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Download Transmission: how do we measure infectiousness?
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Transmission: how do we measure infectiousness? Judith Glynn Infectiousness How well infection spreads: • Organism: probability of transmission given contact, how long a case remains infectious • Environment: probability of contact, type and number of contacts in the population, prevention measures Infectiousness How we measure how well an infection spreads: • Secondary attack rate • Case reproduction number Secondary attack rate • secondary attack rate (2o AR) is used in the study of the spread of infectious diseases in small communities • Only for person-person transmission • = the proportion of those exposed to the primary case that develop disease as a result of the exposure Secondary attack rate example First Ebola outbreak Number of family members exposed to cases = 498 No. of new cases arising = 38 Secondary attack rate = 38/498 = 7.6% Secondary attack rate is context specific Depends on: Closeness of contact e.g. First Ebola outbreak 2o AR= 27% for close family Type of contact e.g. For HIV: sharing needles > sexual contact e.g. For Ebola: extensive contact with body fluid > skin contact Stage of illness e.g. Ebola more infectious (higher viral load) as disease progresses Case Reproduction Number = number of secondary cases per case • Basic case reproduction number = R0 Reproduction number at time zero, when all are susceptible • Net case reproduction number = R ( or Rt) Reproduction number at time t, changeable, varies with the proportion still susceptible Basic case reproduction number Average number of secondary cases per case in a totally susceptible population. Can sometimes be measured directly (eg SARS, Ebola) R0 Basic case reproduction number R0 R0 depends on 3 factors • duration of infectiousness • probability of infection being transmitted during contact between a susceptible and infected individual • average rate of contact between susceptible and infected individuals Basic case reproduction number R0 R0 depends on 3 factors • duration of infectiousness • probability of infection being transmitted during contact between a susceptible and infected individual • average rate of contact between susceptible and infected individuals Basic case reproduction number R0 R0 depends on 3 factors • duration of infectiousness • probability of infection being transmitted during contact between a susceptible and infected individual • average rate of contact between susceptible and infected individuals R0 and 2o AR R0 Average number of secondary cases per case in a totally susceptible population 2o AR Proportion of those exposed to the primary case that develop disease as a result of the exposure in a particular situation R0 = 2o AR [household] x no. of contacts [household] + 2o AR [other family] x no. of contacts [other family] + 2o AR [community] x no. of contacts [community] + etc.
