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the ratio differ much from one epidemic to another ; the same is found to be the case
in the years 1920-22, when the population had largely returned to the normal sex
proportion, the excess in deaths among females being accounted for by the greater
number of females in the population. especially at high ages.
Dr. Brownlee concluded from analysis of the epidemics of the nineties that
influenza was apt to recur in periods of 33 weeks or multiples thereof with, however,
the qualification that when the period occurred in the summer months no outbreak
was to be expected. (See Report of the Ministry of Health on the Pandemic of
Influenza, 1918-19, Appendix XII.) The first of the new series of epidemics, that
in June-July, 1918, as well as the third (February-March, 1919), occurred after
an interval which, measured from the epidemics of 1890, is a multiple of 33 weeks,
while the epidemics of 1891, 1892, 1893, and 1895 approximately fulfil the same
condition. The epidemic of October-November, 1918—the greatest of them all—
is, however, an interloper, breaking midway into the period of 33 weeks and the
last epidemic, that of January, 1922, followed this interloper after 165, i.e., 33 by 5,
weeks, and is also aperiodic in respect to the epidemics of the early nineties. Moreover,
there is only one instance, in the last 32 years, in which the actual interval
between two successive epidemics approximates to 33 weeks ; in all other cases
the interval approximates to a multiple of this period. The intervals range in
length from 16 weeks which separated the first, second and third of the recent series to
152 weeks to the fourth, that of 1922. The interval between two epidemics is measured
by the time elapsing between the week of maximum mortality in each, but this is,
of course, only an approximation. In the nineties the periods thus measured were
70, 35, 99 and 64 weeks, respectively between the five epidemics in the period 1890-95.
Periodicity
of epidemics.
In its seasonal incidence as judged by the mortality influenza is similar to typhoid
fever; in both cases there is a definite seasonal recurrence, though the time of year
is different, typhoid being an autumnal and influenza a winter disease. In addition
to this more or less regular annual recurrence, there are mass outbreaks which do
not appear to follow the seasonal incidence of normal prevalences, at least, not very
closely; but if experience were available of a large number of mass outbreaks of
typhoid fever in London, it is probable that in the long run it would be found that
they tend to be more frequent in the autumn. In the case of influenza it is difficult
to draw a hard and fast line of distinction between an "epidemic" and a merely
seasonal prevalence; but if attention be confined to instances in which the deaths
have risen to not less than 100 in one week, it is found that since 1889 there have
been 17 such prevalences and that of these 13 have fallen in the first quarter, one
in each of the second and third quarters and 2 in the fourth. If consideration be
limited to the pandemics in this period, numbering 8, it is found that 5 occurred in
the first quarter and 1 in each of the other quarters of the year. There were no pandemics
in the l0 years 1901-1910, and for this period the week of maximum mortality
was the tenth week in the year. Endemic influenza, therefore, tends to reach a seasonal
maximum about the middle of March; and the facts stated point to the
probability that in England epidemic influenza also tends to a maximum about the
same period.
The contrast in respect of mortality between the pandemics of the nineties
and those of 1918 and following years is remarkable. The total deaths from
"influenza" in the six years 1890-1895, in which there were 5 epidemic prevalences,
numbered 9,658, while in the period 1918-1922, with 4 epidemics, the deaths numbered
22,332. Allowance must be made for varying fashion as regards nomenclature
and for the possibility that the population in 1918 was, owing to the war, less resistant
to influenza than in the nineties. It is interesting to note that in New South Wales,
where the population could hardly in any direction have been affected by the war
to the extent of the population of London, the death-rate in 1919 was four times
that of the great epidemic of 1891.
The post-war mortality of European capital cities also offers some striking