Epidemiology in outbreak

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definition of outbreak by CDC

  • epidemic limited to localised increase in the incidence of disease
  • Epidemic Disease Occurrence - Level of disease
  • Sporadic
    • a disease that occurs infrequently and irregularly
  • Endemic
    • constant presence
    • and/or usual prevalence of a disease
    • or infectious agent in a population within a geographic area
  • Hyperendemic
    • persistent, high levels of disease occurrence.
  • Epidemic
    • increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area
  • Outbreak
    • the same definition of epidemic, but is often used for a more limited geographic area
  • Cluster
    • aggregation of cases grouped in place and time that are suspected to be greater than the number expected, even though the expected number may not be known
  • Pandemic
    • epidemic that has spread over several countries or continents, usually affecting a large number of people.

Epidemics occur when an agent and susceptible hosts are present in adequate numbers, and the agent can be effectively conveyed from a source to the susceptible hosts. More specifically, an epidemic may result from:

   A recent increase in amount or virulence of the agent,
   The recent introduction of the agent into a setting where it has not been before,
   An enhanced mode of transmission so that more susceptible persons are exposed,
   A change in the susceptibility of the host response to the agent, and/or
   Factors that increase host exposure or involve introduction through new portals of entry.(47)

what to do when to investigate an outbreak

  1. Prepare for field work
  2. Establish the existence of an outbreak
  3. Verify the diagnosis
  4. Construct a working case definition
  5. Find cases systematically and record information
  6. Perform descriptive epidemiology
  7. Develop hypotheses
  8. Evaluate hypotheses epidemiologically
  9. As necessary, reconsider, refine, and re-evaluate hypotheses
  10. Compare and reconcile with laboratory and/or environmental studies
  11. Implement control and prevention measures
  12. Initiate or maintain surveillance
  13. Communicate findings

second attack rate (SAR)

  • proportion of individuals who are exposed to an infectious agent who become ill

[math]\displaystyle{ SAR=\frac{number_of_becoming_ill}{number_of_exposed} }[/math]

reproduction number

basic reproduction number

  • average number of secondary cases caused by a single primary case in a fully susceptible population

effective reproduction number

  • actual number of secondary cases per single primary case

time course

  • incubation period
    • followed by symptomatic p.
  • symptomatic p.


  • latent (pre-infectious) p.
    • followed by infectious p.
  • infectious p.


  • serial interval
    • time from illness onset of primary case to illness onset of secondary case caused by primary case
  • generation time
    • time from infection of primary case to /u>infection of secondary case caused by primary case

SIR model

  • S = susceptible
  • I = infected
  • R = recovered
  • β = transmission coefficient (S→I)
  • γ = recovery rate (I→R)