The R0 is the number which represents the average reproduction rate (or the average speed of contagion) of a disease, that is to say how many people on average a single infected individual risks infecting around him.
It is calculated by multiplying three factors:
1 / The transmissibility
, which depends on the mode of transmission of the disease (contact, droplets…) but also on hygiene and prevention measures in the population.
2 / Social contact
, that is to say the number of people crossed on average per day by each individual and the degree of contact between them (kiss, handshake, physical proximity, etc.)
3 / The duration of the contagious period,
specific to each virus, and which varies according to the person and the degree of infection.
If it is greater than 1, the number of new cases increases every day: the dynamic is exponential, it is runaway.
If it is equal to 1, the number of new cases is constant over time: the dynamic is linear, the epidemic under control.
If it is less than 1, the number of new cases gradually decreases and the virus eventually disappears.
