Pandemic Influenza: History & Epidemiology

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Pandemic Influenza History Epidemiology

• Kathryn H. Jacobsen, MPH, PhD
• Dept. of Global Community Health
• George Mason University

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What is normal flu?

• Respiratory disease
• Symptoms fever, muscle aches, cough, sore
  throat, runny nose, and possible nausea
• May develop into pneumonia
• Spread by airborne droplets that reach the eyes,
  nose, or mouth may also be spread by touching
  contaminated surfaces and then the face
• The elderly, very young, and those with weakened
  immune systems are at risk of severe illness
  (especially pneumonia) and death.
• 30,000 40,000 deaths in the USA in a normal
  flu year (and 5 20 of the population sick)

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Why are we susceptible to new flu strains?
Antigenic Drift

• Influenza viruses frequently mutate (antigenic
  drift).
• Surface proteins change in ways that make it
  difficult for our immune systems to quickly
  identify them and fight them off.
• A new flu vaccine is developed every year to
  adjust for the changes in the circulating flu
  viruses.

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Why are we susceptible to new flu strains?
Antigenic Shift

• Two very different types of flu virus that infect
  one animal can merge to create a completely new
  strain (antigenic shift).
• Most common strain in humans in recent decades
  H3N2
• Bird flu H5N1

Domestic birds
Domestic pigs
Humans
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What is bird flu?

• Many flu strains are bird flu or pig flu,
  etc.
• Avian influenza H5N1 is a flu virus type that
  primarily affects birds highly contagious in
  birds and has resulted in many bird deaths
• Bird flu is species specific and rarely infects
  humans only a few hundred humans are known to
  have contracted H5N1
• Genetic changes may cause H5N1 to develop the
  ability to more easily infect humans

Domestic birds
Domestic pigs
Humans
Wild birds
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Why are we concerned about H5N1?

• Most humans have no immunity to H5N1
• About half of the humans known to have been
  infected with H5N1 have died.
• Important note This is an overestimation of the
  true case fatality rate because only severe cases
  are tested.
• All had direct and prolonged contact with poultry
  or bird habitats.
• Most cases have occurred in children and young
  adults.

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Why are we concerned about a possible pandemic?

• Conditions for a pandemic
• (1) Emergence of a new flu strain
• (2) Ability of the strain to infect humans and
  cause severe illness
• (3) Ability to spread easily among humans
• If further mutations of the H5N1 virus occur and
  the virus becomes easy to transmit from human to
  human a global outbreak (pandemic) could occur.

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WHO Pandemic Staging

• Interpandemic Period
• Phase 1 Low Risk No new influenza virus
  subtypes in humans
• Phase 2 New Virus No new flu in humans but an
  animal variant emerges
• Pandemic Alert Period
• Phase 3 Self Limiting Human infections but no
  human-to-human transmission
• Phase 4 Person-to-Person Small clusters with
  limited human-to-human transmission
• Phase 5 - Epidemic Larger but still localized
  clusters
• Pandemic Period
• Phase 6 - Pandemic Increased and sustained
  transmission in the general population

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Pandemics (Global Epidemics) in History

• 541 CE, bubonic plague spreads from Egypt to
  Constantinople then beyond, killing up to ¼ of
  the population of the eastern Mediterranean
• 1300s CE, bubonic plague (Black Death) returns
  to Europe from Asia and kills up to ¼ of the
  population (and more than half in the hardest hit
  areas)
• 1500s CE, typhus pandemics Europeans bring
  smallpox and other epidemic infections to the
  Americas
• A series of cholera outbreaks in the 1800s hit
  all continents

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Pandemic Influenza History

• 1580 Africa ? Europe
• 1729 Russia ? Europe
• 1781-1782 China ? Russia ? Europe
• 1830-1833 China ? Pacific and India ? Russia ?
  Europe
• 1889-1900 Asiatic flu Russia ? Mediterranean ?
  North America ? South America ? India and
  Australia
• 1918-1919 Spanish flu Started in October 1918,
  quickly spread worldwide, and vanished within 18
  months
• 1957-1958 Asian flu
• 1968-1969 Hong Kong flu

Potter CW, A history of influenza, Journal of
Applied Microbiology 2001 91 572-579.
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Why are we concerned about a possible pandemic?

• Modern influenza pandemics tend to occur every 20
  to 30 years

?
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Where is H5N1 now?
Nations With Confirmed Cases H5N1 Avian
Influenza (July 7, 2006)
http//www.pandemicflu.gov/
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How quickly will it spread?

• It may take several weeks for an initial outbreak
  to be announced
• Influenza may be found in dozens of countries
  within a week or a few weeks
• Travel advisories and warnings may be issued
  quickly and restrict international travel and/or
  local travel
• Quarantine orders may be issued within weeks
• Thousands to millions may become infected

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Sample Timelines
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Sample Timelines
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How can we respond to H5N1?

• A flu vaccine might be developed but probably
  not in time to prevent a pandemic because
  human-to-human transmissible H5N1 does not yet
  exist.
• Production capacity may also be an issue.
• Antiviral drugs that mitigate symptoms (like
  Tamiflu and Relenza) may not work against this
  strain.
• We do know that normal preventive measures will
  be helpful in preventing transmission.
• We can plan!

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Areas of Concern Addressed during Todays
Conference

• Clinical features of H5N1
• Comparisons to other EIDs like SARS
• Planning, security, communications, and
  preparation

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