| Type of exposure in 1920. | Per cent. of exposed individuals who had had influenza in 1918–19 and who contracted it again, per cent. | Per cent. of those who had not had a previous attack, and who on exposure contracted influenza, per cent. |
|---|---|---|
| “Sleeping” | 27.0 | 31.0 |
| “Room” | 18.3 | 17.6 |
| “Family” | 12.0 | 11.2 |
On the whole there is no evidence of protection afforded by a previous attack.—Individuals who had had the disease before succumbed to a second attack in the same proportion as those who had not previously had influenza.
Recurrent cases.—In certain families there were individuals who had had influenza during both the 1918 and 1920 epidemics. Were these recurrent cases the first ones to occur in the family, or did they, as a rule, follow other cases in the same household? We have records of 236 recurrent cases in which we know the order of occurrence of the various cases in the family. Out of this total number 57 were the initial cases in the household. One hundred and nineteen were the only cases occurring in the family. Therefore 176 or 74 per cent. of the total number of recurrent cases were either the first or the only cases in the family. Sixteen recurrent cases followed between other cases and 44 occurred as the last of a series of two or more in the household.
SECTION V.
Immunity.
Opinions of all observers who have studied in detail the question of immunity in influenza are remarkedly in accord. The conclusions reached by Parkes in 1876 are valid today, and form as excellent an abstract of our present knowledge as any produced since his time. “There is some discrepancy of evidence, but, on the whole, it seems clear that, while persons seldom have a second attack in the same epidemic (though even this may occur), an attack in one does not protect against a subsequent epidemic. Indeed, it has been supposed rather to render the body more liable.”
In 1890, Abbott wrote: “There is but little if any evidence in support of the protective power of one attack to confer immunity against a second; and hence adults are not exempt, as they usually are in epidemics of scarlet fever or other exanthemata; so that the proportion of adults to children attacked in an epidemic is necessarily greater than that which is observed in epidemics of other infectious diseases.”
Parsons made somewhat similar observations: “One attack of influenza does not seem to be protective against another; the disease in this respect resembling diphtheria, erysipelas, and cholera rather than smallpox, measles, or whooping cough. The duration of the epidemic in a locality is so short that it is difficult to distinguish between second attacks properly so-called, and relapses, which are frequent enough. A case is recorded in the ‘British Medical Journal’ of February 15, 1890, in which a patient who had suffered from influenza in France in December, 1889, had another attack in England in January, 1890. It was noticed in 1837 that many persons suffered from influenza who had had the disease during the previous epidemic in 1834. The shortness of the interval between these two epidemics, as compared with that between 1848 and 1889, seems to show that the periodical return of the disease in an epidemic form does not depend upon the accumulation in the interval of susceptible individuals unprotected against the disease by a previous attack. If one attack afforded protection against another a large proportion of the population in 1837 must have been protected, yet an epidemic occurred, and on the other hand for many years before 1889 a large majority of the population must have been unprotected by a previous attack, yet the epidemic did not recur.
“The persons now living who passed through the disease in 1847 are of course comparatively few, but such persons have not been exempt from the present epidemic.
“I should be inclined to attribute the short duration of the influenza epidemic in a locality to the establishment of a tolerance for the specific poison among the persons exposed to it, similar to the tolerance for dust possessed by workmen in rag factories, as mentioned, but which is soon lost on their ceasing to be exposed to it, rather than to a true immunity being established.