DISEASE E&E (Measuring Disease 2) Flashcards
Epidemic curves:
-graphical depiction of the number of cases of an illness or deaths by date of onset/occurrence
-are ‘incidence curves’, not a rate as we don’t have a denominator
Epidemic curves can provide info on:
-pattern of spread
-magnitude of outbreak
-outliers
-time trends
-exposure and/or disease incubation period?
-effect of control effects and mitigation strategies
Shape of the curve may reveal the type of outbreak: 3 types
-point source or common source epidemic
-continuous common source epidemic
-propagated epidemic
*rarely do they fit perfectly
Point source epidemic:
-pronounced clustering of disease
-common source or event
-brief exposure period
*all infectious cases occur within one incubation period
Ex. food poisoning outbreaks or toxic events
Continuous common source epidemic:
-single source of exposure
-exposure period is prolonged
-relatively abrupt beginning
-no cases arise beyond one incubation period following the termination of exposure
Ex. lead poisoning where source of lead is not discovered
Outbreak pattern of spread-propagated:
-caused by infectious agent
-spread from person to person or animal to animal
-last longer than common source
-may have multiple waves
-primary cases infected susceptible individual which become secondary cases
-‘build up’ or amplification
Propagated epidemic curve shape:
-progressively taller peaks, that are an incubation period apart
Ex. FMD, Ebola
Index case: propagated epidemic:
-first primary case that comes to the attention of the investigators
Propagated curve ‘stages’:
- Exponential
- Saturation and peak
- Declining phase
- What happens next
Exponential stage:
-highly infectious agent with a short incubation period
-rapid spread of infection
-epidemic growing at an exponential rate
*slope is highly dependent on R0
How can we calculate R0?
need to know how long they are infectious
-need to know the contact rate
-multiply that by the infectiousness of the pathogen
Contact rate can be driven by:
-population density
-social and behavioural traits
Equation for R0
R0=pcD
-p: probability of infection on contact
-c: rate of contact
-D: duration of infectiousness
Rt:
-number of individuals that can be infected at any specific time
-Rt>1=epidemic is increasing
-Rt=1: peak of epidemic
-Rt<1: epidemic is waning
Dispersion factor (k):
-that some people will spread it and other won’t spread it at all
-captures extra level of complexity
*not all exposures are created equal
Value of k close to 0:
-a small number of individuals are responsible for a large number of transmissions
Ex. Covid is around 0.1, meaning that 10% of infected individuals may be associated with 80% of the transmissions
‘super spreading’ event:
-when an infected individual enters a high-risk setting (Ex. indoor event) where large numbers of people are clustered together
Saturation and peak:
-after initial phase, not all contacts are with susceptible animals
-rate is slowed and eventually Rt=1
-determined by the proportion of susceptible
Size of the epidemic is determined by:
-proportion of susceptible animals in the population
*herd immunity
Herd immunity:
-when most of the population is immune to an infectious disease, it provides INDIRECT PROTECTION even to those not immune to the disease
-level depends on the infectiousness of the disease
-maybe 70-90%
Epidemics are like a fire:
-susceptible are the fuel
Declining phase:
-cases are still occurring
-approximately half the infections happen
-curve is usually roughly symmetrical
What happens next? (stage 4): 3 potential patterns
-extinction
-stable prevalence and incidence
-endemic cycles
Factors that affect the shape of the curve
- Incubation period
- Infectivity
- Proportion of susceptible animals in the population
- Distance between animals
Extinction:
-if no new susceptibles are provided
-usually you get new ones from: birth or loss of immunity
*susceptibles come in TOO slowly and disease dies out
Stable prevalence:
-Rt=1
-no change in levels of infection
-susceptible entering slowly allowing the disease to become endemic
Endemic cycles:
-created by differences in rates of decline and supply of susceptibles
-susceptibles come in rapidly: sharp increase
-susceptibles decline: sharp decreases
When will cycles occur
-if the decline is sharp due to short infectious period relative to birth rate
*infections that produce GOOD IMMUNITY do NOT tend to cycle
“slide to glory:
-when you come in at the tail of the epidemic curve and you can look really good
Mapping disease:
-can use maps as tools to describe distributions of disease in geographic areas
Ex. John Snow
