The effective reproduction rate of the virus is a fundamental indicator of epidemic surveillance.

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 / transmissibility**, which depends on the mode of transmission of the disease (contact, dropletsâ€¦) but also on hygiene and prevention measures in the population.

**2/ Le contact social**, 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 between people 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 dynamics are linear, the epidemic under control. If it is less than 1, the number of new cases gradually decreases and the virus eventually disappears.

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