Morbidity.
There has been great actual variation in the morbidity from influenza in the various epidemics and even in different localities during single epidemics. Previous to 1889 there were no reliable statistics for the disease incidence, and subsequent to that date the records, for the reasons previously mentioned, have still been not entirely adequate.
In the history of influenza morbidity, as in that of its mortality, we must content ourselves for information prior to the nineteenth century with the very general estimates made by contemporary historians. During the last century the statistics have been more numerous and more nearly correct. As far back as the first recognized pandemic, 1510, the extremely high morbidity has been a recognized characteristic. Thomas Short in speaking of this pandemic says, “The disease ... attacked at once and raged all over Europe, not missing a family and scarce a person.”
Pasquier in 1557 spoke of the disease as common to all individuals, and Valleriola describes the widespread distribution of the epidemic throughout the whole of France during that year. It spared neither sex, age, nor rank, neither children nor aged, rich nor poor. The mortality, however, was low, “children only, dying.” Again, Thomas Short remarks, “This disease seized most countries very suddenly when it entered, catching thousands the same moment.”
Of the second pandemic, 1580, Short says, “Though all had it, few died in these countries except such as were let blood of, or had unsound viscera.”
Thomas Sydenham remarks that in the epidemic of 1675 no one escaped, whatever might be his age or temperament, and the disease ran throughout whole families at once.
Molineux recorded concerning 1693, “All conditions of persons were attacked, those residing in the country as well as those in the city; those who lived in the fresh air and those who kept to their rooms; those who were very strong and hardy were taken in the same manner as the weak and spoiled; men, women and children, persons of all ranks and stations in life, the youngest as well as the oldest.”
Schrock tells us that in Augsburg in 1712 not a house was spared by the disease. According to Waldschmidt in Kiel, ten and more persons were frequently taken ill in one house, and Slevogt says that the disease was fearful because so many persons contracted it at the same time. The disease was, however, not dangerous, for Slevogt continues: “Fear soon vanished when it was seen that although it had spread all over the city, it left the sick with equal rapidity.”
It is estimated that in the epidemic of 1729–1730, 60,000 people developed the disease in Rome, 50,000 in Mayence, and 14,000 in Turin. In London “barely one per cent. escaped.” In Lausanne one-half of the population, then estimated at 4,000, was stricken. In Vienna over 60,000 persons were affected. In the monasteries of Paris so many of the inmates were suffering from the disease that no services could be held.
Huxham is quoted in Thomson’s “Annals” as declaring concerning the epidemic of 1732–33: “Not a house was free from it, the beggar’s hut and the nobleman’s palace were alike subject to its attack, scarce a person escaping either in town or country; old and young, strong and infirm, shared the same fate.”
Finkler writes as follows concerning the epidemic of 1758: “On Oct. 24th, Whytt continues, the pestilence began to abate. He is not sure whether this was due to a change in the weather, or because the disease had already attacked most people, although the latter seems more plausible to him, particularly as he says that ‘in Edinburgh and its vicinity not one out of six or seven escaped,’ and in other localities it is said to have been even worse. In the north of Scotland also, the epidemic was greatly disseminated from the middle of October to the end of November. A young physician wrote to Robert Whytt: ‘It was the most universal epidemic I ever saw, and I am persuaded that more people were seized with it than escaped.’ This same physician reported that ‘it was not at all mortal here.’”
In the epidemic of 1762, we learn from Razoux, de Brest, Saillant, Ehrmann, that the morbidity was great while the mortality was low.
According to Grimm, nine-tenths of the inhabitants of Eisenach contracted the disease in 1767.
Daniel Rainy, of Dublin, in describing the invasion of an institution in 1775–76, tells us that from among 367 persons varying in age from 12 to 90 years, 200 were taken sick. Thomas Glass says: “There sickened in Exeter Hospital all the inmates, one hundred and seventy-three in number; one hundred and sixty-two had coughs. Two or three days after the hospital was invaded the city workhouse was attacked; of the two hundred paupers housed there only very few escaped the disease.”
Gilibert described an extraordinary morbidity in Russia in 1780–81.
Metzger says that in 1782 the Russian catarrh was so universal during the month of March that in many houses all the inhabitants were attacked. During this period, “in St. Petersburg, 30,000, and in Königsberg, 1,000 persons fell ill each day;” in Rome two-thirds of the inhabitants were attacked; in Munich, three-fourths; and in Vienna the severity of the epidemic compelled the authorities to close the theaters for eight days.
The epidemics of 1788–89, 1799–1800 and of 1802–1803 were characterized by a relatively lower morbidity than that of 1830–32, in which the morbidity was again enormous. Likewise in 1833, the morbidity was very great. In Prague “scarcely a house was spared by the plague.” In Petrograd, 10,000 persons were attacked; in Berlin at least 50,000. These are the figures of Hufeland. The Gazette Médicale records the morbidity as being four-fifths of the total number of inhabitants of Paris.
In 1836, according to Gluge, 40,000 persons suffered from the disease in Berlin alone.
In London, in the 1847 epidemic, it has been calculated that at least 250,000 individuals took sick, and in Paris, according to Marc d’Espine, between one-fourth and one-half of the population developed the disease, and in Geneva not less than one-third.
Leichtenstern informs us that in 1890 the early reports were made by clinical men and were mere presumptions. They were almost universally higher than the later statistical findings. The early estimates for the morbidity in several German cities were from 40–50 per cent. On the other hand, one of the highest statistical reports recorded by Leichtenstern was for Strasbourg in which 36.5 per cent. of the individuals became sick. The average morbidity reported by him ran between 20 and 30 per cent. The difference is accounted for in part by the fact that some of the very mild cases were not recorded in the statistics, and in part by the tendency in giving estimates, to exaggerate.
Auerbach has collected the statistics of 200 families distributed throughout the city of Cologne. He found that 149 of these families (75 per cent.) were attacked. In these, 235 were ill—59 men, 95 women, and 81 children. The larger number of women was explained as due to the illness of the female servants. He estimates each family as consisting on an average of six individuals, and concludes that 20 per cent. were taken with the disease.
Following the 1889 epidemic, Abbott concluded, on a basis of questionaires sent out to various individuals and institutions in the State of Massachusetts, that 39 per cent. of the entire population had been attacked, in all about 850,000 persons.
Moody and Capps, in December, 1915, and January, 1916, made a survey of the personnel and inmates of four institutions in Chicago, the Michael Reese Hospital, the Illinois Training School for Nurses, the Old Men’s Home, and St. Luke’s Hospital Nurses Training School, making a total of 677 persons surveyed, of whom 144 developed influenza, making a percentage morbidity of 21. They remarked that there were many others with colds who remained on duty and were not included in the table and were not diagnosed as influenza.
We have already described the relatively low morbidity and mortality in the early spring epidemic in the United States. According to Soper, the total number affected in March, 1918, at Camp Forrest and the Reserve Officers Training Camp in the Oglethorpe Camps was estimated at 2,900. The total strength at that time was 28,586. The percentage morbidity then was probably a little over 10 per cent. Dunlop, in describing the May, 1918, epidemic in Glasgow, says that it was more limited in extent, as well as milder, than the later epidemic.
It has been estimated that in the autumn epidemic in the United States Army Camps one out of every four men had influenza, and one out of every twenty-four men encamped in this country had pneumonia. During the four autumn months of 1918, 338,343 cases of influenza were reported to the Surgeon General’s Office; there were 61,691 cases of pneumonia.
Woolley reports that among the soldiers at Camp Devens, Mass. 30 per cent. of the population was affected.
At Camp Humphreys, 16 per cent. of the entire personnel developed the disease. The camp had an average strength of 26,600 individuals. Fifty-two per cent. of the entire number of cases occurred during the peak week, which ended October 4th. The outbreak began September 13th and ended October 18th.
Hirsch and McKinney report that an epidemic of unusual virulence swept with great rapidity through several organizations in Camp Grant between September 21, 1918, and October 18, 1918. During this time 9,037 patients were admitted to the Base Hospital, representing about one-fourth of the strength of the camp, and of these, 26 per cent. developed pneumonia. About 11 per cent. of the total admissions or 43 per cent. of the total cases of pneumonia died.
Referring to the report of Howard and Love, we quote as follows: “It is probable that practically all susceptible human material in infected camps suffered from an attack of the disease during the continuance of the epidemic. The records from various camps indicate that from 15 to 40 per cent. of commands suffered from an attack of the disease. These records, as previously stated, do not indicate in full the true incidence of the disease. Certain good results were accomplished in some camps by the application of effective and early isolation of patients and suspects and other measures generally recognized as of value. It was sometimes possible to retard the progress of the epidemic and cause it to be spread over a longer period of time. The epidemic thus became less explosive in character, and fewer people were under treatment at the same time. It was possible to take better care of the sick and thus reduce the incidence of and deaths from complicating pneumonia. It has not been shown that such measures accomplished reduction in the absolute number of cases of influenza occurring in one command as compared with another.
“The ‘cantonment’ group of camps gave a much higher death rate from influenza and its complications than the ‘tent’ camp or ‘departmental’ group. At first glance it would appear that the different housing conditions and the more marked overcrowding in cantonments at the time would fully account for this divergence. Closer study, however, leads to the conclusion that geographical location was a factor of equal or greater importance. It is well known that the disease was most virulent and fatal in the northern, eastern and middle west states, a district in which cantonments predominated. In the southern and Pacific coast states, where the most of the tent camps were located, a milder type of the disease prevailed, with fewer resultant fatalities. Camp Lewis, Washington, and Camp Gordon, Georgia (both cantonments), had relatively low death rates, approximating those in nearby tent camps. On the other hand, Camp Syracuse, New York, and Camp Colt, Pennsylvania (both tent camps), suffered severely and reported death rates approximating those of cantonments in the same geographical district.”
Three waves of influenza are reported by Stanley at San Quentin Prison. During the early wave it was estimated that over 500 of the 1,900 men in the prison population were ill. The wave lasted for a little over two weeks. In the second epidemic there were 69 cases in all, ten per cent. of which developed pneumonia, with two deaths. There were fewer ambulatory cases than in the first. Three and seven-tenths per cent. of the population was attacked in the second epidemic, as compared with 27 per cent. in the first. In the third epidemic there were 59 cases, with no pneumonia and no deaths.
Hernando estimates that in the Philippine Islands, 40 per cent. of the total population of 7,000,000 was stricken with the disease. The epidemic began in June, although it did not become severe until October. The group of ages that suffered most were those between ten and twenty-nine years. Hernando does not believe that the disease was imported because cases were reported before ships arrived from infected countries. After the importation of cases from elsewhere the disease assumed the more severe form.
Armstrong, in reporting a survey of 700 influenza convalescents in Framingham, Mass., remarked that 16 per cent. of the entire population were infected with influenza. Reeks, in a house survey in New Britain, Connecticut, found from among 2,757 persons that the morbidity rate reached 234 per thousand. Dr. Niven found in his block census in Manchester, England, that of 4,721 individuals, 1,108 (25 per cent.) had developed the disease. Fourteen and eight-tenths per cent. of the population were attacked in the summer and 10.4 per cent. during the autumn and winter.
Frost found in his survey of 130,033 individuals that the percentage of the population attacked varied from 15 per cent. in Louisville, Ky., to 53.3 per cent. in San Antonio, Texas, the aggregate for the whole group being about 28 per cent. He remarks that this agrees with scattered observations in the first phase of the 1889–1890 epidemic, when the attack rate seems to have varied within these limits. In five of the localities studied, geographically widely separated, the incidence rate varied only within a narrow limit, from 200 to 250 per thousand. Variations in attack rate showed no apparent consistent relation to geographic location or size of community, or to the rapidity of development of the epidemic.
In a house-to-house survey of 10,000 individuals in Boston the author found that in the winter of 1918–19, 19.71 per cent., or one-fifth of the entire population had developed the disease. It should be pointed out that while the standards used in this survey are entirely comparable to those used by Frost, the author has, contrary to Frost’s method, not included in his group of positive cases those classified as “doubtful.” This would raise the total incidence to a certain extent, but we feel convinced that by omitting the doubtful cases we have approached nearer to a correct picture of the epidemic as it actually occurred. As will be seen from Chart XVI there was no great variation in the different districts studied, with the exception of Districts IV and V. Districts I, II and III were in the tenement section of the city, while District VI was in one of the finest residential parts of Brookline. Districts IV and V were midway between these two extremes as regards economic and sanitary status, as well as extent of crowding. The lowest incidence was in the Irish tenement district. The highest in a middle class Jewish population.
CHART XV.
CHART XVI.
In the 1920 recurrence we found that 9.55 per cent., or one-tenth of the entire population, suffered from the disease, and the arrangement of districts in order of incidence was very little changed. The Irish community suffered least; the two middle class communities most. The well-to-do district in Brookline had the next lowest incidence in 1920. That the high recorded incidence in middle class districts was not due to more accurate or more thorough work on the part of the inspectors is indicated by the fact that a great part of the work on Districts IV and V was done by the same individuals who inspected Districts II and III.
One-fifth of the population studied developed the influenza in 1918–19, and one-tenth of the same population suffered in 1920.
We may agree with Winslow and Rogers, who conclude that the proportion of the population actually affected by the influenza epidemic in 1918 varied between 200 and 400 per thousand.
Relation of sex to morbidity.—Abbott concluded from his studies in 1890, that the weight of testimony appears to favor the statement that persons of the male sex were attacked in greater number and with greater severity than females. Leichtenstern reached similar conclusions. In the epidemic of 1889, the males were attacked more frequently than the females. He attributes this to two causes: first, the greater exposure to infection, and; second, the fact that strong, robust individuals are more frequently attacked.
It is amusing to compare this explanation with another found in the Medical Supplement to the Review of the Foreign Press for March, 1919. “A Spanish mission composed of Maranon, Pittaluga and Falco visited Paris last October to collect information as to the identity of the Spanish epidemic with the world pandemic of influenza. They found that the epidemics in France and Spain were absolutely identical from the epidemiologic, bacteriologic and clinical standpoint. The great majority of the severe cases in both countries occurred between the ages of 16 and 40. Both in France and Spain more females than males were attacked, which was possibly explained by the greater tendency of the former to lead an indoor existence.”
Jordan, Reed and Fink, working in Chicago, found very different results. They could discover no noteworthy difference among the pupils in high school and elementary school. The attack rate was 230 for the boys and 231 for girls. One sex was presumably as much exposed as the other.
Among the employees of the Chicago Telephone Company, on the other hand, the men were affected in considerably lighter proportion than the women (151 per 1,000 as compared with 233 per 1,000 for women). Jordan believes that the age factor was largely responsible for the difference as the women employees are as a rule of much lower average age than the men.
Frost found that with few exceptions the attack rate at all ages was somewhat higher in females than in males. The total excess of incidence in females was six per cent., which ranged from an excess of nineteen per cent. in the highest locality to a deficiency of two per cent. in the lowest. Only two of the eleven localities surveyed showed a lower incidence among females than among males.
When the sexes were compared in different age groups, the female was found to be higher than the male in each age period except under 5, 10 to 14, 40 to 44, and 70 to 74. The excess of incidence in males in these groups is relatively small, and is hardly significant in the highest age groups, where the rates are computed from small figures. Frost found the most striking excess of incidence in females occurring between the ages of fifteen and forty, the difference between the sexes being relatively slight in age periods above and below these limits. Females over the age of fifteen and especially between the ages of 15 and 45 were either more susceptible to infection, or more generally and more intimately exposed than males of corresponding age.
Our own records by the different age groups were remarkably similar. We have found an excess among the females in every age except under five years, 10 to 14, 50 to 54, and 60 to 64. In 1920 we found a slight excess among the males up to the age of 15, and again at the ages 55 to 65. Females predominated in all other ages (Chart XVII). Among those individuals who had attacks of influenza during both epidemics females again predominated except in the ages under 5 years, 10 to 14 and 55 to 59. In our own results we find that ages above 65 show a predominance of females.
After considering both series of results it is safe to generalize in saying that above the age of 15 the female sex tends to acquire the disease in slightly greater proportion than the male sex.
Chart XV shows the predominance of the female incidence in both epidemics.
CHART XVII.
CHART XVIII.
CHART XIX.
CHART XX.
CHART XXI.
Relationship of sex to severity.—In classifying cases as to severity, we have followed the standard previously described. Our results have shown that the 1920 recurrence in our group of individuals has been decidedly milder than the earlier 1918 spread. The proportion of mild cases in 1920 is nearly twice that of mild attacks in 1918–19. The proportion of severe cases was twice as great in 1918 as in 1920. The actual severity in 1918 was even greater than would be indicated by these figures. The last column in Chart XVIII is a combination of the two preceding, and while the 1920 column includes all classed as severe, pneumonia, and fatal, that for 1918 only includes the severe and pneumonia cases, but does not include the fatal cases for that year, because those who died during the 1918–19 epidemics are not counted in our 10,000 individuals surveyed. If these were included the percentage of total severe, or average severity would be greater than 42.70.
We find that in 1918 the female sex had a higher proportion of severe cases according to our standard than did the male. This was equally true in 1920. We should emphasize here that we are not comparing only the fatal cases in the two sexes, but all classified as severe, and including fatal in 1920.
Not only was the female sex attacked in slightly greater proportion, but also the individual cases appear to have been on the aggregate somewhat more severe in that sex.
Morbidity by age.—Before discussing the incidence of influenza in the various age periods we should explain that the charts for 1920 are based on the ages given by the individuals, and those for 1918 upon these ages, corrected by the subtraction of 15 months from the age as given. In our study of cases recurring during both epidemics the age used in the calculations is that of 1920. It is for this reason that in all of our age charts we have a first age period from zero to 15 months. Infants of less than 15 months at the time of our survey were born subsequent to the peak of the 1918–19 epidemic, and are not included in computations for that time.
The general similarity of the age incidence in the six districts studied (Charts XIX and XX) is evident. As a rule two peaks can be discerned, one falling somewhere between 15 months and 9 years, and the other between 20 and 39 years. There are individual variations in the different districts, and in Districts IV, V and VI there is a tendency toward a peak in the period 55 to 64. This, however, disappears when the total 10,000 is tabulated, when the two peaks, 15 months to 9 years, and 20 to 39, show out clearly for the year 1918 (Chart XVII).
Frost found for the same epidemic that the attack rate was highest in the age group 5 to 9, declining with almost unbroken regularity in each successive higher age group, with the exception of the groups 25 to 34, in which the attack rates were higher than in the age groups 15 to 24, but not as high as that of 5 to 9.
Both series of observations agree in finding relatively high incidence in early childhood and in early adult life.
For 1920 (Chart XVII) we find that these peaks, although present, have become decidedly less prominent, and that there is a relatively higher incidence in individuals past the age of 40 (Charts XXI and XXII). There is some tendency toward straightening out of the curve; age appears to have played a less important part, and those higher ages which were relatively insusceptible in 1918 have become more susceptible in 1920. We cannot generalize in the statement that all ages which were lightly attacked in the first epidemic were more severely attacked in the 1920 spread, because the ages from 10 to 19 are found to be relatively lower during both epidemics.
Other observations have been made regarding the age incidence particularly during the 1918 pandemic. Jordan’s figures for the October epidemic show a higher incidence among school children of ages 4–13 than among those of higher school age, 14–18. The teachers in these schools had a lower attack rate than the pupils. The pupils in both school groups were from the same section of the city and to a large extent from the same families and were presumably exposed in similar degree.
Lynch and Cumming found that of 49,140 children in public institutions the influenza rate was 412 per 1,000, while among 703,006 adults in similar institutions the rate was 263 per 1,000. These figures include children in a large number of institutions scattered throughout the United States, and would indicate that in childhood the susceptibility is much greater than in adults.
Many writers agree that nursing infants show a relative insusceptibility. However Abt records a case of an expectant mother who, within two weeks of term, developed influenza, and during the course of her illness gave birth to a baby boy, who at birth was found to be suffering from bronchitis and bronchopneumonia, but who lived for three days, finally dying of bronchopneumonia. Abt concludes from a review of all of the facts that the newly born infant had influenza and that the baby had become infected before birth.
CHART XXII.
According to Carnwath, the age incidence showed curious changes. During the 1918 summer wave the ages most affected were 15 to 45. In the winter of 1918–19 there was a considerable shifting toward the extremes of life and particularly toward the younger years. The susceptibility of young children was the subject of a special inquiry in London. Though the attack rate was below the average, the chances of recovery were less than in other age groups. Of breast-fed infants, 30 per cent. contracted the disease; of artificially-fed 54 per cent. The opposite, however, occurred in lying-in homes. An inquiry in Cheshire revealed that 25.4 per cent. of expectant mothers affected died.
Renon and Mignot have made a report on the 1920 recurrence. According to them the grip of 1920 attacked all ages, in contradistinction to the 1918 epidemic, which attacked especially the young and vigorous. One-third of their group were over 40 years of age, while some were 70 and 80 years old. In spite of this the disease remained relatively mild.
Age morbidity in previous epidemics.—Previous to the epidemic of 1889–93, the various recorded observations regarding morbidity, and particularly regarding age morbidity, have consisted often of records made by practising physicians, and are merely estimates based upon their clinical experience and varying with the type of individual treated by them. Or else they have been records made by non-medical historians. During the 19th century, the tendency toward statistical enumeration becomes more and more prominent, but the first statistical studies of real value to the epidemiologist were made in the epidemic of thirty years ago. Statistical study must begin with this last epidemic. Observations of the earlier epidemics, while very interesting for reference and comparison, are no longer acceptable as unquestioned statements of fact. Even at the present time and with all of the emphasis that is now being laid upon statistical procedure the records are far from perfect, and it is to be hoped that in years to come the improvement will be so decided that the records even of the 1918–20 epidemics will appear crude.
Buoninsegni remarks of the 1387 epidemic that many individuals of all ages died, but the deaths were particularly prevalent among the aged.
Jacob, of Königshofen, writes that “there came a general pestilence in the whole country, with cough and influenza, so that hardly one among ten remained healthy,” and that old and debilitated persons were frequently the victims.
Balioanus tells us that the epidemic of 1404 let not rank, age nor sex escape its effect.
In 1557, according to Valleriola, the disease appeared with pestilential rapidity, and spared neither sex, nor any age, nor rank, neither children nor old persons, rich nor poor, but that it was not as a general rule dangerous; children only, who could not freely cough out the phlegm, dying.
The same story is told by Molineux, for 1693, “All conditions of persons were attacked, those residing in the country as well as those in the city; those who lived in the fresh air and those who kept to their rooms; those who were very strong and hardy were taken in the same manner as the weak and spoiled; men, women and children, persons of all ranks and stations in life, the youngest as well as the oldest.” Molineux, however, added that, “it rather favored the very old who seldom were attacked with it.”
These observations are but broad generalizations; if we pause to study the psychology of the historian we are tempted to conclude that his primary object was to impress his readers with an idea of the enormousness of the dissemination of the disease during his period. That being the main endeavor, a tendency to exaggerate for the sake of rhetoric and yet remain within the limits of truth may be considered excusable. But during the 1889–93 epidemic there was ample opportunity to compare the estimates made by the practising physicians with the later statistical tabulations. As a rule the former were higher both as regards morbidity and mortality.
In the 1830–32 epidemic an interesting observation has been made. While Kahlert says that no distinction between age, sex nor rank occurred, Leberscht stated that persons of middle age, especially women in the climacteric period, were attacked with special frequency. This is of interest in view of the findings in the 1918 epidemic. Krimer states for the same epidemic that children under 14 years of age and adults over 45 years were spared by the epidemic.
For the 1836–37 epidemic Finkler records the following: “Most of the patients were adults from 20 to 40 years of age, and of these more women than men were attacked. Curiously, however, the physicians of Würtenberg speak of the great dissemination of the disease among children.”
In 1847–48, among the adult influenza patients, there are said to have been more women than men. According to Canstatt, there were proportionately more children than adults attacked.
In 1889–90, according to Finkler, no age was spared, but persons between 20 and 50 years of age were attacked by preference. No trade was a sure protection. The course of the disease in general was favorable and also quite rapid, unfavorable only in many children during the first few years of life, in many old people, in many debilitated persons, and especially in those suffering from chest affections.
An interesting table of this kind is given us by Leichtenstern. His hospital material included 439 influenza patients, and these he carefully grouped according to age.
| Age. | Influenza admissions, per cent. | General average of admissions, per cent. |
|---|---|---|
| Under 10 years | 0.9 | 0.7 |
| 10–20 years | 14.7 | 8.8 |
| 20–30 years | 40.3 | 27.5 |
| 30–40 years | 19.1 | 23.3 |
| 40–50 years | 10.1 | 15.7 |
| 50–60 years | 7.4 | 12.3 |
| 60–70 years | 5.3 | 8.9 |
| 70–80 years | 1.7 | 2.6 |
| Above 80 years | 0.4 | 0.2 |
Comby found that in Paris only the new-born were noticeably insusceptible to influenza, that children up to 15 years were attacked in the proportion of 40 per cent., and adults in the proportion of 60 per cent. Danchez believed that in families in which all the adults became ill, the little children usually escaped.
Finkler states that in the schools at Bordeaux the older children were first and most frequently attacked. Of the 248 male and female teachers in 41 schools, 153 (61.7 per cent.) developed the disease. Children up to five or six years of age at any rate seem to have been very little affected, while older children were no less susceptible than adults.
Among 47,000 cases of influenza treated by physicians in Bavaria in 1889, the various ages were as follows:
| 1 year | 1.5 per cent. |
| 2–5 | 5.4 per cent. |
| 6–10 | 6.6 per cent. |
| 11–15 | 7.2 per cent. |
| 16–20 | 11.4 per cent. |
| 21–30 | 22.2 per cent. |
| 31–40 | 19.3 per cent. |
| 41–50 | 12.6 per cent. |
| 51–60 | 7.7 per cent. |
| 61–70 | 3.6 per cent. |
| 71–80 | 2.0 per cent. |
| Above 80 | 0.5 per cent. |
Leubuscher recorded that in Jena the proportion of cases in the individual age classes did not correspond with the figures reported from other localities. Children, and especially very young children, suffered relatively less than adults.
The following statistics of the 1889–90 incidence of influenza among school children in Cologne were collected by Lent:
| Attendance. | Ill of influenza. | |||
|---|---|---|---|---|
| Class | I–13 | to 14 years of age | 3,002 | 1,015 33.8 per cent. |
| Class | II–11 | to 12 years of age | 5,737 | 1,835 31.9 per cent. |
| Class | III–10 | years of age | 3,701 | 1,130 30.5 per cent. |
| Class | IV– 9 | years of age | 3,590 | 930 25.9 per cent. |
| Class | V– 8 | years of age | 2,929 | 822 28.0 per cent. |
| Class | VI– 7 | years of age | 3,388 | 758 22.3 per cent. |
These may be compared with figures for the public schools in the suburbs of Cologne:
| Attendance. | Ill of influenza. | |||
|---|---|---|---|---|
| Class | I–13 | to 14 years of age | 1,609 | 689 42.9 per cent. |
| Class | II–11 | to 12 years of age | 2,885 | 1,094 37.9 per cent. |
| Class | III–10 | years of age | 1,683 | 626 37.1 per cent. |
| Class | IV– 9 | years of age | 1,758 | 552 31.4 per cent. |
| Class | V– 8 | years of age | 1,771 | 502 28.2 per cent. |
| Class | VI– 7 | years of age | 1,938 | 510 26.3 per cent. |
The increase of disease incidence with age is apparent. Finkler’s explanation for the higher incidence among the children of the suburbs, “that the children in the country had usually to walk a greater distance to school” does not appear to be complete.
Comby found that out of 3,411 school children in Lausanne 1,840 contracted influenza. This shows a relatively high incidence in children of school age in that city.
Concerning age distribution in 1889–90 Leichtenstern remarks that the greatest morbidity incidence was in school children, adolescents and young adults, especially the last. Nursing infants were attacked in considerably less degree than any of these other ages. Also in the higher ages those above sixty were attacked in lesser degree. The greatest morbidity frequently was between the ages of twenty and forty. Abbott concluded on the basis of estimates furnished him from various institutions and individuals in the State that people of all ages were attacked but the ratio of adults was greatest, of old people next, and of children and infants least.
Relationship of occupation to morbidity incidence.—Leichtenstern found that the only apparent influence of occupation on the incidence of influenza depended upon the liability to exposure in the various occupations. He remarks particularly on the large incidence of influenza among physicians. In contrast was the low incidence in lighthouse keepers. In 1889–90 among 415 dwellers on 51 lightships and 20 isolated lighthouses on the English coast only 8 persons developed influenza and these in four localities, and in every instance there was traceable direct communication from some other source. There is contradictory evidence as to whether individuals working out of doors are more apt to develop influenza. Certain statistics show that postmen and individuals working on railroads were attacked more frequently and earlier than others, while other statistics show that in railroads the office personnel was attacked earlier than individuals on the trains and those working on the tracks.
Abbott concluded that special occupations did not appear to have had a marked effect in modifying the severity of the epidemic. At the Boston Post Office in 1889–90, of the indoor employees, 475 in number, 25 per cent. were attacked. Of the carriers, 450 in number, 11 per cent. were affected with the disease. But there were other reports of the same period which stated that the ratio of the persons employed at outdoor occupations who were attacked was greater than that of indoor occupation.
Finkler has discussed the influence of occupation at some length:
“When we compare the statistics of the last pandemic concerning the influence of vocation, we see in the first place that those first and chiefly were attacked whose occupation compelled them to remain in the open air. This was shown especially by Neidhardt, who studied the influenza epidemic in the Grand Duchy of Hesse. His conclusions, however, were disputed by others. Thus, the prejudicial influence of exposure to the open air was not supported by the statistics of railroad employees in Saxony. Of those who were employed in the outdoor service, 32 per cent. became ill; of those employed in office work, on the other hand, 40 per cent. The statistics of the local benefit societies in Plauen show that the percentage of the sick among farm hands and builders was not greater than that among the members of other benefit societies who worked indoors. In Schwarzenberg the laborers in the forest who were working in the open air all day were affected less than others, and there was no sickness whatever in some forest districts. Lancereaux, of Paris, states that most of the railroad employees who suffered from influenza were those engaged in office work and not those who worked in the open air. The preponderance of influenza patients among the factory hands may be seen from a table prepared by Ripperger:
| A. In the open air. | |
|---|---|
| Occupation. | Per cent. attacked. |
| Workmen and laborers of Niederbayern | 7 |
| Railway officials in Amberg | 9 |
| Peasants in Niederbayern | 11.7 |
| Workmen in the Salzach-Correction | 20 |
| B. In closed rooms. | |
| Slag mills in St. Jugbert | 15 |
| Cotton mill in Bamberg | 20 |
| Cotton mill in Bayreuth | 33 |
| Sugar factory in Bayreuth | 36 |
| Aniline works in Ludwigshafen | 38.8 |
| Cotton mill in Zweibrücken | 50 |
| Tinware factory in Amberg | 60 |
| Factory in Schweinfurth | 62 |
| Gun factory in Amberg | 70 |
| Gold beaters in Stockach | 80 |
“Many peculiar records of how individual classes of occupation have fared are obviously to be explained by the fact that the infection manifested its action in very different degrees. Thus, among the workmen on the Baltic ship-canal only those became ill who lived in the town of Rendsburg; those who had been housed in barracks outside of the city were not affected. Of the 438 lead workers of Rockhope, which is situated in a lonely valley in Durham, all remained perfectly free from the disease during the three epidemics of 1889–92.
“Some occupations are said to afford protection against influenza. Thus workmen in tanneries, chloride of lime, tar, cement, sulphuric acid, glass, and coke works, are said to have escaped the disease with extraordinary frequency.
“We shall be compelled perhaps to agree with Leichtenstern in his conclusion that occupation and social position only in so far exert an influence on the frequency of the disease as certain occupations in life lead to more or less contact with travellers.
“Very remarkable is the proportionately small number of soldiers affected, at least in the Prussian army, where, according to the official record, the epidemic from its beginning to its end attacked only 101.5 per thousand of the entire forces.”
Comparison of morbidity by occupation necessarily includes so many variables and so many factors other than occupation that the results are decidedly unsatisfactory. An example is found in Jordan, Reed and Fink’s report of the incidence among troops in the Student Army Training Camps in Chicago. They found a strikingly different attack rate in the various groups studied. In the Chicago Telephone Exchange they ranged from 30 to 270 per 1,000, although the working conditions in the various exchanges were not materially different. In the Student Army Training Corps at the University of Chicago the lowest was 39 and the highest 398 per 1,000. The higher rate group was particularly exposed to infection while the lower, although composed of men of similar ages, living under similar conditions, were guarded to a considerable extent against contact with beginning cases.
Woolley has made an interesting observation on the effect of occupation: “The disease was no respecter of persons except that it was more severe in those who were hard workers. Those who tried to ‘buck the game’ and ‘stay with it’ showed the highest mortality rates. So, the non-commissioned officers and the nurses suffered more severely than the commissioned officers and privates.
“The annual morbidity rate per 1,000 was as follows:
| For | commissioned officers | 261 |
| Non-commissioned officers | 208 | |
| Nurses | 416 | |
| White enlisted men | 568 | |
| Black enlisted men | 1,130 |
“The annual mortality rate per 1,000 was:
| For | commissioned officers | 69 |
| Non-commissioned officers | 83 | |
| Nurses | 77 | |
| White enlisted men | 145 | |
| Black enlisted men | 253 |
“The case mortalities were:
| Per cent. | ||
|---|---|---|
| For | commissioned officers | 26.8 |
| Non-commissioned officers | 40.0 | |
| Nurses | 33.3 | |
| White enlisted men | 26.0 | |
| Black enlisted men | 22.5 |
“The above figures are for the period of five weeks from August 28th to October 1st, 1918, and cover the most active portion of the epidemic, but are obviously incomplete. They are given for purposes of comparison.”
Woolley makes the observation that the organizations which spent most of the time in the open and which were therefore most exposed to the weather suffered least during the epidemic. This was particularly true in the Remount Depot.
In our work we have attempted to classify our population according to occupation along very broad lines.
“Infant” includes all individuals up to the age of two years. In these the exposure is limited by the fact that they are either relatively isolated at home, or when abroad, are still under relative isolation in a perambulator or under the eye of a nurse. There is relatively little commingling with the older age groups.
“Child,” refers to all children up to the age of school years. There is relatively much greater commingling, particularly with other individuals of the same age.
“School” refers to all children and adolescents who were reported as attending school.
“Home” includes not only the housewife, the housekeeper, but also servants and invalids; all who in their daily routine spend the greater part of the time in the home.
“Manual Indoors” refers chiefly to laborers in factories and includes all manufacturing occupations in which the work is of a manual character no matter what the particular branch.
“Manual Outdoors” refers to such occupations as ditch diggers, street cleaners, conductors and motormen, longshoremen, trucksters and teamsters, telephone and telegraph linemen, etc.
“Retail Sales Indoors” refers to clerks in stores and all other individuals who, working indoors, come into about the same degree of contact with the public-at-large.
“Retail Sales Outdoors” includes sales agents, life insurance agents, traveling salesman, pedlers, newsboys, etc.
“Office,” officials, secretaries, stenographers, telephone operators, telegraph operators, etc.
We have observed that in 1918 infants presented the lowest incidence and school children the highest. Occupations designated Home and Office were surprisingly high. Children also showed a high incidence, one out of every five developing the disease. The records show that manual labor, both indoors and outdoors, was associated with a higher incidence than less strenuous work, as retail sales, indoors and outdoors (Chart XXIII).
The attack rates in most of the occupations are so nearly the same as to lead to no certain conclusions. It would appear from our records that individuals working out of doors were less frequently attacked than those whose occupation kept them in doors. The groups at the two extremes of incidence correspond to what we should expect when considering opportunities for contract. The infant has least direct contact. His contact is only with one or a few individuals, the mother or the nurse. This group developed the disease in 5.8 per cent. The school child not only has the same degree of contact as do adults, but also in the tussle and scramble of play the contact becomes much closer. The factor of age plays a large part in the occupational distribution and the apparent occupational susceptibility is influenced by the age susceptibility.
When we consider the occupational incidence in the various districts we find that the only constant feature in the relatively small groups is the low incidence among infants (Chart XXIV).
CHART XXIII.
CHART XXIV.
CHART XXV.
CHART XXVI.
The first fact gained from a study of the 1920 occupational case rate is that just as was the case in age incidence there is less variation between the highest and the lowest than in 1918–19. While in the first epidemic the highest occupational rate was five times the lowest, in the second it was only twice the lowest (Chart XXV). But at the extremes of the two charts we see some tendency to an inversion of the order. In 1918–19 those occupied in “retail sales” outdoors showed a low incidence, while in 1920 they were the highest. So also, the incidence in the school group changed from highest in 1918 to lowest in 1920. The incidence in infants increased; that in the office workers decreased. No general conclusions are warranted from these results.
In comparing the sex incidence by age groups we have found that females as a rule showed a slightly greater incidence than males. That this is not due fundamentally to occupational differences is suggested by a comparison of the sex incidence in the two epidemics studied. In 1918 the distribution is practically the same in the two sexes in all occupations except “Home,” “Manual Outdoors,” “Retail Sales Indoors,” “Retail Sales Outdoors” (Chart XXVI). In the first the number of males is so small and in the second and fourth the number of females is so small that these cannot justly be compared. The group, “Retail Sales,” consists in 1918–19 of 69 males and 27 females, out of a total distribution in the population of 426 males and 107 females. This is the only occupation that showed a definite higher incidence among the females, and even here the number is too small for accuracy. In 1920 this difference has practically disappeared.
Effect of race stock.—Leichtenstern remarks in his monograph that the reported differences in influenza morbidity among different races, such for instance as European and other nationalities, doubtless are due to factors other than genetic racial differences, such as different modes of living, commerce, etc. The work of the last two years calls for a reconsideration of this idea.
Frost in his valuable work found that “in the seven localities with considerable colored population the incidence rates among the colored were uniformly lower than among the whites, the difference persisting after adjustment of the rate to a uniform basis of sex and age distribution. The extent of the difference varied, being relatively great in Baltimore, Augusta and Louisville, and very small in Little Rock. This relatively low incidence in the colored race is quite contrary to what would have been expected a priori, in view of the fact that the death rate from pneumonia and influenza is normally higher in the colored than in the white, and that the colored population lived generally under conditions presumably more favorable to the spread of contact infection.”
Brewer, in his study of influenza in September, 1918, at Camp Humphreys, finds that the colored troops showed a decidedly lower rate than the white troops throughout the epidemic. He finds that the incidence among colored troops was only 43 per cent. of that among whites. The difference between colored and white organizations was probably not due to difference in housing. Most of the colored troops were in tents and the white troops were all in barracks. But the 42d Company composed of negroes was housed in barracks under the same conditions as the white troops of other organizations and they had next to the lowest incidence of all organizations. Brewer concludes that the colored race when living under good hygienic conditions is not as susceptible to influenza as the white race under the same conditions. The age distribution was the same in both groups.
Armstrong concluded from figures based on reported cases of influenza that in the autumn of 1918 proportionately four times as much influenza and pneumonia was reported among the Italians as was reported for the rest of the Framingham community, made up largely of Irish or Irish-American stock. On the contrary, an examination of a large proportion of the population of that town showed a tuberculosis incidence among the Italian race stock of .48, in contrast to an incidence among the Irish of 4.85 per cent. and of 2.16 per cent. in the entire population. Armstrong contrasts the relative insusceptibility of Italian stock to tuberculosis, with the apparent marked susceptibility to acute disease of the respiratory tract, such as influenza and pneumonia; and the high susceptibility of the Irish to tuberculosis, with their low susceptibility to acute respiratory infection.
With regard to our work it is sufficient to state that the lowest incidence in both epidemics, as well as in recurrent cases, was in the Irish tenement districts. Both the Jewish and the Italian tenement districts were slightly higher in both epidemics (Charts XIX and XXI). The age distribution of the entire population of each of these three districts was about the same, so it does not appear that the slightly lower incidence among the Irish is due to a variation in the age distribution of the population.
The subject of race in relation to influenza will be discussed further under mortality.