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

• Steps of an Outbreak Investigation

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)