Lecture 24. Epidemic Patterns

Question 1

What is an epidemic?

Answer 1

An increase in incidence of disease in excess of that expected

Question 2

What is the sequence of host stages?

Answer 2

Susceptible (S) → Infected (I) → Recovered (R)

Question 3

What is R0?

Answer 3

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

Question 4

What does Pc mean?

Answer 4

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

Question 5

What is the formula for R0?

Answer 5

R0 = p x c x D where:
R0 = The reproductive number of the infectious agent
p = Probability that a contact results in transmission
c = The frequency of host contacts between infectious and susceptible individuals
p x c = Effective contact rate (rate of movement from ‘S’ to ‘I’)
D = The average amount of time the host is infectious

Question 6

What is effective R (Re)?

Answer 6

The restrained growth rate, the true reproductive rate

Question 7

What is the formula for Re?

Answer 7

Re = R0 x fraction of susceptible individuals (S)

Question 8

What causes a decreases in Re?

Answer 8

When the fraction of susceptible declines

Question 9

When does an epidemic occur in terms of Re?

Answer 9

When the number of secondary cases is on average >1

Question 10

Why do epidemics end?

Answer 10

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

Question 11

How do epidemics continue?

Answer 11

Susceptibles increase (born, migrate into a population)
No immunity (SI model)
Pathogen mutates (e.g. antigenic drift) and can re-infect/or continually infect individuals
Immunity wanes

Question 12

Why are there recurrent epidemics in small populations?

Answer 12

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 13

How does epidemic fade-out occur?

Answer 13

In small populations rather than large populations
Generation (birth) of threshold susceptibles is low and numbers of infected low

Question 14

What does waning immunity mean?

Answer 14

Loss of immunity post recovery from infection

Question 15

What can patterns in epidemic data show?

Answer 15

Infecteds through time: prevalence & incidence
Origin of the outbreak
Mode of spread through the population
Potential incubation period and time of exposure
Clues to identify the infectious agent

Question 16

What does incubation period mean?

Answer 16

The period between infection and clinical onset of the disease

Question 17

What does latent period men?

Answer 17

The time from infection to infectiousness

Question 18

What is a point epidemic?

Answer 18

Single common exposure and number of cases incubation period
Does not spread by host-to-host transmission
E.g. food-borne disease outbreaks

Question 19

What is a continuous common source epidemic?

Answer 19

Prolonged exposure to source over time
Cases do not occur within the span of a single incubation period
Curved decay may be sharp or gradual
E.g water-borne cholera: 1-3 days incubation

Question 20

What is a propagated progressive source epidemic?

Answer 20

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 21

What is cholera?

Answer 21

Cholera, caused by a bacteria (Vibrio cholerae)
Cholera is a micro-parasite, infecting the small intestine
Cholera toxin inhibits water absorption