Lec 20-21: Epidemiology

Question 1

What did Daniel Bernouli (1766) do for the field of epidemiology?

Answer 1

created math models to analyze dynamics of smallpox epidemic in Paris

Question 2

Epidemic or outbreak=

Answer 2

disease occurrence among a population that is above what’s expected at a given time & place

Question 3

Cluster=

Answer 3

a group of cases in a specific time & place that might be more than expected

Question 4

Endemic=

Answer 4

disease or condition present among a population at all times (constantly present at a steady rate)

Question 5

pandemic=

Answer 5

a disease or condition that spreads across regions/ countries

Question 6

rate (in terms of epidemiology)=

Answer 6

of cases occurring during a specific period; always dependent on the size of the pop during that period
- prevalence vs incidence

Question 7

T/F

1. An endemic disease is constantly present in a pop, with a predictable rate of spread
2. An epidemic disease is characterized by a sudden increases in cases across the world
3. A pandemic disease is characterized by a sudden increase in cases across several countries or the world

Answer 7

1. true
2. false. epidemic= sudden increase in cases spreading through a large population (but not worldwide)
3. true

Question 8

Explain the epidemiological triangle

Answer 8

This is used as a tool to address the 3 components that contribute to the spread of the disease

1. Environment (climate, sanitation, health care, etc)
2. Host (genetics, age, sex, health, previous infections, etc)
3. Agent (pathogen, parasite, virus)

Question 9

The reproductive number (R0) represents a threshold for disease spread. Explain the different values and what the thresholds might be

Answer 9

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

Question 10

Is the basic reproductive number fixed?

Answer 10

No!

Different aspects of parasite transmission can affect it, and may change over time

Question 11

For microparasites, what determines its persistence in host populations?

Answer 11

Can each infection replace itself?

Determined by R0= # of secondary infections produced by one infected host introduced into a pop comprising only susceptible hosts

Question 12

For macroparasites, what determines its persistence in host populations?

Answer 12

Can each individual replace itself?
Determined by R0= # of new females produced by 1 female in absence of density-dependent constraints

Question 13

Give the 3 factors that influence R0

Answer 13

1. duration of infectious period
2. # of susceptible people in the population (contact rate)
3. mode of transmission (eg airborne vs spread by bodily fluids etc)

Question 14

Rank the following diseases from highest to lowest reproductive number

Ebola
Rabies
Fly
Measles
Chicken Pox

Answer 14

Measles
Chicken pox
Ebola
Flu
Rabies

Question 15

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?

Answer 15

higher R0= higher vacc % required

eg. Malaria and Measles required near 100% vacc to eradicate

Question 16

T/F

R0 can estimate the proportion (p*) to be vaccinated or otherwise treated in order to eradicate the infection

Answer 16

true

p*= 1-1/R0

Question 17

What does R0 determine?

Answer 17

The ability of parasites to regulate host population

Outbreak of epidemics, persistence of endemic infections, & whether or not the parasite dies out

Question 18

Give the proportionality for the basic reproductive number

Answer 18

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)

Question 19

Give the equation to calculate Ro for MICROparasites

Answer 19

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

Question 20

Give the equation to calculate Ro for MACROparasites

Answer 20

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

Question 21

What kind of important questions can modelling epidemics answer? Lots of options, give 2-3

Answer 21

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?

Question 22

For microparasites, β x SI=

Answer 22

rate at which susceptible hosts become infected

β = infectiousness (transmission rate)
SI= contact rate

Question 23

Give an SIR compartmental model

Answer 23

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

Question 24

What is a variation of the SIR model?

Answer 24

SEIR model

Susceptible, exposed, infected, recovered

Question 25

T/F In general, in highly endemic areas, children become infected at a very early age

Answer 25

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)

Question 26

Immuno-epidemiology=

Answer 26

how immune responses, especially acquired immunity, affect the population dynamics of host-parasite interactions and parasite population structure

Question 27

Give 4 factors that need to be considered for immuno-epidemiology

Answer 27

- extent of herd immunity - degree of cross-immunity - genotypes of hosts and parasites - stochastic effects

Question 28

T/F Particular contacts and the frequency of contacts varies considerably among host individual. BUT this variation doesn't follow any certain pattern

Answer 28

false this variation does follow certain patterns

Question 29

Does latency reduce or increase Ro?

Answer 29

higher latency period= reduced Ro time between recurrences of the epidemic waves extended

Question 30

Do sexually transmitted infections depend on frequency or density dependent transmission?

Answer 30

frequency-dependent transmission directly proportional to # of sexual contacts

Question 31

Life expectancy and age of infection are added as extensions of the standard SIR model. Give the new equation

Answer 31

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

Question 32

T/F If the average age of infection is higher, Ro would increase

Answer 32

false would decrease

Question 33

Give 5 problems with the basic SIR model

Answer 33

1. 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 2. Assumes direct mode of transmission - even within subgroups contact modes can vary - depends on non-biological aspects of transmission risk (change in contact patterns) 3. Role of chance is not accounted for! 4. The population is fixed 5. Latent period= 0

Question 34

When considering the behavior or a microparasite endemic, what are the 2 possible outcomes?

Answer 34

1. epidemic followed by parasite disappearance 2. epidemic followed by endemic infection

Question 35

Describe the behavior of a microparasite in outcome 1: epidemic followed by parasite disappearance

Answer 35

1. intro of parasite to entirely susceptible host pop. Inc contact rate b/w infected & noninfected= rapid spread 2. pool of susceptible hosts depleted rapidly --> immunity increases, so # of infected individuals decreases 3. lower contact rate= fewer new cases. Too few new susceptibles to sustain epidemic 4. epidemic dies out 5. # of susceptibles will begin to increase

Question 36

Describe the behavior of a microparasite in outcome 2: epidemic followed by endemic infection

Answer 36

1. intro of parasite to entirely susceptible host pop. Inc contact rate b/w infected & noninfected= rapid spread 2. Pool of susceptibles is depleting rapidly, fewer infected hosts as immunity sets in; lower contact rate 3. epidemic dies out 4. intro of new susceptibles --> new infections @ a lower rate 5. susceptibles are not depleted as rapidly, so some level of new infections can persist