Influenza

by Warren T. Vaughan

Death rates from all causes by weeks in certain large cities of the United States during the winter of 1918–19. (Pearl.)

Concerning geographical position, he did find some slight relationship with linear distance from the city of Boston, where the epidemic was supposed first to have begun in this country:

“This result means that the greater the linear distance of a city from Boston the less explosive did the outbreak of epidemic mortality in that city tend to be. This is in accord with the general epidemiological rule that the force of an epidemic tends to diminish as it spreads from its primary or initial focus. It must be noted, however, that the correlation coefficient in this case is not large. It is barely past the value where it may safely be regarded as statistically significant. This fact may probably be taken to mean that influenza does not follow the epidemiological law referred to with anything like such precision as do some other epidemic diseases, notably poliomyelitis.”

These factors having been found to be of little value in his attempt to explain the varying curves in the 39 different cities, Pearl next correlated the explosiveness of the epidemic mortality with deaths from all causes, deaths from pulmonary tuberculosis, from organic heart disease, from acute nephritis and Bright’s disease, from influenza, from pneumonia (all forms), from typhoid fever, from cancer and from measles, in the various cities.

“The outstanding fact which strikes one at once from this table is the high order of the correlation which exists between the explosiveness of the outbreak of epidemic mortality in these communities and the normal death rate from certain causes of death in the same communities. In the first four lines of the table the correlation coefficients range from about 6 to more than 10 times the probable errors. There can be no question as to the statistical significance of coefficients of such magnitude.

“The highest correlation coefficient of all is that on the first line of the table, for the correlation of epidemicity index with death rate from all causes. The existence of this high correlation at once indicates that an essential factor in determining the degree of explosiveness of the outbreak of epidemic influenza in a particular city was the normal mortality conditions prevailing in that city. In the group of communities here dealt with, those cities which had a relatively high normal death rate had also a relatively severe and explosive mortality from the influenza epidemic. Similarly, cities which normally have a low death rate had a relatively low, and not sharply explosive, increase in mortality during the epidemic.

“It will also be noted that the correlation in the next three lines of the table, namely those of pulmonary tuberculosis, so-called, organic diseases of the heart, and chronic nephritis and Bright’s disease, are of the same order of magnitude as that between the death rate from all causes and the explosiveness of the epidemic outbreak of influenza.”

Pearl suggests that this correlation might arise because of differences in the constitution of populations in the different cities, or, that it was a factor of geographical position, such as the distance from the Atlantic seaboard; but that even after correction of the results for age distribution and geographical position, the net correlations were actually higher than were the gross uncorrected correlations.

“We may conclude that the most significant factor yet discovered in causing the observed wide variation amongst these 39 American cities in respect of the explosiveness of the outbreak of epidemic influenza mortality in the autumn of 1918 was the relative normal liability of the inhabitants of the several cities to die of one or another of the three great causes of death which primarily result from a functional breakdown of one of the three fundamental organ systems of the animal body, the lungs, the heart and the kidneys.”

Winslow and Rogers studied the relation of the pneumonia death rate from 1901 to 1916 to the influenza death rate of 1918 in 40 large cities of the United States and found a distinct correlation. The cities which have been characterized by a high pneumonia rate in the past are precisely the cities which suffered most severally in the 1918 outbreak. This is not due especially to virulent types of pneumonia organisms in certain sections of the country because they found this same high correlation between total death rates and influenza death rates, in the same cities.