Lec 20-21: Epidemiology Flashcards
What did Daniel Bernouli (1766) do for the field of epidemiology?
created math models to analyze dynamics of smallpox epidemic in Paris
Epidemic or outbreak=
disease occurrence among a population that is above what’s expected at a given time & place
Cluster=
a group of cases in a specific time & place that might be more than expected
Endemic=
disease or condition present among a population at all times (constantly present at a steady rate)
pandemic=
a disease or condition that spreads across regions/ countries
rate (in terms of epidemiology)=
of cases occurring during a specific period; always dependent on the size of the pop during that period
- prevalence vs incidence
T/F
- An endemic disease is constantly present in a pop, with a predictable rate of spread
- An epidemic disease is characterized by a sudden increases in cases across the world
- A pandemic disease is characterized by a sudden increase in cases across several countries or the world
- true
- false. epidemic= sudden increase in cases spreading through a large population (but not worldwide)
- true
Explain the epidemiological triangle
This is used as a tool to address the 3 components that contribute to the spread of the disease
- Environment (climate, sanitation, health care, etc)
- Host (genetics, age, sex, health, previous infections, etc)
- Agent (pathogen, parasite, virus)
The reproductive number (R0) represents a threshold for disease spread. Explain the different values and what the thresholds might be
R less than 1: self-sustaining epidemic is not possible (parasite will disappear)
R=1: parasite will barely persist
R greater than 1: parasite will persist and tend to spread (epidemic)
** transmission threshold**
R much greater than 1: parasite will be difficult to eradicate
Is the basic reproductive number fixed?
No!
Different aspects of parasite transmission can affect it, and may change over time
For microparasites, what determines its persistence in host populations?
Can each infection replace itself?
Determined by R0= # of secondary infections produced by one infected host introduced into a pop comprising only susceptible hosts
For macroparasites, what determines its persistence in host populations?
Can each individual replace itself?
Determined by R0= # of new females produced by 1 female in absence of density-dependent constraints
Give the 3 factors that influence R0
- duration of infectious period
- # of susceptible people in the population (contact rate)
- mode of transmission (eg airborne vs spread by bodily fluids etc)
Rank the following diseases from highest to lowest reproductive number
Ebola
Rabies
Fly
Measles
Chicken Pox
Measles
Chicken pox
Ebola
Flu
Rabies
If a disease has a higher reproductive number, does it require a higher or lower % of the population to be vaccinated in order to eradicate it?
higher R0= higher vacc % required
eg. Malaria and Measles required near 100% vacc to eradicate
T/F
R0 can estimate the proportion (p*) to be vaccinated or otherwise treated in order to eradicate the infection
true
p*= 1-1/R0
What does R0 determine?
The ability of parasites to regulate host population
Outbreak of epidemics, persistence of endemic infections, & whether or not the parasite dies out
Give the proportionality for the basic reproductive number
R0 is directly proportional to (infection/contact) x (contact/time) x (time/infection)
basically, transmissibility x avg rate of contact x duration of infectiousness
It’s directly proportional, so if one of these increases, so does R0 (and vise versa)
Give the equation to calculate Ro for MICROparasites
Ro=β(H) / b + a + y
β= rate of transmission
H= density of susceptible hosts
b= natural host mortality
a= parasite-induced host mortality
y= host recovery rate
B and H= directly related to Ro
b, a, and y= inversely related to Ro
Give the equation to calculate Ro for MACROparasites
Ro=βλH / (μ+b+α) (y+βH)
β= rate of transmission
λ= parasite per capita birth rate
H= density of susceptible hosts
b= natural host mortality
μ= parasite per capita death rate
α= parasite-induced host mortality
What kind of important questions can modelling epidemics answer? Lots of options, give 2-3
What is the risk of an epidemic to occur?
How far will it spread?
How long will it last?
What impact does a particular intervention have on the risk, severity, and duration of the epidemic?
For microparasites, β x SI=
rate at which susceptible hosts become infected
β = infectiousness (transmission rate)
SI= contact rate
Give an SIR compartmental model
Susceptible —–> Infectious —-> Recovered
S–> I is based on β
I –> R is based on y (rate of recovery)
R –> S is the rate of recovery with no acquired immunity
What is a variation of the SIR model?
SEIR model
Susceptible, exposed, infected, recovered
T/F
In general, in highly endemic areas, children become infected at a very early age
true
BUT
- rates of infection or recovery are likely to vary w age
- assumes a type II survival curve (constant risk of mortality independent of age)
–> type I survival curve is more realistic (mortality increases with age)
Immuno-epidemiology=
how immune responses, especially acquired immunity, affect the population dynamics of host-parasite interactions and parasite population structure
Give 4 factors that need to be considered for immuno-epidemiology
- extent of herd immunity
- degree of cross-immunity
- genotypes of hosts and parasites
- stochastic effects
T/F
Particular contacts and the frequency of contacts varies
considerably among host individual. BUT this variation doesn’t follow any certain pattern
false
this variation does follow certain patterns
Does latency reduce or increase Ro?
higher latency period= reduced Ro
time between recurrences of the epidemic waves extended
Do sexually transmitted infections depend on frequency or density dependent transmission?
frequency-dependent transmission
directly proportional to # of sexual contacts
Life expectancy and age of infection are added as extensions of the standard SIR model. Give the new equation
Ro = 1 + (β(I)/µ) which = 1 + (L/A)
β(I)= rate susceptibles become infected
1/µ= life expectancy= L
1/ β(I)= A= age of infection
T/F
If the average age of infection is higher, Ro would increase
false
would decrease
Give 5 problems with the basic SIR model
- Assumes every person will encounter every other with equal probability (not true)
- heterogenous contact patterns: some contacts more likely than others, & contact patterns change with habits etc - Assumes direct mode of transmission
- even within subgroups contact modes can vary
- depends on non-biological aspects of transmission risk (change in contact patterns) - Role of chance is not accounted for!
- The population is fixed
- Latent period= 0
When considering the behavior or a microparasite endemic, what are the 2 possible outcomes?
- epidemic followed by parasite disappearance
- epidemic followed by endemic infection
Describe the behavior of a microparasite in outcome 1: epidemic followed by parasite disappearance
- intro of parasite to entirely susceptible host pop. Inc contact rate b/w infected & noninfected= rapid spread
- pool of susceptible hosts depleted rapidly –> immunity increases, so # of infected individuals decreases
- lower contact rate= fewer new cases. Too few new susceptibles to sustain epidemic
- epidemic dies out
- # of susceptibles will begin to increase
Describe the behavior of a microparasite in outcome 2: epidemic followed by endemic infection
- intro of parasite to entirely susceptible host pop. Inc contact rate b/w infected & noninfected= rapid spread
- Pool of susceptibles is depleting rapidly, fewer infected hosts as immunity sets in; lower contact rate
- epidemic dies out
- intro of new susceptibles –> new infections @ a lower rate
- susceptibles are not depleted as rapidly, so some level of new infections can persist
