transmission dynamics and disease spread
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Transmission Dynamics and Disease Spread. General issues in transmission. Contagion Spread, if we have time. Modeling Contagious Disease. R 0 = “Basic reproductive number” Average number of people that an infected person will infect. What Determines R 0 ?. R 0 = cpd where - PowerPoint PPT PresentationTRANSCRIPT
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TRANSMISSION DYNAMICS AND DISEASE SPREAD
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General issues in transmission
Contagion
Spread, if we have time
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Modeling Contagious Disease R0 = “Basic reproductive
number”
Average number of people that an infected person will infect
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What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
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What happens when:
R0 < 1?
R0 = 1?
R0 > 1?
R0 >> 1?
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R0 < 1
Epidemic will: Grow quickly? Grow (transmission sustained)? Remain stable? Disappear?
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R0 < 1
Epidemic will: Grow quickly? Grow? Remain stable?
Disappear
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R0=1?
Epidemic will: Grow quickly? Grow (transmission sustained? Remain stable? Disappear?
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R0=1?
Epidemic will: Grow quickly? Grow (transmission sustained?
Remain stable Disappear?
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R0>1?
Epidemic will: Grow quickly? Grow (transmission sustained? Remain stable? Disappear?
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R0>1
Epidemic will: Grow quickly?
Grow (transmission sustained?
Remain stable? Disappear?
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R0 >>1 (much greater than)
Grow quickly? Grow? Remain stable? Disappear?
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R0 >>1 (much greater than)
Grow quickly Grow? Remain stable? Disappear?
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Gets more complex
More than 1 region Changes in the parameters in different
places In-migration
Recovery period and mortality (people removed from the population)
Changes in any of the parameters over time
Population growth and shrinkage Incubation period
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Musher, N Engl J Med 2003;348:1256-66.
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GOAL: REDUCE R0 TO < 1
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What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
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Herd Immunity
Not just individual immunity
Population immunity such that infection will disappear (temporarily)
Endemic vs. epidemic
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“SIR” Models
Susceptibles: Number of people who are susceptible to the disease
Infectives: Number of people who are infected
They can pass the disease on to susceptibles
Recovereds: Number of people who have recovered—they are immune
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INFLUENZAPatterns and Geography
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Objectives today
Describe some of the basic features of influenza and its transmission
Describe and illustrate some of the general considerations in the “spatial epidemiology” of communicable disease
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INFLUENZA
Why study it and understand it?
What can be done?
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Source: Pyle, Diffusion of Influenza
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Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18
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Influenza Strain Variants, Animal-Human
Source: Kilbourne, Influenza, p. 273.
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Schematic: Known Events, Cross Species Transmission
Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18
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Temporal Patterns of Influenza, 1999-2003
Source: MMWR, April 25, 2003
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Temporal Pattern of Influenza, Houston
Source, Kilbourne, Influenza, p. 259.
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INFLUENZA MORTALITY, BRESLAU, GERMANY
Source: Taubenberger andMorens, Public HealthReports, 2010
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Kilbourne’s Generalization of Epidemics
Source: Kilbourne, Influenza, p. 274
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Harmonic Analysis, First Autumn Wave,1918-19
Source: Pyle, Diffusion of Influenza,
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Foreign Born Population in Seattle, 1920
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Deaths by Week, Seattle, 1918-19
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Deaths by Weeks, 1918-1919, Seattle
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Age-Specific Death Rates, Influenza, Seattle
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General issues in transmission Contagion
Spread, if we have time
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Modeling Influenza
R0 = “Basic reproductive number”
Average number of people that an infected person will infect
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What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
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Musher, N Engl J Med 2003;348:1256-66.
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Musher, N Engl J Med 2003;348:1256-66.
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Incubation period makes it even more complex
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Lessler et al, Lancet Infectious Disease 2009;9:291-300
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Lessler et al, Lancet Infectious Disease 2009;9:291-300
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SARS Transmission Chain, Beijing(superspreaders at nodes A, H, D, I)
Source: Emerging Infectious Diseases 2004;10:256-60