Epidemic patterns

Epidemic

• an increase in incidence of disease in excess of that expected

A compartmental model

• sequence of host stages

1. susceptible individuals becomes infected

2. Infected people recover

3. People leave the sequence upon death

Effective contact rate

• susceptible people becoming infected

Recovery rate

• people recovering from being infected

R0

• basic case reproduction number

• Not a rate

• Given a pathogen enters susceptible host pop and spreads

• Definition - average no. Of new cases arising from one infectious case introduced into pop of wholly susceptible individuals

Basic case reproduction number

• R0

• How many people are infected by 1 infectious person

Epidemic characteristics

• when number of secondary cases is on average >1

• Considers R0

Pc

• percentage of pop likely to get a disease in a fully susceptible population

Estimating R0

• p x c x D

• P - probability that contact results in transmission

• C - frequency of host contacts between susceptible and infected

• P x c = effective contact rate

• D = average amount of time the host is infectious

R e

• Restrained growth rate

• Effective R

• True reproductive rate

• R0 x fraction of susceptible individuals

• Better than R0 since R0 is for a virgin population

R e calc

• R0 x susceptible

How epidemics end

• pool of susceptible depleted

• Declines to <1

• Can’t increase unless new susceptible generation

Continuing epidemics

• susceptible increase

• No immunity Endemic Continual

• Mutation of pathogen

• Immunity wanes

Recurrent epidemics in small pop

• slow regeneration of susceptibles

• Successive epidemics when enough susceptibles

Epidemics fade out

• slow regeneration due to low birth rate (small pop

• Number of infected is low

• Elimination of infectious agent due to chance

Waning Immunity

• Loss of immunity post recovery from infection

• Recovered individuals becomes susceptible

What do we learn from epidemic patterns

• prevalence and incidence through time

• Origin

• Mode of spread

• Incubation period

• Time of exposure

• Compare R0 to identify infectious agent

Incubation period

• period between infection and clinical onset of disease

Latent period

• time from infection to infectiousness

Point epidemic

• single common exposure and incubation period

• Not spread by host to host transmission

• Eg food borne outbreaks

Continuous common source epidemic

• prolonged exposure to source over time

• Cases don’t occur within span of a single incubation period

• Curve decay may be sharp or gradual

Propagated progressive source epidemic

• spread between hosts

• Larger curves (graph of incidence and time) until susceptibles are depleted or intervention is made

• Most common in small populations

• In a larger population, all curves would merge

Cholera

• caused by bacteria

• Micro parasite

• Infects small intestine

• The toxin inhibits water absorption

Snows grand experiment

• tested number of houses and cholera deaths in different water company areas

• Identified cause

Broad st pump cholera

• source identified

• Pump handle removed to prevent outbreak

• Demonstrated cholera was waterborne