Session 7 Infectious diseases epidemiology

• It is consistent presence of a specific disease or infectious agent within a particular geographic area, population, or community. - endemic
• Examples of endemic diseases in UK?

  Lyme Disease, Seasonal Influenza, Common Cold in UK.
• It is sudden occurrence of a particular disease in a large number of people within a specific region, community, or population. - epidemic
• the average number of new infections (secondary cases) that arise from a single infectious case introduced into a population where everyone is susceptible - Basic reproductive Rate
• • R0 < 1 - the disease will disappear.
  • R0 = 1 - the disease will become endemic.
  • R0 > 1 - there will be an epidemic
• R0 = β * κ * D

  • β- risk of transmission per contact.
  • Condoms, face masks, hand washing - β ↓
  • κ-average number of contacts per time unit.
  • Isolation, closing schools, public campaigns - κ ↓
  • D- duration of infectiousness measured by the same time units as κ
  • Specific for an infectious disease
  • Early diagnosis and treatment, screening, contact tracing - D ↓
• If the population is not fully susceptible, the average number of secondary cases is less than R0. This is the Effective Reproduction Number (R).
• Waning immunity refers to the gradual decrease in the effectiveness of the immune response over time after vaccination or natural infection.
• Herd Immunity Threshold?

  Minimum proportion (p) of population that needs to be immunized in order to obtain herd immunity. (p = 1 - 1/R0).
• Multi Locus Sequence Typing (MLST): characterisation of bacterial isolates based on the combination of alleles at multiple loci (gene fragments).
• Whole Genome Sequencing (WGS): characterisation of isolates based on unique whole genome sequences. There is some debate about how to interpret WGS in their entirety.
• Common Point Source: Commonly associated with food-borne illness, where many people are exposed for a short period of time. Lack of person-to-person transmission.

  
• Common Persistent Source: Outbreak due to exposure of persons to a noxious influence that is common to the individuals in the group.

  
• Propagated Source: An epidemic that arises when a primary case introduces an infectious agent to a population and subsequent host-to host transmission results in ‘waves’ of cases.