Lecture 24: Epidemic patterns

Question 2

What is an epidemic?

Answer 2

An increase in incidence of disease in excess of that expected

Question 3

What is the incidence?

Answer 3

number of new cases per unit time

e.g number of new cases of cholera per day

Question 4

Draw out the epidemic compartmental model

Answer 4

Answer on paper

Question 5

What is R0?

Answer 5

The basic reproduction number (not a rate).

“the average number of new cases arising from one infectious case introduced into a population of wholly susceptible individuals”

(number of secondary cases)

Question 6

For an epidemic to occur, what does the R0 value have to be?

Answer 6

An epidemic occurs when the number of secondary cases is on average >1

Question 7

What is Pc?

Answer 7

percent of population likely to get disease in a fully susceptible population

Question 8

What is the R0 for smallpox?

Answer 8

3-5

Question 9

What equation can be used to estimate the R0 of a pathogen?

Answer 9

R0 = p x c x D

Question 10

what is p?

Answer 10

Probability that a contact results in transmission

Question 11

What is c?

Answer 11

The frequency of host contacts between infectious and susceptible individuals

Question 12

What is p X c?

Answer 12

Effective contact rate (rate of movement from ‘S’ to ‘I’)

Question 13

What is D?

Answer 13

The average amount of time the host is infectious

Question 14

Does introduced infection move individuals between compartments?

Answer 14

Yes

Question 15

What is Effective R (Re)?

Answer 15

Effective R (Re) is the restrained growth rate

Question 16

Describe what Effective R (Re) is?

Answer 16

• R0 is defined for a “virgin” population with all individuals susceptible.
• “Effective” R (Re) is the true reproductive rate:
  R0 x fraction of susceptible individuals (S)

Question 17

Why do epidemics end?

Answer 17

• The pool of susceptible individuals is depleted.
• Re declines to < 1
• Re cannot return to >1 until new susceptibles are generated

Question 18

How do epidemics continue?

Answer 18

• Number of suceptibles increase through:
• More births
• Migration into a population
• No immunity (SI) model
• Pathogen mutates (e.g. antigenic drift) and can re-infect/or continually infect individuals
• Immunity wanes
  (pathogen may change and so immunity may become less and less effective).

Question 19

Recurrent epidemics in small populations

Answer 19

Slow regeneration (birth) of susceptibles due to small population size.

Successive epidemics follow re-introduced measles virus by visitors when Re>1 (there are enough susceptibles)

Question 20

What is epidemic fade-out?

Answer 20

“The elimination of the infectious agent due to chance”

Question 21

Describe epidemic fade out

Answer 21

n small populations rather than large populations:
generation (birth) of threshold susceptibles is slow
numbers of infecteds is low

Question 22

What is waning immunity?

Answer 22

Loss of immunity post recovery from infection

e.g. flu virus

Question 23

What can patterns in epidemic data tell us?

Answer 23

• Infecteds through time: prevalence & incidence
  (Retrospective analysis of previous infections)
• Origin of the outbreak
• Index case – the first case in an outbreak of disease
• Mode of spread through the population
• Potential incubation period and time of exposure
• Clues to identify the infectious agent
  (R0 value comparisons)

Question 24

What is the incubation period?

Answer 24

the period between infection and clinical onset of the disease

Question 25

What is the latent period?

Answer 25

the time from infection to infectiousness.

Question 26

Describe a point epidemic

Answer 26

Single common exposure and incubation period

Does not spread by host-to-host transmission

E.g. food-borne disease outbreaks

Question 27

Describe a Continuous Common Source Epidemic

Answer 27

Prolonged exposure to source over time

Cases do not all occur within the span of a single incubation period

Curve decay may be sharp or gradual

E.g. water-borne cholera: 1-3 days incubation

Question 28

Describe a Propagated Progressive Source Epidemic

Answer 28

E.g. measles: 10 days incubation

Spread between hosts.

Larger curves until susceptibles are depleted, or intervention is made.

This pattern most likely in a small population.

In a larger population, it would all ‘merge’ together.

Question 29

Describe cholera

Answer 29

Cholera, caused by a bacteria (Vibrio cholerae)

Cholera is a micro-parasite, infecting the small intestine

Cholera toxin inhibits water absorption

Question 30

Broad St. Pump

Answer 30

John Snow identified source, removed pump handle, prevented major cholera epidemic, and importantly demonstrated that cholera is water borne.

Vibrio cholerae discovered as causative agent of cholera 30 years later (in 1884) by Robert Koch

Question 31

Summary

Answer 31

R0 = p x c x D

Re = R0S

Re >1

increase in disease incidence 🡪 epidemic