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be understood " that the intervals between epidemics are never exact; only their
average approaches exactitude. A month either way is of no moment." Again:
"Epidemics seem to be of extreme rarity between the end of June and the beginning
of December, so rare in fact as to be almost non-existent. . . . The great
epidemic which occurred in October, 1919, falls completely out of line." Round
about this epidemic the intervals between it and the two nearest epidemics are much
shorter than 33 weeks, approximately about half that period. In London the
period 1889-1896 contains "the most typical years." In Boston during the years
1899-1900 "the centre of the main epidemic varies very little from its theoretical
placing. In only two cases is it more than a month out of place."
Dr. Brownlee finds, moreover, that in the years 1889-1896 "the deaths from
pneumonia precede the deaths from influenza by a little over a week, while the
deaths ascribed to bronchitis have their epidemic rise a full fortnight before the
rise of influenza." Dr. Stallybrass gives further details of a like sort for Liverpool,
but one of his influenza peaks, which appeared when it should not have done (December
18th, 1920), was he holds really "an outbreak of primary non-influenzal pneumonia."
He adds : "A number of the outbreaks of respiratory disease, which do not
correspond to the periodicity of 33 weeks of influenza, occur at intervals of 39 weeks
dating from the week ending December 18th, 1920." These respiratory outbreaks
sometimes appear a week or two after the influenza outbreaks, whereas Dr. Brownlee's
respiratory outbreaks in London tended to occur a week or two weeks, on the average,
before influenza.
Mr. Spear has pointed out that in influenza annual periodicity is of not infrequent
occurrence; it is the rule, of course, in respiratory diseases (bronchitis and pneumonia).
Dr. Brownlee says on this: " If a 33 week period exists with the further
requirement that every third epidemic is missed, the form of the curve arising . . . . .
is identical with that in this case." Three times 33, and 66 added to 33,
are both very nearly twice 52, and thus both approximately fit in with annual
periodicity.
Inspection shows that the intervals, between epidemics in London from December,
1889, onwards, approximate, quite roughly speaking, to two alternating periods of
about four-thirds and two-thirds of a year (a brace of two such intervals makes up
2 years); after about 8 or 10 years, however, the rhythm tends to become frankly
an annual one, in fact the maxima then correspond closely with those of pneumonia
and bronchitis ; as the next pandemic prevalence approaches the periodicity quickens
up again. In 1918-19 there were three waves within one period of, roughly speaking,
33 weeks. Following upon this the recent pandemic prevalences recurred at approximately
annual intervals for three years, and then, between 1922 and 1925, the
alternating four-thirds and two-thirds rhythm again appears.
The impressions left upon the mind by study of the periodicities are first that,
as Brownlee says, the period from the end of June to the beginning of December
is a very unfavourable one for influenza. The early weeks of the year, particularly
if the extreme rigour of winter is then experienced, constitute the chosen time for
influenza, bronchitis and pneumonia; cerebro-spinal meningitis and encephalitis
lethargica tend to prevail in the late winter or early spring season. Another point
of time favourable to development of influenza is June, and poliomyelitis is also
wont to occur then. In the second place loss of immunity conferred by attack,
owing to lapse of time, is also a potent factor in determining periodicity; but,
thirdly, over and above these two factors, the special phenomena noted round
about the pandemic prevalences inevitably suggest the need for postulating mutation
of the virus itself.
In summary, then, the normal periodicity exhibited by influenza tends to be
an annual one, and this is particularly the case in the mid-period between pandemic
prevalences. This rhythm is apt to be speeded up in the nearer neighbourhood
of the pandemics, and quite close to the times of pandemic spread a still shorter
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