Question 1: This epidemic seems to spread like wildfire. Why?

The speed at which the virus causing this epidemic is spreading is indeed very fast. The high contagiousness of the virus is the main cause of course, but we also have our share of responsibility. Let’s demonstrate it!

Text : Arnaud B. / Simulation : Arnaud B., Benoît G, Arthur B. / Illustration : Odile P. / Translation : Clémentine C., Kevin M.
the 24 March 2020 · 4 minute read

The spread of the virus causing this epidemic is indeed very fast. The high contagiousness of the virus is responsible for it, of course, but we also have our share of responsibility. Let’s demonstrate it!

Let’s start by simplifying the problem and focus on two key aspects: on one side we have a group of people pursuing their daily activities and therefore coming in contact with one another, and on the other the virus is introduced by an infected person (its host). Since it is a new disease that no one is immune to, everyone is susceptible to contract the virus.

For this experiment, you will need: A healthy and mobile population. An infected and mobile person. An environment.

Let’s also simplify the mechanism of virus transmission: from the moment a person is infected by the virus, they are immediately contagious and can transmit it to healthy people they come in contact with. Obviously, in reality this is a bit different because the probability of transmission in each contact is less than the one chosen for the simulation, also we need to include the incubation time.

We just formulated the basis of a very simple model, which could be further developed later in order to better understand the specificities of the COVID-19 epidemic.

One of the big advantages of this model is that we can simulate it and therefore visualise the spread of this virtual epidemic (Simulation 1a: The virus and us).

The two curves (in red are the people who are infected, in green are those who aren’t) evolve symmetrically. They cross when half of the population is infected. Once the number of people susceptible to the infection is too small, the spread of the virus slows down and the epidemic disappears by itself.

Naturally, the speed of transmission depends on the characteristics of the virus, as well as the frequency of contacts between people.

If we run the same simulation with ten times more people, we immediately see that the dynamics of the epidemic is a lot faster: the crossing of the two curves, the point at which half the population is infected happens earlier (Simulation 1b: The more the merrier…).

We also notice that the speed of transmission of the virus in the population (the red curve) is directly linked to the number of infected people and to the number of people still healthy, in a multiplicative manner. This dynamics works a little like a snowball rolling down a snowy slope: once the number of infected people is large enough, the number of people infected at each subsequent stage going forward will only grow. This is the accelerating phase of the epidemics, the one which is qualified as exponential growth.

To put it simply, the more we come in contact with one another, the more opportunities there are for the virus to spread.

This is as simple as that.

Reminder: the models developed on this website are for educational purposes only. They are a lot simpler than models built and deployed by other teams working on COVID-19. They are not substitutes for these reference models and cannot be used to make diagnoses or forecasts. Our goal instead is to raise awareness about this epidemic and its drivers.