5: Infectious Disease Epidemiology

Question 1

Define

Susceptible
Immune
Pathogen
Dose
Vector
Reservoir
Source
Zoonosis

Answer 1

Susceptible
A person that could become infected if exposed

Immune
opposite to suceptible, non-susceptible would mean that person lacks required receptor for pathogen

Pathogen
microorganism that could cause disease

Dose
Amount of pathogens that attack the person

Vector
Animal that transmit pathogen to a susceptible

Reservoir
Natural occurence / ecological niche of the pathogen outside of a human

Source
object, animal, person from where pathogen is acquired

Zoonosis
infection between animals and humans, infectious agent that can overcome the species barrier

Question 2

Order of cases

Answer 2

Primary case
Case that brings the disease into population

Secondary case
People infected by primary case

Index case
The first case discovered by the health care system in an outbreak, often this is not the primary case

Question 3

Define time periods

Latent
Infectious
Incubation
Serial interval
Generation time

Answer 3

Latent
time between infection and start of infectious period

Infectious
length of time period during which a person can transmit disease

Incubation
Time between infection and start of disease symptoms, often depends on infectious

Serial interval
time between appearence of similar symptoms in successive generations

Generation time
Time from infection in primary case until time of infection in secondary case

Question 4

Infectious Disease
Communicable Disease
Transmissible Disease
Non infectious / communicable Disease

Answer 4

Question 5

5 special things about Infectious Disease epidemiology

Answer 5

1. Case may be a risk factor
2. Case can be a source without being recognized as a case
3. People can be immune
4. There is sometimes need for urgency
5. Preventive measures usually have a good scientific basis

Question 6

Reproductive rate R

Answer 6

measures the potential for infection to spread in a population, depending on:

• probability of transmission in a contact between infected and a susceptible (ß)
• Frequency of contacts (k)
• infectious period (D)
• proportion of susceptibles in population (P)

R = ßkD*P

Question 7

Basic reproductive rate

Answer 7

Ro
average numbers of persons directly infected by infectous case during entire infectious period in a totally susceptible population, same formula but without P

R < 1: Disease may dissapear
R = 1: endemic
R > 1: epidemic

Question 8

Application of reproductive rate to vaccinaion

Answer 8

proportion of p can be immunized, out of Ro people who could get infected by primary case, p x Ro will escape

primary case will infect Ro - Ro x p people

For prevention, number of secondary cases must be < 1

Ro - p*Ro < 1

Solve for p > 1 - 1/Ro

e.g. if Ro is 5, 80% of people needs to be vaccinated

Question 9

How to estimate Ro?

Answer 9

1. Data from early stage of outbreak (susceptible population)

generation time g, which factor has incidence increased in time of g

2. Use data about mean age of infection

Ro = 1 + L/A

L = average life span of individuals
A = average age of infection

Question 10

What are compartent models?

Formula
Good for what?
Limitations

Answer 10

A population size N can be divided into:

S: Proportion that is susceptible
I: Proportion that is infectious
R: Proportion that is immune (removed)

S(t+1) = S(t) - ßK* S(t)I(t)
I (t+1) = I(t) + ßKS(t)*I(t) - I(t)/D

Good for
- Good for reaction (providing sufficeint health care) because you can estimate height and timing of incidence peak

• Good for action (predicting and comparing effects of vaccinations or other interventions)

Limitations
- fixed population
- latent period is 0
- durations of infectivity = clinical disease length
- epidemic will not change habits in populaions
- whole model is deterministic
- every person meets every person
with equal probability
- all individuals have same probability
to contract disease when exposed