Lecture 24. Epidemic Patterns Flashcards
What is an epidemic?
An increase in incidence of disease in excess of that expected
What is the sequence of host stages?
Susceptible (S) → Infected (I) → Recovered (R)
What is R0?
The average number of new cases arising from one
infectious case introduced into a population of wholly susceptible individuals
What does Pc mean?
Percent of population likely to get disease in a fully susceptible population
What is the formula for R0?
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
What is effective R (Re)?
The restrained growth rate, the true reproductive rate
What is the formula for Re?
Re = R0 x fraction of susceptible individuals (S)
What causes a decreases in Re?
When the fraction of susceptible declines
When does an epidemic occur in terms of Re?
When the number of secondary cases is on average >1
Why do epidemics end?
The pool of susceptible individuals is depleted
Re declines to < 1
Re cannot return to >1 until new susceptibles are generated
How do epidemics continue?
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
Why are there recurrent epidemics in small populations?
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)
How does epidemic fade-out occur?
In small populations rather than large populations
Generation (birth) of threshold susceptibles is low and numbers of infected low
What does waning immunity mean?
Loss of immunity post recovery from infection
What can patterns in epidemic data show?
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
What does incubation period mean?
The period between infection and clinical onset of the disease
What does latent period men?
The time from infection to infectiousness
What is a point epidemic?
Single common exposure and number of cases incubation period
Does not spread by host-to-host transmission
E.g. food-borne disease outbreaks
What is a continuous common source epidemic?
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
What is a propagated progressive source epidemic?
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
What is cholera?
Cholera, caused by a bacteria (Vibrio cholerae)
Cholera is a micro-parasite, infecting the small intestine
Cholera toxin inhibits water absorption
