The COVID-19 pandemic has put the world to the test. Never before have we been so aware of the many ways we come into contact with innumerable others. We have been forced to reassess and retrain common habits, from the handshake to the simple act of opening a door.

This is because the virus spreads via physical proximity: direct contact between people (handshakes, kisses, and hugs), coughs, or even touching objects with contaminated droplets. The sum of all of these contacts forms a large and dynamic network – just like Facebook maps out our social interactions online. Disconnecting or weakening this extensive network is the key purpose of social distancing measures, currently experienced across the world.

How we come out of lockdown is the next challenge. It is important to avoid a resurgence of the virus while minimizing the societal and economic damage. Proposals range from creating herd immunity to keeping the lockdown intact until the development of a treatment or vaccination.

We are part of a group of mathematicians and economists specializing in modeling random and uncertain situations. We propose an exit strategy that mitigates the health risks of the former, and the social and economic risks of the latter. This is in line with other ideas such as keeping limitations only for high-risk groups.

Our recently published proposal is based on two key elements. First, identifying green zones, meaning areas where the sanitary system is operational, the growth rate of infections is low, and the future risks appear manageable. And second, progressively joining these green zones together once it is safe to do so.

Zoning

The purpose of a lockdown is to partition the world population into disconnected sub-networks, between which movement is limited. But such partitions are far from complete.

In France, where we are based, people are asked to stay within 1km of their homes. Although such a measure significantly slows down the spread, the virus can still travel through the entire network. In a city like Paris, two people who are 2km apart may still share the same grocery shop, and so everyone in this 10km city is connected within five degrees of separation.

Instead of enforcing a radius of movement for each individual, as is the case in France, we propose that it would work better if people were allowed to move within disconnected areas, such as counties, towns, or boroughs. Obviously, such a division would be easier to enforce between separate towns than between boroughs of one single town. The division between some zones may, therefore, need to be legally enforced, while divisions between other zones merely recommended. This would, of course, represent a step up in state-enforced control in many countries.

Such zoning measures have been successfully implemented in several areas in China, notably Wuhan. This would reduce links between different areas and prevent the virus from traveling throughout the territory. As it is impossible to cut links completely due to the need for key workers to travel, and some people ignore the rules, we incorporated this into our analysis.