The Inspector General of Health, in Spain, reported in January, 1918, that those cities which had the disease in May, 1918, suffered lightly in the autumn of that year, while others of the large cities which had been spared in the first invasion suffered most in the second.
Maillard and Brune report an epidemic of influenza in an epileptic colony. There were 32 deaths among the 63 cases. None of the inmates of the hospital who had influenza during the June epidemic contracted it anew during the October wave.
Ovazza records that although a number of persons contracted the influenza anew on its return in the fall after having had it in the spring, yet the return cases were strikingly mild, and always free from complications.
Barthélemy describes the successive waves of epidemic influenza at Bizerte. He found that the doctors and nurses who had been through the first epidemic did not develop influenza in the second one a few months later, even though they came in the closest contact with the patients.
Hamilton and Leonard have studied two successive outbreaks due to lapses in a rigid quarantine in an institution of 180 girls between 12 and 18 years of age. The girls were distributed through six cottages. In the first epidemic November, 1918, 76 girls contracted the disease, at which time it was entirely limited to the occupants of cottages 2, 3, and 4. The second outbreak occurred in January, 1919, when 82 took ill. Only five of these were located in cottages 2 and 4, the remainder being in 1, 5 and 6. No cases occurred in cottage 3 during the second spread. Both epidemics lasted a little under two weeks. Those who had suffered in the first spread appeared to be immune to the second. There were no recurrences. The second epidemic was much milder in character. Twelve per cent. of the total remained well throughout both epidemics.
Dr. Niven, in his study of 1,021 households previously described, found that 105 families suffered in both the summer and autumn 1918 epidemics. “They comprised a population of 565 persons, of whom 205 suffered in summer and 360 escaped. In the autumn epidemic eighty-two (or 40 per cent.) of the presumably ‘protected’ persons succumbed again, whereas only 120 (or 33 per cent.) of the ‘unprotected’ suffered. Of the former, however, only one died, while five of the latter terminated fatally. These are interesting figures. If they are borne out by subsequent inquiry, they are somewhat difficult of explanation. The persistent susceptibility to the primary disease and yet comparative immunity from the fatal sequel, would seem to suggest a dual infection, against one element of which the body is able to produce protection, while it is unable to do so against the other.”
Frost made a canvass of 33,776 individuals in Baltimore between November 20th and December 11th, 1918. The same population was again covered in January, 1919, to determine the extent of the recrudescence reported in December. Among 32,600 people, 724 cases of influenza had occurred in the interval since the first survey. Of this number only 26 or 3.6 per cent. were definitely cases of second attack in the same individual. Even in these cases the diagnosis is necessarily uncertain. Frost says that considering that 23 per cent. of the population had had influenza prior to December 11th, the proportion of second attacks should have been much greater if no immunity had been acquired. A second canvas in San Francisco gave generally corresponding results.
Our own experience was quite similar. We have divided the whole period from March, 1918 to March, 1920, into two portions separated at August 1, 1919. In the first portion we have knowledge of but four individuals suffering from what the records would indicate to be two genuine attacks of influenza. Similarly, five individuals appeared to have had two attacks within the second interval. These are to be contrasted with a total incidence in the fall and winter of 1918–1919 of 1,971 cases, and in the winter of 1919–20, of 965 cases. Among the total nine individuals the intervals between attacks varied from 26 days to five months. All except one had an interval of one month or over. In two cases there was an interval of one month, in one an interval of two months, in two an interval of three months, in one of four months, and in one of five months. None of the four individuals who had two attacks in the first group of months had a subsequent attack in the second. On the contrary, two of the five suffering two attacks in the second group of months had one previous attack in the first. The second attack, following the first by a relatively short interval tended to be milder than the first. In five out of the entire nine the second attack was milder, in two it was of the same degree of severity, and in only two was it more severe than the first. The order of severity in the two individuals having three attacks each was, in the first, severe, mild, severe; in the second, severe, average, average.
Zinsser makes the following remark: “The writer himself believes that he had three attacks during the last epidemic. The first and second were mild ones and the third complicated and therefore severe; and innumerable others with whom he has spoken have had similar experiences.”
From a consideration of these reports by divers authorities it is reasonable to conclude that for a period of a few months at least, one attack of influenza protects against a second. As is to be expected, this relative immunity is not of constant duration in all individuals. If there were no lessened susceptibility following an attack we would be faced with the phenomenon of individuals succumbing time and again to rapidly successive attacks of the disease. Such a circumstance is very rare.