R nought Flashcards
What is R0?
Basic reproduction number - A key epidemiological metric used to describe the transmissibility of an infectious disease
What does R0 represent?
The average number of secondary infections produced by one infected individual in a completely susceptible population
What does it mean for R0 to be less than 1?
Infection will likely die out over time
What does it mean for R0 = 1?
Stable transmission, disease becomes endemic (constant, stable presence in population/area)
What does it mean for R0 to be greater than 1?
Disease will likely spread, potentially causing an epidemic or pandemic
What is an example of a disease that has a high R0 value?
Measles, which has a R0 value of 12-18. It spreads rapidly in susceptible (unvaccinated) populaitons
What are the 3 main principles of R0?
R0 = B x k x D
Where
B = Transmission probability per contact
k = Number of contacts per unit time
D = Duration of infectiousness
What is herd immunity?
Where enough of the population is immune (via vaccination or prior infection) to block chains of transmission, protecting susceptible individuals
What is the critical vaccination ratio?
The proportion of the population that must be vaccinated to achieve herd immunity
What is the formula for critical vaccination ratio (Vc)?
Vc = 1 - (1/R0)
E.g. for measles:
Vc = 1 - (1/18)
Vc = 0.94 = 94% vaccination
How does infection type (e.g. acute vs chronic) relate to R0?
Acute infections (e.g. those by norovirus and measles) tend to have intense but short bursts of transmission, therefore a high R0 is needed for sustained spread.
Chronic infections (e.g. HIV, HBV) can persist even with low R0, due to long infectious periods
What is the role of WHO’s global influenza surveillance system?
It adjusts vaccines based on predicted antigenic drift to keep R0 < 1 in vaccinated populations
What is density-dependent transmission?
As population density increases, so does the rate of contact between infected and susceptible individuals
I.e
Transmission is proportional to population density
For what diseases is density-dependent transmission important?
Airbourne or droplet-spread diseases like:
- Sars-COV-2
- Influenza
- TB
What is frequency-dependent transmission?
Transmission dependent on proportion of infected individuals (not raw density)- each person has a fixed number of contacts
For what diseases is frequency-dependent transmission important?
- STIs (e.g. HIV)
- Vector-borne diseases (e.g. malaria)
- Some zoonotic diseases
What would be some examples of control strategies for density-dependent transmission diseases?
- Reduce crowding
- Masks
- Increase ventilation
- Social distancing
(just think COVID)
What would be some examples of control strategies for frequency-dependent transmission diseases?
- Vector control
- Partner tracing (STIs)
- Zoonotic reservoir culling
