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Paragraph: The differences between the
waves thus seemed to be primarily in the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves. 

But 3 extensive pandemic waves of inﬂuenza within 1
year, occurring in rapid succession, with only the briefest
of quiescent intervals between them, was unprecedented. The occurrence, and to some extent the severity, of recur-
rent annual outbreaks, are driven by Viral antigenic drift,
with an antigenic variant Virus emerging to become domi-
nant approximately every 2 to 3 years. Without such drift,
circulating human inﬂuenza Viruses would presumably
disappear once herd immunity had reached a critical
threshold at which further Virus spread was sufﬁciently
limited. The timing and spacing of inﬂuenza epidemics in
interpandemic years have been subjects of speculation for
decades. Factors believed to be responsible include partial
herd immunity limiting Virus spread in all but the most
favorable circumstances, which include lower environ-
mental temperatures and human nasal temperatures (bene-
ﬁcial to thermolabile Viruses such as inﬂuenza), optimal
humidity, increased crowding indoors, and imperfect ven-
tilation due to closed windows and suboptimal airﬂow. However, such factors cannot explain the 3 pandemic
waves of 1918-1919, which occurred in the spring-sum-
mer, summer—fall, and winter (of the Northern
Hemisphere), respectively. The ﬁrst 2 waves occurred at a
time of year normally unfavorable to inﬂuenza Virus
spread. The second wave caused simultaneous outbreaks
in the Northern and Southern Hemispheres from
September to November. Furthermore, the interwave peri-
ods were so brief as to be almost undetectable in some
locales. Reconciling epidemiologically the steep drop in
cases in the ﬁrst and second waves with the sharp rises in
cases of the second and third waves is difﬁcult. Assuming
even transient postinfection immunity, how could suscep-
tible persons be too few to sustain transmission at 1 point,
and yet enough to start a new explosive pandemic wave a
few weeks later? Could the Virus have mutated profoundly
and almost simultaneously around the world, in the short
periods between the successive waves? Acquiring Viral
drift sufﬁcient to produce new inﬂuenza strains capable of
escaping population immunity is believed to take years of
global circulation, not weeks of local circulation. And hav-
ing occurred, such mutated Viruses normally take months
to spread around the world. At the beginning of other “off season” inﬂuenza pan-
demics, successive distinct waves within a year have not
been reported. The 1889 pandemic, for example, began in
the late spring of 1889 and took several months to spread
throughout the world, peaking in northern Europe and the
United States late in 1889 or early in 1890. The second
recurrence peaked in late spring 1891 (more than a year
after the ﬁrst pandemic appearance) and the third in early
1892 (21 ). As was true for the 1918 pandemic, the second
1891 recurrence produced of the most deaths. The 3 recur-
rences in 1889-1892, however, were spread over >3 years,
in contrast to 191871919, when the sequential waves seen
in individual countries were typically compressed into
z879 months. What gave the 1918 Virus the unprecedented ability to
generate rapidly successive pandemic waves is unclear. Because the only 1918 pandemic Virus samples we have
yet identiﬁed are from second-wave patients ([6), nothing
can yet be said about whether the ﬁrst (spring) wave, or for
that matter, the third wave, represented circulation of the
same Virus or variants of it. Data from 1918 suggest that
persons infected in the second wave may have been pro-
tected from inﬂuenza in the third wave. But the few data
bearing on protection during the second and third waves
after infection in the ﬁrst wave are inconclusive and do lit-
tle to resolve the question of whether the ﬁrst wave was
caused by the same Virus or whether major genetic evolu-
tionary events were occurring even as the pandemic
exploded and progressed. Only inﬂuenza RNAipositive
human samples from before 1918, and from all 3 waves,
can answer this question. What Was the Animal Host
Origin of the Pandemic Virus? Viral sequence data now suggest that the entire 1918
Virus was novel to humans in, or shortly before, 1918, and
that it thus was not a reassortant Virus produced from old
existing strains that acquired 1 or more new genes, such as
those causing the 1957 and 1968 pandemics. On the con-
trary, the 1918 Virus appears to be an avianlike inﬂuenza
Virus derived in toto from an unknown source (17,19), as
its 8 genome segments are substantially different from
contemporary avian inﬂuenza genes. Inﬂuenza Virus gene
sequences from a number ofﬁxed specimens ofwild birds
collected circa 1918 show little difference from avian
Viruses isolated today, indicating that avian Viruses likely
undergo little antigenic change in their natural hosts even
over long periods (24,25). For example, the 1918 nucleoprotein (NP) gene
sequence is similar to that ofviruses found in wild birds at
the amino acid level but very divergent at the nucleotide
level, which suggests considerable evolutionary distance
between the sources of the 1918 NP and of currently
sequenced NP genes in wild bird strains (13,19). One way
of looking at the evolutionary distance of genes is to com-
pare ratios of synonymous to nonsynonymous nucleotide
substitutions. A synonymous substitution represents a
silent change, a nucleotide change in a codon that does not
result in an amino acid replacement. A nonsynonymous
substitution is a nucleotide change in a codon that results
in an amino acid replacement. Generally, a Viral gene sub-
jected to immunologic drift pressure or adapting to a new
host exhibits a greater percentage of nonsynonymous
mutations, while a Virus under little selective pressure
accumulates mainly synonymous changes. Since little or
no selection pressure is exerted on synonymous changes,
they are thought to reﬂect evolutionary distance. Because the 1918 gene segments have more synony-
mous changes from known sequences of wild bird strains
than expected, they are unlikely to have emerged directly
from an avian inﬂuenza Virus similar to those that have
been sequenced so far. This is especially apparent when
one examines the differences at 4-fold degenerate codons,
the subset of synonymous changes in which, at the third
codon position, any of the 4 possible nucleotides can be
substituted without changing the resulting amino acid. At
the same time, the 1918 sequences have too few amino acid
diﬁerences from those of wild-bird strains to have spent
many years adapting only in a human or swine intermedi-
ate host. One possible explanation is that these unusual
gene segments were acquired from a reservoir of inﬂuenza
Virus that has not yet been identiﬁed or sampled. All of
these ﬁndings beg the question: where did the 1918 Virus
come from? In contrast to the genetic makeup of the 1918 pandem-
ic Virus, the novel gene segments of the reassorted 1957
and 1968 pandemic Viruses all originated in Eurasian avian
Viruses (26); both human Viruses arose by the same mech-
anismireassortment of a Eurasian wild waterfowl strain
with the previously circulating human H1N1 strain. Proving the hypothesis that the Virus responsible for the
1918 pandemic had a markedly different origin requires
samples of human inﬂuenza strains circulating before
1918 and samples of inﬂuenza strains in the wild that more
closely resemble the 1918 sequences. What Was the Biological Basis for
1918 Pandemic Virus Pathogenicity? Sequence analysis alone does not oﬁer clues to the
pathogenicity of the 1918 Virus. A series of experiments
are under way to model Virulence in Vitro and in animal
models by using Viral constructs containing 1918 genes
produced by reverse genetics. Inﬂuenza Virus infection requires binding of the HA
protein to sialic acid receptors on host cell surface. The HA
receptor-binding site conﬁguration is different for those
inﬂuenza Viruses adapted to infect birds and those adapted
to infect humans. Inﬂuenza Virus strains adapted to birds
preferentially bind sialic acid receptors with 01 (273) linked
sugars (27729). Human-adapted inﬂuenza Viruses are
thought to preferentially bind receptors with 01 (2%) link-
ages. The switch from this avian receptor conﬁguration
requires of the Virus only 1 amino acid change (30), and
the HAs of all 5 sequenced 1918 Viruses have this change,
which suggests that it could be a critical step in human host
adaptation. A second change that greatly augments Virus
binding to the human receptor may also occur, but only 3
of5 1918 HA sequences have it (16). This means that at least 2 H1N1 receptor-binding vari-
ants cocirculated in 1918: 1 with high—afﬁnity binding to
the human receptor and 1 with mixed-afﬁnity binding to
both avian and human receptors. No geographic or chrono-
logic indication eXists to suggest that one of these variants
was the precursor of the other, nor are there consistent dif-
ferences between the case histories or histopathologic fea-
tures of the 5 patients infected with them. Whether the
Viruses were equally transmissible in 1918, whether they
had identical patterns of replication in the respiratory tree,
and whether one or both also circulated in the ﬁrst and
third pandemic waves, are unknown. In a series of in Vivo experiments, recombinant inﬂuen-
za Viruses containing between 1 and 5 gene segments of
the 1918 Virus have been produced. Those constructs
bearing the 1918 HA and NA are all highly pathogenic in

mice (31). Furthermore, expression microarray analysis
performed on whole lung tissue of mice infected with the
1918 HA/NA recombinant showed increased upregulation
of genes involved in apoptosis, tissue injury, and oxidative
damage (32). These ﬁndings are unexpected because the
Viruses with the 1918 genes had not been adapted to mice;
control experiments in which mice were infected with
modern human Viruses showed little disease and limited
Viral replication. The lungs of animals infected with the
1918 HA/NA construct showed bronchial and alveolar
epithelial necrosis and a marked inﬂammatory inﬁltrate,
which suggests that the 1918 HA (and possibly the NA)
contain Virulence factors for mice. The Viral genotypic
basis of this pathogenicity is not yet mapped. Whether
pathogenicity in mice effectively models pathogenicity in
humans is unclear. The potential role of the other 1918 pro-
teins, singularly and in combination, is also unknown. Experiments to map further the genetic basis of Virulence
of the 1918 Virus in various animal models are planned. These experiments may help deﬁne the Viral component to
the unusual pathogenicity of the 1918 Virus but cannot
address whether speciﬁc host factors in 1918 accounted for
unique inﬂuenza mortality patterns. Why Did the 1918 Virus Kill So Many Healthy
Young Ad ults? The curve of inﬂuenza deaths by age at death has histor-
ically, for at least 150 years, been U-shaped (Figure 2),
exhibiting mortality peaks in the very young and the very
old, with a comparatively low frequency of deaths at all
ages in between. In contrast, age-speciﬁc death rates in the
1918 pandemic exhibited a distinct pattern that has not been
documented before or since: a “W—shaped” curve, similar to
the familiar U-shaped curve but with the addition of a third
(middle) distinct peak of deaths in young adults z20410
years of age. Inﬂuenza and pneumonia death rates for those
1534 years of age in 191871919, for example, were
20 times higher than in previous years (35). Overall, near-
ly half of the inﬂuenza—related deaths in the 1918 pandem-
ic were in young adults 20410 years of age, a phenomenon
unique to that pandemic year. The 1918 pandemic is also
unique among inﬂuenza pandemics in that absolute risk of
inﬂuenza death was higher in those <65 years of age than in
those >65; persons <65 years of age accounted for >99% of
all excess inﬂuenza—related deaths in 191871919. In com-
parison, the <65-year age group accounted for 36% of all
excess inﬂuenza—related deaths in the 1957 H2N2 pandem-
ic and 48% in the 1968 H3N2 pandemic (33). A sharper perspective emerges when 1918 age-speciﬁc
inﬂuenza morbidity rates (21) are used to adj ust the W-
shaped mortality curve (Figure 3, panels, A, B, and C
[35,37]). Persons 65 years of age in 1918 had a dispro-
portionately high inﬂuenza incidence (Figure 3, panel A). But even after adjusting age-speciﬁc deaths by age-specif—
ic clinical attack rates (Figure 3, panel B), a W—shaped
curve with a case-fatality peak in young adults remains and
is signiﬁcantly different from U-shaped age-speciﬁc case-
fatality curves typically seen in other inﬂuenza years, e.g.,
192871929 (Figure 3, panel C). Also, in 1918 those 5 to 14
years of age accounted for a disproportionate number of
inﬂuenza cases, but had a much lower death rate from
inﬂuenza and pneumonia than other age groups.

Question: What was the primary difference between the first wave and the 2nd and 3rd wave of the 1918-1919 swine flu pandemic?
the much higher fre-
quency of complicated, severe, and fatal cases in the last 2
waves.