Influenza

by Warren T. Vaughan

“In the English Responsoria (1, 54) the epidemic constitution of 1679 is described as a recurrence of that of 1675—that is, as having the features of primary epidemic influenza. In the five following years intermittents prevailed, and in one (1684) the mortality much exceeded that of 1679, although the deaths from smallpox were fewer. Again, a hundred years later, in 1782, there was a famous summer epidemic of influenza in London which gave rise to much discussion. The London mortalities in 1782 and 1783 were, however, almost equal, when the smallpox deaths (which were nearly three times as numerous in 1783 as in 1782) are subtracted from the total mortality of each year.

“Whether these vague indications are sufficient to permit of our thinking that the epidemic constitution of 1889–94 was not entirely unprecedented is disputable. But the contrast of the latter period with the preceding single epidemic of 1847–48 is striking; that was a primary epidemic without important sequelae.

“We have now to consider whether our experience this year is concordant with that of the early nineties, a reversion to the earlier type, or a new phenomenon.”

After comparing the 1889 curves with those for the July, 1918, outbreak in England, Greenwood concludes: “I believe that the evidence just presented establishes a substantial identity between the summer outbreak of 1918 and the primary wave of 1889–90. We do not need to appeal to any new factor arising out of the war to account for it.

“I next consider the secondary epidemic which we are now experiencing (October, 1918). Evidently our knowledge of the events in 1891 would lead us to feel no surprise at the emergence of a secondary wave, although we could not be sure that the precedent of 1847 would not be followed.

“The summer epidemic of 1918 in the Royal Air Force included nearly 80 per cent. of the total incidence within the three weeks containing the maximum, and the Munich epidemic included just over 80 per cent. within the same limits. Now if the current epidemic has reached its maximum, not more than 65 per cent. of the incidence will probably be so concentrated, and the duration will therefore be longer than in the summer; if, as suggested by the ratio of the last two ordinates, the maximum is not yet attained, then the quota of the three first weeks is likely to be still smaller and the complete duration still longer.

“The diagram of factory sickness leads to the same inference, which is that, from the standpoint of prevalence, the present is a typical secondary epidemic, congruent with that of 1891.

“It appears, then, that the origin of the summer epidemic must be explained upon such epidemiological principles as will account for the primary wave of 1889–90, that the current outbreak is in pari materia with that of 1891, its excessive mortality being mainly due to the accident of season, aided by the special circumstances of overcrowding and fuel shortage which are due to the war. In a word, this is not essentially a war epidemic.”

Wutzdorff found that in some towns, particularly in North Germany, the 1891–1892 wave was almost as extensive as that of 1889–90 had been in other places, but that in general the morbidity in Germany was much lower. He bases these conclusions on a study of the extent of crowding in the hospitals in the two years, on statistics of government physicians, etc.

In Europe the recurrent epidemics of 1891 increased as a rule very gradually, developed slowly, reached their high point frequently after many weeks, and as gradually decreased. The epidemic duration in the winter of 1891–92 lasted four or five months. The morbidity in spite of the longer duration was decidedly less. This is very different from the explosive appearance of 1889 when the peak was reached in fourteen days and the whole epidemic had been completed in six to eight weeks. There were some exceptions to this rule, as in Yorkshire, England, where the epidemic broke out suddenly between the 11th and 13th of April, 1891, had reached its peak after ten days, and for another twenty days declined. Especially interesting was Sheffield, where the first spread began gradually and ran a slow course, while the second epidemic of 1891 began explosively, lasted a short time and declined rapidly, but showed a significantly greater mortality than that of 1889.