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Transmission Dynamics and Disease Spread

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

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Transmission Dynamics and Disease Spread

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  1. Transmission Dynamics and Disease Spread

  2. General issues in transmission • Contagion • Spread, if we have time

  3. Modeling Contagious Disease • R0 = “Basic reproductive number” • Average number of people that an infected person will infect

  4. What Determines R0? • R0 = cpd where • c=contact rate • P=probability of transmission (any given contact) • d = duration of contact

  5. What happens when: • R0 < 1? • R0 = 1? • R0 > 1? • R0 >> 1?

  6. R0 < 1 • Epidemic will: • Grow quickly? • Grow (transmission sustained)? • Remain stable? • Disappear?

  7. R0 < 1 • Epidemic will: • Grow quickly? • Grow? • Remain stable? • Disappear

  8. R0=1? • Epidemic will: • Grow quickly? • Grow (transmission sustained? • Remain stable? • Disappear?

  9. R0=1? • Epidemic will: • Grow quickly? • Grow (transmission sustained? • Remain stable • Disappear?

  10. R0>1? • Epidemic will: • Grow quickly? • Grow (transmission sustained? • Remain stable? • Disappear?

  11. R0>1 • Epidemic will: • Grow quickly? • Grow (transmission sustained? • Remain stable? • Disappear?

  12. R0 >>1 (much greater than) • Grow quickly? • Grow? • Remain stable? • Disappear?

  13. R0 >>1 (much greater than) • Grow quickly • Grow? • Remain stable? • Disappear?

  14. 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

  15. Musher, N Engl J Med 2003;348:1256-66.

  16. GOAL: REDUCE R0 TO < 1

  17. What Determines R0? • R0 = cpd where • c=contact rate • P=probability of transmission (any given contact) • d = duration of contact

  18. Herd Immunity • Not just individual immunity • Population immunity such that infection will disappear (temporarily) • Endemic vs. epidemic

  19. “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

  20. Patterns and Geography INFLUENZA

  21. 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

  22. INFLUENZA • Why study it and understand it? • What can be done?

  23. Source: Pyle, Diffusion of Influenza

  24. Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18

  25. Influenza Strain Variants, Animal-Human Source: Kilbourne, Influenza, p. 273.

  26. Schematic: Known Events, Cross Species Transmission Source: Forrest+ Webster, Animal Health Res Revs 2010;11:3-18

  27. Temporal Patterns of Influenza, 1999-2003 Source: MMWR, April 25, 2003

  28. Temporal Pattern of Influenza, Houston Source, Kilbourne, Influenza, p. 259.

  29. INFLUENZA MORTALITY, BRESLAU, GERMANY Source: Taubenberger and Morens, Public Health Reports, 2010

  30. Kilbourne’s Generalization of Epidemics Source: Kilbourne, Influenza, p. 274

  31. Harmonic Analysis, First Autumn Wave, 1918-19 Source: Pyle, Diffusion of Influenza,

  32. Foreign Born Population in Seattle, 1920

  33. Deaths by Week, Seattle, 1918-19

  34. Deaths by Weeks, 1918-1919, Seattle

  35. Age-Specific Death Rates, Influenza, Seattle

  36. http://www.cdc.gov/flu/weekly/

  37. http://www.google.org/flutrends/

  38. General issues in transmission • Contagion • Spread, if we have time

  39. Modeling Influenza • R0 = “Basic reproductive number” • Average number of people that an infected person will infect

  40. What Determines R0? • R0 = cpd where • c=contact rate • P=probability of transmission (any given contact) • d = duration of contact

  41. Musher, N Engl J Med 2003;348:1256-66.

  42. Musher, N Engl J Med 2003;348:1256-66.

  43. Incubation period makes it even more complex

  44. Lessler et al, Lancet Infectious Disease 2009;9:291-300

  45. Lessler et al, Lancet Infectious Disease 2009;9:291-300

  46. SARS Transmission Chain, Beijing (superspreaders at nodes A, H, D, I) Source: Emerging Infectious Diseases 2004;10:256-60

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