R0 Flashcards
Define R0
The basic reproductive ratio/ number.
It is the average number of new cases that arise directly from an infectious individual which enters an entirely susceptible population
Pretty much the secondary infections!
R0 = DXkXβ D = duration of infection k = number of contacts per unit time β = probability of infection if contact occurs
Define R
the effective reproductive ratio (or number)
- Represents the potential for a pathogen to spread from individual to individual.
- It is the average number of new cases that arise from an infected case, and this varies depending on the proportion of the population that is susceptible
Define herd immunity
o the indirect protection from infectious disease that occurs when a large percentage of a population has become immune to an infection. The remaining susceptibles are protected by others’ immunity
o The herd immunity threshold is where R drops below 1 – and is the same as the critical proportion to vaccinate.
Define infectious disease
a disease caused by a microorganism and therefore potentially transferable to new individuals.
Define communicable/ transmissible disease
an infectious disease that can be transmitted from one individual to another.
Define contagious disease
: a very communicable disease capable of spreading directly between individuals (somewhat obsolete).
Define the index
also known as primary case: the first infected person of a disease
Define secondary case
those infected by the primary case
What is the basic reproduction number a product of?
R0
Duration (how many days they are infectious)
k- how many people they contact per day
beta- the chances of transmission to those contacts
What are properties of R
- R0 varies among diseases
- R changes during an outbreak
- R has a threshold value
- R is time independent
- R is an average
- R is reduced by control measures
- R determines the target vaccine proportion
- R determines the herd immunity threshold
Equation for R
R = R0 X S
Basically R =DXkXBXS
D= duration of infection k = number of contacts (per unit time) B = probability transmission if contact occurs S = proportion of the population that is susceptible
If R is 1, greater than 1, less than 1 what deos this mean?
- R = 1 is an epidemiological threshold
- R>1 = outbreak will grow. Epidemic possible
- R = 1 = Outbreak stable, under control
- R<1 = Outbreak reducing, extinction possible
What to remember about R
It tells us about change in GROWTH of an epidemic, not change in SIZE of an epidemic
It relates to the incidence and RATE of increase
Cases can GROW even if R is less than 1
One case giving rise to 0.9 cases makes 1.9 cases. This is about NEW cases.
Super spreaders
Some individuals pass infection to many others, while others pass it to few or none.
Why are come individuals super spreaders?
• Behaviour – individual behaviour may lead to transmission
o Have high rates of contact (kappa)
• Physiology
o Have high rates of transmission, given contact (beta)
o Some hosts develop higher pathogen titres (ie, more infectious)
o Long duration of infectiousness (D)
Epidemic definition
o A disease where R>1
