“During the summer this camp received 20,000 recruits. In October, 1918, the virulent form of influenza struck this camp. It confined itself almost exclusively to the recruits of the summer and scarcely touched the men who had lived through the epidemic of April. Not only the 2,000 who had the disease in April, but the 24,000 who apparently were not affected escaped the fall epidemic. It appears from this that the mild form of influenza of April gave a marked degree of immunity against the virulent form in October. There is another observation which points the same way. Looking over the statistics of the fall epidemic in cities in the United States we find that certain cities had a low death rate, while others had a relatively high rate. Among those cities which had a low death rate we will mention Atlanta, Ga.; Kansas City, Mo.; Detroit, Mich., and Columbus, Ohio. Going to the spring records of these cities we find that in all of them in March and April of 1918 there was an unusually high death rate from pneumonia and undoubtedly in these cities at that time there was a relatively mild epidemic of influenza. In this way I am inclined to account for the relatively low death rate in these cities in the fall of 1918. I make no claim that this and other instances of a similar kind prove that the mild and virulent forms of influenza are manifestations of the same disease, but I do hold that the evidence points that way.”
Lemierre and Raymond report the following observation in favor of the development of a certain degree of immunity in the French troops in April, 1918. After an intervening period of quiescence there was a manifest recrudescence at the end of August. Many military formations were attacked during both periods. This was true especially in three groups of an artillery regiment under their observation. In the first of these groups there were three cases in April, while 114 men were attacked in August. In the third group there were 100 cases in April and only 3 in August. In the second group there were 20 cases in April and 59 in August. Their report does not state the total number of individuals in each of the three groups.
Joltrain and Baufle discuss the flaring up of the epidemic in October, and relate that a troop of soldiers from Indo-China nearly all had the disease lightly in the spring, but when the disease appeared again it spared this troop completely, while troops and civilians around developed it in a severe form.
Gibbon writes: “During the last three waves of the epidemic I had to deal with the sick of 2,000 troops, and during this time we treated in hospital over 400 cases. No cases admitted in June, July or August were re-admitted in October, November, or December, and no cases admitted in either of these two periods were re-admitted in February this year. Unfortunately I am unable to trace the cases into March as the troops were changed.”
Dopter reports recurrent epidemics of influenza in a French Army Division in 1918. The division, of which he was surgeon, was one of the first to contract la grippe at the time of its first appearance in the zone of the armies at the end of April, 1918. At this time nearly the entire body of infantry troops was attacked. The disease was mild, and without complications. The regiment of artillery escaped nearly entirely. This epidemic subsided very rapidly, and by the end of May it had entirely disappeared. Early in August a group of heavy artillery was attached to the division, bringing influenza with it. Then a few cases appeared in the regiment of light artillery which had hitherto escaped. By the end of August all three groups of this regiment had been attacked. In this second epidemic the men who had come through the first unattacked were very severely ill in the second.
With rare exceptions those sick in the first did not contract it again. Dopter notes that in the battery the most severely affected in August, of which the effectives were reduced almost to none, only those men were considered well enough for duty who had had influenza in the first period. They escaped the second in spite of the close contact with their comrades. The infantry regiments, which were in close association with the artillery, remained unaffected.
Finally, toward the middle of September new troops were attached to the division, in view of an imminent attack by the enemy. These troops, coming from neighboring and distant formations were suffering at the time from grip, and continued to have the disease in the new sector. Again, those attacked in May passed without damage through this new epidemic. Among them there were only rare isolated mild cases. The recurrences made only 1.6 per cent. of the total incidence.
Opie and his associates found that at Camp Funston after the first wave of influenza in March and April, 1918, the succeeding waves usually affected only new recruits, who had not been in camp during previous waves.
In Calcutta influenza appeared as an epidemic in July, 1918, and in November, 1918. During the first quarter of 1919, at Calcutta as elsewhere, many cases were still recurring. Malone investigated the incidence of the disease in three institutions of Calcutta: He found that in the Gourepore Jute Mills where the population was practically stationary, those individuals who were attacked in July, 1918, passed through two later epidemics, in December, 1918, and February, 1919, without contracting the disease a second time, in spite of intimate contact with infected persons. The same was true according to Malone in the Alipore Central Jail and the Presidency Jail in Calcutta. He believes that his evidence strongly suggests an immunity lasting for at least nine months.
Dunlop found that Glasgow had a mild epidemic in the month of May, 1918, in which the death rate rose from 14.1 to 20.1. There is no record of any similar outbreak in Edinburgh. In the July epidemic the Glasgow death rate rose from 11.7 to 15.9, while the Edinburgh death rate went from 11.3 to 18.0, a higher increase. In the October-November epidemic the Glasgow rate rose from 11.0 to 38.4, while the Edinburgh rate went from 10.8 to 46.2. In the February-March epidemic the Glasgow rate rose from 14.9 to 48.3, and the Edinburgh rate from 18.9 to 52.1 In the July and October epidemics Edinburgh showed a greater increase in death rate, while in February, 1919, the increase in the two cities was the same. However, in this case probably other factors play a part. Also, we must remember that here we are dealing with death rates, not with incidence rates.