The R number

Question 1

Define an infectious disease

Answer 1

A disease caused by a microorganism which can therefore be potentially transferrable to new individuals

Question 2

Compare a transmissible vs a contagious disease

Answer 2

Transmissible = infectious disease that can be transmitted from one individual to another
Contagious = a communicable disease capable of spreading directly between individuals

Question 3

What is a primary case?

Answer 3

The first infected passenger - they infect secondary cases

Question 4

What is infection a product of when linked to the primary case?

Answer 4

How many days they are infectious, how many people they have effective contact with and the chance of transmission to those contacts

Question 5

What is R0?

Answer 5

The average number of new cases that arise directly from an infectious individual which enters an entirely susceptible population

Question 6

How is R0 calculated?

Answer 6

Duration of infectiousness (D) x Number of contacts per unit time (K) x probability of transmission if contact occurs (B)

Question 7

Is R0 constant?

Answer 7

No, it varies among disease, there is variation even among similar diseases

Question 8

Why does R change during an outbreak?

Answer 8

Number of susceptible people will eventually run out

Question 9

What is the epidemiological threshold of R?

Answer 9

1

Question 10

What do the follwoing mean:
R > 1
R = 1
R < 1

Answer 10

• Outbreak will grow, epidemic is possible
• Outbreak is stable, ‘under control’
• Outbreak is reducing, extinction is possible

Question 11

If R < 1 can cases grow?

Answer 11

Yes.
R is about the rate of increase/incidence
It tells us about the rate of growth

Question 12

Define R?

Answer 12

The number of new cases generated during the period that the index case is infectious
The average number of new cases that arise directly from an infectious individual

Question 13

What are some reasons for an individual being a super spreader?

Answer 13

• Individual behaviour (have high rates of contact)
• Have high rates of transmission
• Higher pathogen titres
• Long duration of infectiousness

Question 14

For diseases with a small variation in secondary cases, how can transmission be dramatically reduced?

Answer 14

• Preventing super-spreading events

- Identifying and isolating super-spreaders

Question 15

What is the primary objective of control programs?

Answer 15

Reduce R to below 1

Question 16

What determines the target vaccine proportion?

Answer 16

R

Question 17

What are the aims of vaccination?

Answer 17

• Protect individuals from becoming infected
• Reduce severity of disease if infection occurs
• Reduce the proportion of the population susceptible to disease

Question 18

What is herd immunity?

Answer 18

The indirect protection from infectious diseases that occurs when a large percentage of a population has become immune to infection, remaining susceptibles are protected by others immunity

Question 19

If R0 = 3, how much of the population will have to be immune for R to be below 1?

Answer 19

2/3 of the population

One third will still be susceptible but the epidemic will be declining

Question 20

What are the 2 ways that immunity develops?

Answer 20

Natural exposure

Vaccination

Question 21

What is the simple equation for R?

Answer 21

R0 x S (proportion susceptible)

- R will always change as the susceptible population changes

Question 22

What are the 2 ways that immunity develops?

Answer 22

Natural exposure

Vaccination

Question 23

What is the simple equation for R?

Answer 23

R0 x S (proportion susceptible)

- R will always change as the susceptible population changes