The Wrong Lessons - The Monster at Our Door: The Global Threat of Avian Flu - page 15

high-risk population, as well as lobbying for a much more timely and aggressive Washington response to
the next pandemic. 63
New discoveries soon supported the case for taking influenza more seriously. After 1968, researchers
made a number of dramatic breakthroughs. Virologists, for the first time, were actually able to see the
distinctive shapes of the HA and NA molecules. They also confirmed that antigenic drift was the result of
point mutation (amino acid substitution) and that HA and NA mutated independently. Even more import-
antly, they identified genetic reassortment as the probable mechanism for the emergence of new pandem-
ic subtypes; with the parallel discovery of influenza's natural home in ducks and waterfowl, researchers
could begin to trace the virus's modern family tree (see Table 3.1 ). They ultimately identified 15 antigen-
ically distinctive HAs and 9 different NAs in the avian reservoir, for a total of 135 hypothetical subtypes.
The evolution of human influenza, it was now clear, was primarily driven by the crossing over of new HA
proteins from waterfowl—after its deadly debut, each pandemic form then settled down to earn its living
by modest mutation. Blood sera studies of the 1957 and 1968 pandemics, moreover, indicated that elderly
people had some immunity to the new pandemic HAs. Researchers, accordingly, postulated that an H2
subtype had caused the 1889 pandemic, and an H3 the 1898 pandemic. Their reemergence in the postwar
period was interpreted as evidence that influenza had hiding places or cryptic reservoirs where it could
slumber for decades, or even generations. 64
Table 3.1.
Influenza A Dynasties
The vistas opened by influenza research in the 1970s were breathtaking, but new knowledge only
seemed to deepen the fundamental mysteries. The molecular basis of flu virulence remained unknown,
as did the viral component responsible for catastrophic cyanosis. No one could convincingly explain why
new subtypes usually extinguished the old, or how seemingly extinct lineages could suddenly reemerge,
nor could researchers predict which avian hemagglutinin (an H5 or H7, for example) would next cross the