DISEASE E&E (Measuring Disease 2)

Question 1

Epidemic curves:

Answer 1

-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

Question 2

Epidemic curves can provide info on:

Answer 2

-pattern of spread
-magnitude of outbreak
-outliers
-time trends
-exposure and/or disease incubation period?
-effect of control effects and mitigation strategies

Question 3

Shape of the curve may reveal the type of outbreak: 3 types

Answer 3

-point source or common source epidemic
-continuous common source epidemic
-propagated epidemic
*rarely do they fit perfectly

Question 4

Point source epidemic:

Answer 4

-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

Question 5

Continuous common source epidemic:

Answer 5

-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

Question 6

Outbreak pattern of spread-propagated:

Answer 6

-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

Question 7

Propagated epidemic curve shape:

Answer 7

-progressively taller peaks, that are an incubation period apart
Ex. FMD, Ebola

Question 8

Index case: propagated epidemic:

Answer 8

-first primary case that comes to the attention of the investigators

Question 9

Propagated curve ‘stages’:

Answer 9

1. Exponential
2. Saturation and peak
3. Declining phase
4. What happens next

Question 10

Exponential stage:

Answer 10

-highly infectious agent with a short incubation period
-rapid spread of infection
-epidemic growing at an exponential rate
*slope is highly dependent on R0

Question 11

How can we calculate R0?

Answer 11

need to know how long they are infectious
-need to know the contact rate
-multiply that by the infectiousness of the pathogen

Question 12

Contact rate can be driven by:

Answer 12

-population density
-social and behavioural traits

Question 13

Equation for R0

Answer 13

R0=pcD
-p: probability of infection on contact
-c: rate of contact
-D: duration of infectiousness

Question 14

Rt:

Answer 14

-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

Question 15

Dispersion factor (k):

Answer 15

-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

Question 16

Value of k close to 0:

Answer 16

-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

Question 17

‘super spreading’ event:

Answer 17

-when an infected individual enters a high-risk setting (Ex. indoor event) where large numbers of people are clustered together

Question 18

Saturation and peak:

Answer 18

-after initial phase, not all contacts are with susceptible animals
-rate is slowed and eventually Rt=1
-determined by the proportion of susceptible

Question 19

Size of the epidemic is determined by:

Answer 19

-proportion of susceptible animals in the population
*herd immunity

Question 20

Herd immunity:

Answer 20

-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%

Question 21

Epidemics are like a fire:

Answer 21

-susceptible are the fuel

Question 22

Declining phase:

Answer 22

-cases are still occurring
-approximately half the infections happen
-curve is usually roughly symmetrical

Question 23

What happens next? (stage 4): 3 potential patterns

Answer 23

-extinction
-stable prevalence and incidence
-endemic cycles

Question 24

Factors that affect the shape of the curve

Answer 24

1. Incubation period
2. Infectivity
3. Proportion of susceptible animals in the population
4. Distance between animals

Question 25

Extinction:

Answer 25

-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

Question 26

Stable prevalence:

Answer 26

-Rt=1
-no change in levels of infection
-susceptible entering slowly allowing the disease to become endemic

Question 27

Endemic cycles:

Answer 27

-created by differences in rates of decline and supply of susceptibles
-susceptibles come in rapidly: sharp increase
-susceptibles decline: sharp decreases

Question 28

When will cycles occur

Answer 28

-if the decline is sharp due to short infectious period relative to birth rate
*infections that produce GOOD IMMUNITY do NOT tend to cycle

Question 29

“slide to glory:

Answer 29

-when you come in at the tail of the epidemic curve and you can look really good

Question 30

Mapping disease:

Answer 30

-can use maps as tools to describe distributions of disease in geographic areas
Ex. John Snow