Infectious disease epi

Question 1

When are you dealing with epidemic vs endemic?

Answer 1

More than expected cases: epidemic. If it is not (with or without peak): endemic

Question 2

What are the five stages of an epidemic?

Answer 2

Spark -growth – peak – decline – dies out eventually

Question 3

At the spark, there is often an interest in..

Answer 3

 Often there is an interest in index cases and primary cases

Question 4

What is an index case?

Answer 4

• Index case: First noticed outbreak patient by authorities > awareness that outbreak might be emerging.  Also outbreaks of disease not spread from human-human.

Question 5

What is a primary case?

Answer 5

• Primary case: First documented individual who brings the disease into a population (e.g., school class, all passengers on a ship, an entire country). Can only apply to infectious diseases that spread human-human.

Question 6

Why is it not helpful to list conditions that led to start of epidemic to predict the next?

Answer 6

for every successful zoonotic jump from animal to human, there are millions that fail to infect people at all

Question 7

How to forecast epidemics?

Answer 7

• Forecasting epidemics is possible by measuring their growth:
• Initial growth is exponential

Question 8

What is the latent period?

Answer 8

Period between infection and the onset of the infectious period

Question 9

What is the infectious period?

Answer 9

Time period where transmitting the infectious agent to another host is possible

Question 10

What is the incubation period?

Answer 10

Infection till onset of symptoms

Question 11

What is a serial interval?

Answer 11

Time between appearance of similar symptoms in successive generations

Question 12

What is the generation time?

Answer 12

beginning of infection in an individual to the time that person transmits to another individual

Question 13

What does the basic reproduction number R0 indicate?

Answer 13

Transmission potential of an infectious agent

Question 14

How is R0 defined?

Answer 14

• expected number of new infectious hosts
• directly infected by a randomly selected infectious host
• during period of infectiousness
• when randomly mixing in a large, homogenous population
• where everyone is completely susceptible

Question 15

By what is the R0 influenced?

Answer 15

DOTS

• Duration of infectiousness
• Opportunity: number of contacts in the host population during the time of infectiousness
• Transmission probability: probability of infection being passed on during contact
• Susceptibility: probability of becoming infectious after infection

Question 16

What can you do to intervene with the duration of infectiousness?

Answer 16

Rapid diagnosis
Rapid isolation following diagnosis

Question 17

What can you do to intervene with Opportunity: number of contacts in the host population during the time of infectiousness?

Answer 17

Social distancing
Quarantine
Isolation in single-bed, negative pressure rooms

Question 18

What can you do to intervene with Transmission probability: probability of infection being passed on during contact?

Answer 18

Personal hygiene
Avoiding handshakes

Question 19

What can you do to intervene with Susceptibility: probability of becoming infectious after infection

Answer 19

Vaccine
Prompt and effective treatment

Question 20

R0 does not necessarily concern velocity over time, why?

Answer 20

(r0 = 7, means that within the infectious period, 7 people will be infected. Measles has a very short infection time, therefore the disease will spread quicker. R0 = 7 for leprocy, with a long infectious period for a year, will infect 7 people in a year.

Question 21

R0 < 1 = ?
R0 = 1 = ?
R0 > 1 = ?

Answer 21

• R0 < 1: the disease will eventually disappear (pathogen dies out)
• R0 = 1: the disease will become endemic
• R0 > 1: the epidemic will continue to grow

Question 22

What is the effective reproduction number (Re)?

Answer 22

▪ Effective reproduction number, Re : as R0, but in a population that includes people who have developed immunity to the pathogen.

Question 23

Fraction that needs to be immunized = ?

Answer 23

f = 1-1/R0

Question 24

What does the SIR model stand for?

Answer 24

Susceptible -r- Infectious -a- Recovered and immune

r = rate of transmission between ‘susceptibles’ and ‘infectious’
a = recovery rate of infected individuals into recovered/immune

Question 25

What are six assumptions of SIR closed cohort?

Answer 25

1. Population is constant (closed cohort) and comprises only ‘susceptibles’, ‘infectious’ and ‘recovered’: 𝑆 + 𝐼 + 𝑅 = 1
2. Rate of increase in the infectious is constant and proportional to contacts between susceptibles and infectious
3. Infectious recover and become immune at a constant rate
4. (People meet each other at random)
5. (Latent period is zero, i.e., people become infectious as soon as they are infected)
6. (The duration of infectiousness is equal to the time of illness)

Question 26

What is herd immunity?

Answer 26

Protection against a pathogen due to a high proportion of immune population (vaccination/previous infections), reducing the likelihood of infection for susceptibles.