Exam 1: Transmission Models

Question 1

What is communicability?

Answer 1

The ease at which a disease agent is spread within a population

Question 2

What does transmission require (epidemiologic triad)?

Answer 2

A susceptible host, an agent, and the right environment to establish an effective contact

Question 3

What is effective contact?

Answer 3

Contact between infected and susceptible animals

Contact that results in transmission of disease agent

Question 4

What does the Reed Frost model predict?

Answer 4

The number of new cases in the next incubation period interval

Question 5

The Reed Frost model is Ct+1 = St (1-q^Ct). What do the variables mean?

Answer 5

Ct+1: cases in the time t+1, where 1 represents an interval of time equal to the incubation period of the agent
St: number of susceptible at time
q: the probability of no contact
Ct: cases at time t

Question 6

What is the R0 transmission model?

Answer 6

Intrinsic (or basic) reproductive rate
Average number of secondary cases that develop from one primary case during its entire communicable period in a population of susceptible hosts

Question 7

How do you calculate R0?

Answer 7

Number of contact per unit time x probability of transmission per contact x duration of infectiousness

Question 8

What does it mean if R0 is greater than 1?

Answer 8

The infection is maintained

Question 9

What does it mean if R0 is less than 1?

Answer 9

The infection cannot be maintained

Question 10

How can you change the number of contacts per unit time in R0?

Answer 10

Reduce the number of animals
Add nonsusceptible animals
Change susceptible to nonsusceptible
Reduce animal density
Restrict animal to animal contact

Question 11

How can you change the probability of transmission per contact in R0?

Answer 11

Restrict contact to be non-effective

Alter mucosal defense

Question 12

What is herd immunity?

Answer 12

The ability to protect susceptible individuals within a group due to the high proportion of immune individuals in the same population

Question 13

What is a sufficient herd immunity level?

Answer 13

70-80%

Question 14

What does herd immunity depend on?

Answer 14

Properties of the agent (infectious dose, duration of communicably, methods of contact, routes of entry)

Question 15

What is the SIR model?

Answer 15

Each individual is in one of 3 classes:
Susceptible
Infected
Recovered

Question 16

What are the assumptions of the Reed Frost Model that limits its application in the real world?

Answer 16

Infection spread solely by adequate contact between individuals
All individuals are susceptible- there is no prior immunity
Once adequate contact is achieved, individual develops infection and serves as a case for the following interval
Assumes a fixed probability of adequate contact, which does not vary over time or between individuals
The population is closed

Question 17

What does it mean if R0 is equal to 1?

Answer 17

One infected animal can infect one susceptible