A recurrent influenza epidemic usually takes the form of a secondary wave, particularly so if it follows the primary wave within a short period of time. The difference in the character of the wave is due to the fact that there is still a comparatively large concentration of immune individuals, immunized by having had the disease during the primary spread. The secondary type of the influenza wave corresponds with the measles curves for the urban camps. There are all gradations in influenza from the typical primary wave down to a very much flattened wave of relatively long duration, and even on to the stage of endemicity, with no discernible wave. In 1920 the recurrent epidemic partook more of the form of a primary wave, because in most individuals the period of immunity had been completed by January and February, 1920.
If we could, by some means, induce an immunity which would last for long periods of time, pandemic influenza would disappear from the earth and the disease would be relegated to the comparatively minor position now occupied by measles. The disease would be constantly endemic, frequently breaking out in small epidemics, but never becoming pandemic. This is one object that should be held in view by the immunologists and bacteriologists. But it is not so simple. Even were a successful vaccine discovered, it is doubtful whether any considerable group of the population could be persuaded to take it as often as would be necessary. Universal vaccination against small pox has never been carried out. The same would be true at the present time with regard to influenza.
There is another similarity between measles and influenza. Measles is as infectious as is influenza. It is as readily transmitted and the mode of transmission is probably the same or very similar. In both diseases we are made poignantly aware of the great contagiousness of the disease, and yet in neither disease has there ever been conclusive evidence of experimental transmission from man to man. Several have reported attempts to transfer measles, but in each case the evidence of infection has been incomplete. The work of Hektoen has been quoted in particular, but Sellards, after carefully reviewing his work, concludes that the evidence of infection is insufficient. Moreover Hektoen’s patients were not exposed subsequently to measles infection in the natural way.
SECTION VIII.
The Prevention and Control of Influenza.
Dr. Hamer has visualized the present state of our knowledge of epidemic influenza in a manner which can scarcely be improved upon. We, therefore, quote him at length: “It seems to me that, during the last thirty years or more, we have been making fairly steady uphill progress along the road which constitutes the boundary between the county of epidemiology and the county of bacteriology, and that we have at length reached, at a height considerably above sea level, the foot of the mountain, on the very top of which lies hidden the secret of an ‘epidemic constitution,’ and now we are face to face with a parting of ways. Straight ahead is the frowning height, its summit in cloud. On the right hand stretches away a fine road skirting the base of the mountain. Along this road we have recently seen Dr. Brownlee whirled away in his new car ‘periodogram.’ We are all hoping to hear more from him, but as he is still insisting upon the primary, if not the exclusive, importance of continuous variation in the virulence of the germ, we have to realize that for the time Dr. Brownlee’s road is going down-hill. On the other hand, on the left, there swerves away, through the territory of the old epidemiology, another fine road, which has been explored more particularly by believers in ‘skiey influences.’ So far as it has been traced this road is as flat as flat can be, but of course there is always the possibility that after a while it will begin to rise, as it skirts round the mountain, and leads to a good vantage point from which to start climbing. At the risk, however, of being laughed at, I venture to bring under notice the very rough and at first sight unpromising ground directly in front of us. Along this can be seen two obscurely marked sheep tracks proceeding at any rate onwards and upwards. One diverges slightly towards the left hand and it has been followed at various times by De Schweinitz and others, naturalists bent on collecting ‘ultravisible viruses;’ the second track, directed rather more to the right, has been explored by Reiner, Müller, Massini, Penfold, and others, workers at the problem of discontinuous variation by ‘mutation.’ As a matter of fact I have reason to believe that two travellers, each of whom follows one of these tracks, will keep in sight of and after a while will find that they are approaching one another, and will ultimately meet at a small and retired upland farm; then after passing some dogs and following the track until clear of all stone walls, they will come right out in open ground on the face of the mountain and can start straight up the steep. But it must here be pointed out that there remains to be considered a fourth method of approach to the mountain, the most direct of all; but that is by aeroplane and is of course only open to those trained in metaphysics and statistical methods. Investigators thus equipped are able to rise in the air, to survey with careful scrutiny the whole of the ground beneath them and to make the best use of details of information obtained by scouting parties below. It is to be hoped that at no very distant date a survey of the top of the mountain will thus become an accomplished fact. Meantime, those who cannot fly may find useful employment in examining the track beyond the farm. There is the chance there of picking up facts relating to such questions as the ‘parasites associated with a parasite,’ symbiosis, and the like; take, for example, a suggestion made fourteen or fifteen years ago that the influenza organism may at one time live in association with Pfeiffer’s bacillus, at another with the Micrococcus catarrhalis, and so on; or the throat distemper organism may be yoked now with the diphtheria bacillus and now with the Streptococcus conglomeratus. (Is that, I wonder, now to be regarded as a concept or as an occurrence or happening?)”
At best our knowledge of the cause and manner of spread of influenza is fragmentary and insufficient. Attempts to outline a system of control and prevention based upon present concepts are met with many discouragements. The next pandemic will not be prevented. The disease will surely return. If the interval be sufficiently long it may find us quite as unprepared as we were in 1918. Discouraging as the outlook is there are many bright points upon which we must base our hopes for future results.
The difficulties are many: First the diagnosis of influenza is difficult either in the individual case or in the form of a mild epidemic. Even in 1918 the identification was often not definitely made until after weeks had elapsed. Second, we know little concerning the mode of transmission of the disease. We speak of “respiratory infection.” We believe that the transmission is by a mechanism similar to that for measles, but we have never experimentally transmitted either disease. The short incubation period places us at a great disadvantage. Were the interval between the occurrence of the first case and the development of additional cases as long as it is in measles, the problem of isolation and quarantine would be simplified. As in measles the disease is probably very early infectious, presumably before acute symptoms develop. The majority of cases of influenza are ambulatory. Many individuals do not take to their beds, but continue about their work, spreading the disease wherever they go.
Again, we are ignorant of the period of infectiousness. Dr. Meredith Davies has made observations indicating that a patient becomes non-infectious within one week after the temperature has become normal. How many influenza patients remain isolated throughout this period?
A carrier state probably exists and plays a most significant part in the spread.