable form. Sixty-nine per cent. of all individuals in 1920 who had not had the disease previously and who slept with cases did not develop evidences of the disease.
It is difficult to conceive of a degree of exposure much closer than that of sleeping in the same bed with a sick individual. And yet it is equally conceivable that many individuals sleeping in the same bed with a patient were not penetrated by the virus of influenza. This does not aid us in answering our question. We do not know whether the more important factor is that of a natural immunity or that of absence of actual invasion by the virus.
These results with sleeping contacts form an interesting link in the chain of evidence started during 1918 by the U. S. Navy and Public Health Service, and reported by Rosenau and by McCoy and others. These experimenters working in Boston and in San Francisco carried out inoculation experiments on human volunteers. The work in Boston, as reported by Rosenau, was carried on with 100 volunteers from the Navy between the ages of eighteen and thirty, most of them between eighteen and twenty-five; all of them entirely well, and with the exception of a few controls, none having experienced known attacks of influenza previously. First, suspensions of thirteen different strains of influenza bacilli, all from cases of influenza during the epidemic, were sprayed into the nose, eyes and throat of nineteen volunteers. None of them took sick. Next, secretions from the mouth, nose and throat and bronchi of acute cases of influenza were collected, pooled, and without filtration sprayed into each nostril, into the throat during inspiration, and onto the conjunctiva of each of ten volunteers. None of them took sick. Some of this same material was filtered through a porcelain filter and administered in the same manner, with similar results. One cubic centimeter of each type was administered to each individual. The interval between the time of collection and time of inoculation was then decreased to one hour and forty minutes, the minimum time in which the material could be transferred from hospital to experiment station. The same results were obtained. This time six cubic centimeters were administered to each individual. Finally, transfer was made directly with swabs from the nose, throat and nasopharynx of one individual to another in nineteen cases. None developed the disease.
The next series of experiments consisted in an attempt to inoculate volunteers with influenza by injecting into them 10 cc. of citrated blood, which was the pooled collection from five cases of acute influenza. Ten volunteers were inoculated. None took sick. Next, the secretions from the upper respiratory tract of acute cases were injected subcutaneously into ten volunteers, each receiving 3.5 cc. This material was first put through a porcelain filter. None took ill. In an attempt to reproduce the disease in imitation of nature, ten individuals were exposed to cases of acute influenza in hospital wards. Each volunteer was placed very near to the patient, shook hands with him, talked and chatted with him, for five minutes, after which he received the patient’s breath full in his face five times while he inhaled, and finally the patient coughed five times directly into the subject’s face. Each volunteer did this with each of ten different patients, all of them acutely ill, none more than three days sick. No volunteers developed the disease. All cases of influenza used throughout the period of these experiments were typical acute cases selected from a distinct focus or outbreak of the disease. Sometimes, for example, they would select four or five typical cases from an epidemic in a school with a hundred cases.
In February, 1919, the experiments were continued at Portsmouth, where the secretions were transferred direct from individual to individual. In about thirty-six hours half of the number came down with streptococcus sore throat, but not with influenza. One of the medical officers, however, who had been very active in the experiments, and who had come into intimate contact with the disease since early in October, but who had not been inoculated, developed, during this experiment, typical influenza. The explanation for these failures is not certain. The experiments were started rather late after the onset of the epidemic, and the volunteers may have developed some immunity, although they had not developed the disease. Or, they may never have been susceptible.