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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 Presentation
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TRANSMISSION DYNAMICS AND DISEASE SPREAD
General issues in transmission
Contagion
Spread, if we have time
Modeling Contagious Disease R0 = “Basic reproductive
number”
Average number of people that an infected person will infect
What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
What happens when:
R0 < 1?
R0 = 1?
R0 > 1?
R0 >> 1?
R0 < 1
Epidemic will: Grow quickly? Grow (transmission sustained)? Remain stable? Disappear?
R0 < 1
Epidemic will: Grow quickly? Grow? Remain stable?
Disappear
R0=1?
Epidemic will: Grow quickly? Grow (transmission sustained? Remain stable? Disappear?
R0=1?
Epidemic will: Grow quickly? Grow (transmission sustained?
Remain stable Disappear?
R0>1?
Epidemic will: Grow quickly? Grow (transmission sustained? Remain stable? Disappear?
R0>1
Epidemic will: Grow quickly?
Grow (transmission sustained?
Remain stable? Disappear?
R0 >>1 (much greater than)
Grow quickly? Grow? Remain stable? Disappear?
R0 >>1 (much greater than)
Grow quickly Grow? Remain stable? Disappear?
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
Musher, N Engl J Med 2003;348:1256-66.
GOAL: REDUCE R0 TO < 1
What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
Herd Immunity
Not just individual immunity
Population immunity such that infection will disappear (temporarily)
Endemic vs. epidemic
“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
INFLUENZAPatterns and Geography
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
INFLUENZA
Why study it and understand it?
What can be done?
Source: Pyle, Diffusion of Influenza
Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18
Influenza Strain Variants, Animal-Human
Source: Kilbourne, Influenza, p. 273.
Schematic: Known Events, Cross Species Transmission
Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18
Temporal Patterns of Influenza, 1999-2003
Source: MMWR, April 25, 2003
Temporal Pattern of Influenza, Houston
Source, Kilbourne, Influenza, p. 259.
INFLUENZA MORTALITY, BRESLAU, GERMANY
Source: Taubenberger andMorens, Public HealthReports, 2010
Kilbourne’s Generalization of Epidemics
Source: Kilbourne, Influenza, p. 274
Harmonic Analysis, First Autumn Wave,1918-19
Source: Pyle, Diffusion of Influenza,
Foreign Born Population in Seattle, 1920
Deaths by Week, Seattle, 1918-19
Deaths by Weeks, 1918-1919, Seattle
Age-Specific Death Rates, Influenza, Seattle
http://www.cdc.gov/flu/weekly/
http://www.google.org/flutrends/
General issues in transmission Contagion
Spread, if we have time
Modeling Influenza
R0 = “Basic reproductive number”
Average number of people that an infected person will infect
What Determines R0?
R0 = cpd where c=contact rate P=probability of transmission (any given contact)
d = duration of contact
Musher, N Engl J Med 2003;348:1256-66.
Musher, N Engl J Med 2003;348:1256-66.
Incubation period makes it even more complex
Lessler et al, Lancet Infectious Disease 2009;9:291-300
Lessler et al, Lancet Infectious Disease 2009;9:291-300
SARS Transmission Chain, Beijing(superspreaders at nodes A, H, D, I)
Source: Emerging Infectious Diseases 2004;10:256-60