Epidemic Curves

Question 1

Epidemic curve

Answer 1

A graphical depiction of the number of cases of an illness or deaths by the date of onset/occurrence
- An anatomy of an epidemic
- An Incidence curve
- Shows increase and decrease of new disease cases, and the rate of change in disease occurrence

Question 2

What kind of information can you get from epidemic curves?

Answer 2

• Pattern of spread
• Magnitude of outbreak
• Outliers
• Time trends
• Sometimes can determine Exposure and disease incubation period
• Effects of control efforts and mitigation strategies

Question 3

Point source epidemic curve

Answer 3

Appears bell- shaped (sharp upward and downward curve); clustering of disease
- Peak usually at average incubation period. Variation in number of cases will occur due to other factors like immune system efficiency

Common source and event; often a brief exposure period. All cases occur within one incubation period of the casual event
- Ex. Food poisoning event

Question 4

Continuous common source epidemic curve

Answer 4

Single source of exposure but with a prolonged exposure period occurring over a long time so the outbreak persists longer (no cases beyond one incubation period following termination of exposure)

Curve: no steep or gradual numbers. Numbers quite high throughout entire time of exposure period

Relatively abrupt beginning (exposure simultaneously rather than spreading through transmission)

Ex. Lead poisoning exposure where source of lead is not discovered OR contaminated water supply

Question 5

Spread-propagated outbreak curve

Answer 5

Epidemic caused by infectious agent. Spread from individual to individual (primary and secondary cases).

• Can last longer than common source outbreaks.
• May have multiple waves
• Curve: Progressively taller peaks (incubation period apart). Characterized by build-up or amplification

Question 6

Spread-propagated outbreak examples

Answer 6

• COVID-19
• Foot and Mouth Disease
• Ebola

Question 7

Index case

Answer 7

The first primary case that comes to the attention of the investigators during a Spread-propagated outbreaks

Question 8

What sort of things does an Ebola Epidemic curve tell you?

Answer 8

1. Case fatality
   - Saw a decrease further on in outbreak. Could this be because hospital closed?
   - Survivors more likely further on in epidemic
2. Length of outbreak

Question 9

Why are some epidemic curves over years instead of days, weeks, or months?

Answer 9

Longer outbreaks are linked to longer incubation periods

Ex. BSE (Mad cow)

Question 10

Parts of epidemic curve

Answer 10

• Exponential
• Saturation and peak
• Declining phase
• What happens next

Question 11

Exponential stage

Answer 11

• Highly infectious agent with short incubation period produced a steep curve on a relatively small time scale
• From rapid spread of infection among the population
• Indicates exponential or doubling rate
• Rate of this curve highly dependent on the Basic Reproductive Number (R0)

Question 12

Basic Reproductive Number (R0)

Answer 12

The number of cases that are expected to occur on average in a homogenous population as a result of infection by a single individual, when the population is susceptible at the start of the epidemic before widespread immunity, immunization or other control methods.

Question 13

Calculating R0

Answer 13

R0= pcD

(probability of infection on contact) x (number of contacts that occur during the infectious period)

Note: rate of contact (number of contacts per unit of time) is the hardest variable to determine

Question 14

What effects rate of contacts?

Answer 14

• Behaviour
• Transmission route
• Density
• Environment

Question 15

Effective Reproductive number (Re or Rt)

Answer 15

The number of individuals in a population who can be infected by an infectious individual at any specific time (not during very start of epidemic)

When greater than 1, epidemic increasing. Peak= 1. When less than 1, epidemic is waning

Question 16

Dispersion factor (k)

Answer 16

Looks at the idea that some individuals will spread the infection to many people while others won’t spread it to anybody
- “Super-spreaders”- one person at large indoor event

If k is close to 0, means small number of individuals are responsible for a large number of transmissions (ex. COVID k=0.1 means that as few as 10% of infected individuals may be associated with 80% of transmissions)

Question 17

Saturation and Peak Stage

Answer 17

After the initial phase, not all contacts are with susceptible individuals

The rate of the epidemic is slowed and eventually Rt=1

Question 18

What determines the saturation and peak/size of population?

Answer 18

Determined by the proportion of susceptibles in the population

Question 19

Herd immunity

Answer 19

When most of population is immune to an infectious disease, this provides indirect protection to those that are not immune

Occurs because more immune individuals decreases chances of individuals getting infected by encountering infectious individuals

**Driven by fact that the size of epidemic is driven by number of susceptibles in population

Question 20

What level of herd immunity is needed?

Answer 20

• Depends on infectious disease
• 70-90% of population needs to be immune

Question 21

Decline Stage

Answer 21

Cases still occurring; approximately half of the infections are going to occur after the peak during the decline

Curve is usually roughly symmetrical

Question 22

What happens next? Stage

Answer 22

Three potential patterns:
1. Extinction
2. Stable prevalence and incidence
3. Endemic cycles (comes and goes)

Question 23

Factors affecting the shape of the curve

Answer 23

• Incubation period of the disease
• Infectivity of the agent
• Proportion of susceptible animals in the population
• Distance between animals (animal density effecting contact rate)

Question 24

Extinction pattern of curve

Answer 24

• Occurs if no susceptibles. Difficult because there is almost always births and loss of immunity
• When it does occur, happens because susceptibles come in too slowly and disease dies out. Ex. Ebola (swoops into villages quickly, high mortality, dies out quickly because susceptibles are wiped out)

Question 25

Stable prevalence (equilibrium) pattern of curve

Answer 25

Rt=1

No change in levels of infection, most likely due to susceptibles continually entering slowly into population allowing the disease to become endemic

Question 26

Endemic cycles pattern of curve

Answer 26

Cycles are created by difference in rates of decline and supply of susceptibles
- Susceptibles come into population sharply and then decline/die off (short infectious period, birth rate slower)

Infections that don’t produce good immunity tend not to cycle

“Slide to glory” on tail of the epidemic- Epidemic dies off but actually due to less susceptibles. Can come back when susceptible population increases.

Question 27

Tips to making an epidemic curve

Answer 27

• Time axis will depend on incubation period (1/4 length of incubation period should be unit)
• If incubation period not known, graph several epi curves with different time units
• Epi curves are histograms… no space between time units
• Include pre-epidemic period to show baseline number of cases

Question 28

Mapping disease

Answer 28

• Epidemiologists use maps to describe distributions of disease in geographic areas
• Use Geographical information systems (GIS)- computerized systems for storing, managing and displaying spatial data

Ex. John Snow and Cholera outbreak
Ex. Bovine TB
Ex. Shiga toxin producing E. coli in humans and livestock density