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Emerging Infections: Influenza Pandemic Facts
December 31, 1997
Contact: Media Relations Division
(404) 639-3286
- Pandemics result from the emergence of an influenza A virus that is novel for the human
population.
- The hallmark of pandemic influenza is excess mortality --- the number of deaths observed
during an epidemic of influenza-like illness in excess of the number expected.
- During this century, pandemics occurred in 1918, 1957, and 1968.
- 1918-19 “Spanish flu” A(H1N1) --- caused the highest known influenza-related
mortality --- at least 500,000 deaths in the United States, and 20 million worldwide.
- 1957-58 “Asian flu” A(H2N2) --- 70,000 deaths in the United States.
- 1968-69 “Hong Kong flu” A(H3N2) --- 34,000 deaths in the United States.
- Although mortality rates associated with the recent pandemics of 1957 and 1968 were
confined primarily to the elderly and chronically ill, both pandemics were associated with
high rates of illness and social disruption, with combined economic losses of
approximately $32 billion (in 1995 dollars).
- The potential impact of an influenza virus in humans depends on whether there is
protective immunity in the population, the virus’ ability to be passed from person to
person and its virulence (ability to cause severe illness or death).
- Influenza viruses undergo two kinds of change. One is a series of mutations overtime
that cause a gradual evolution of the virus, known as antigenic drift. The other is an
abrupt change in the surface antigen proteins, known as anitgenic shift, thus suddenly
creating a new subtype of the virus.
- When antigenic shift occurs, the population does not have antibody protection against
the virus.
- Birds are the primary reservoir for influenza viruses. All 15 recognized influenza A
subtypes have been found in birds. Only 3 influenza subtypes have circulated widely in
humans until this summer: H3N2, H1N1, and H2N2.
- In most years in the United States, influenza is responsible for 10,000-40,000 excess
deaths, 50,000-300,000 hospitalizations, and approximately $1-3 billion in direct costs
for medical care.
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