Influenza

by Warren T. Vaughan

Pontano in Italy is quoted by the Office International d’Hygiène Publique as having observed in his epidemiological study that there was a constant connection between the living conditions and the severity of the complications. Notable differences were observed in neighboring houses according to the hygienic conditions of the various households.

Healthy carriers and convalescents.—Leichtenstern, who apparently accepted the Pfeiffer bacillus as the cause of influenza, did not believe that the disease could be transmitted by healthy carriers. He based this assumption on the statement, made by Pfeiffer, that the influenza bacillus was only found in acute influenza cases. In the past few years it has been abundantly shown, however, that the influenza bacillus can and does exist on the mucous membranes of healthy individuals.

The outbreak in an orphan asylum in New Haven has been previously described. There the probable source of the sharp outbreak of December 27th seemed to be the sister who, on convalescence, resumed her duties in the kitchen. There she inspected and handled the milk served to the children. This suggests the possibility of infection being propagated by convalescents and by food.

At present we do not know whether or not a patient remains infectious after the acute symptoms have subsided; we are ignorant as to whether a convalescent patient can transmit the disease; and we are not certain whether the organism found in healthy carriers is virulent or not. The information at hand strongly indicates that apparently healthy individuals may transmit the infection, but the wide distribution of the disease, with multiple possible sources of infection for each individual, and the relative insusceptibility of experimentally exposed individuals has made it impossible so far to answer these questions satisfactorily.

General Manner of Spread in Individual Localities.

Having discussed the mode of propagation of influenza among individuals we will follow the disease as it attacks one person after another in a community and study the epidemiologic picture, drawn no longer with the individual as a unit, but with the community as the unit.

We must here distinguish between a primary epidemic, the first wave of a progressing pandemic, and the secondary type in which may be grouped those large or small recurrences which light up for a period of one to three or more years after the primary wave.

Primary type of epidemic.—One of the first important statistical studies on this subject was that of P. Friedrich who charted the influenza morbidity in Munich between the months of December, 1889, and February, 1890. Similar observations have been made by Parsons, Raats, Linroth, and H. Schmid, following the 1889 epidemic.

Between the occurrence of the first known case of influenza and the time of the first very definite increase in influenza incidence in a community, which interval may be termed the invasion period, there is as a rule two weeks. During this period, of course, more and more cases are occurring, but remain usually sufficiently isolated to attract no public notice. From this point the epidemic develops very rapidly and reaches its peak, usually within two or at most three weeks. In another two or three weeks the incidence has fallen away nearly to normal. The epidemic period comprises from four to six weeks, or, including the invasion period, an entire duration of six to eight weeks. This is the picture produced in a community by a primary uncomplicated epidemic of influenza. Greenwood well describes the salient features of a primary epidemic as “first a rapid and quasi-symmetrical evolution, and second, a frequency closely concentrated around the maximum.” In other words the duration is short, the rise to a peak rapid, and the subsequent fall equally rapid. He showed that in the July and August, 1918 epidemic in Great Britain nearly 80 per cent. of the total incidence in the localities studied was grouped within three weeks time. His curve corresponds so well with that of the Munich epidemic that he is able to superimpose them (Chart I). The rapid rise to a peak, almost explosive in character, more characteristic of this disease than of any other, is to be explained by the high degree of invasiveness of the organism, by the short period of incubation, by the fact that many of the sick continue at their work, thus spreading the disease, and by the non-immunity of large masses of people, together with the fact that the transmission of a respiratory infection is accomplished much more easily than is any other type of infection.

The author holds that the infrequency of immunity is a most important factor in the production of this type of outbreak. The mode of transmission of influenza is the same as that of other respiratory diseases. The infectivity is probably no greater than that of measles, although that indeed is relatively great. The means of transmission are presumably the same in each. Were we able to develop an immunity for influenza of as high degree and permanence as we possess against measles, pandemics of influenza would disappear. We wish to emphasize that the primary type of curve is a phenomenon not peculiar to influenza, but that under certain circumstances it may be found in other infectious diseases, and that it would be found more frequently in the other diseases if the immunity developed against them was of as short duration as it appears to be against influenza. If, for example, measles were to break out in a large group of individuals, none of whom had had the disease, the type of curve would be the same. We will produce evidence supporting our theory under another subject. Of course, other factors such as short incubation period and unusual opportunities for spread through mildly ill individuals play a not unimportant role.