The coronavirus hits the east of France harder than the west. Climate, borders, population density, collective immunity … Why such a clear territorial distribution in the circulation of the virus? Renaud Piarroux, epidemiologist at Assistance Publique – Hôpitaux de Paris (AP-HP), answers our questions.
For several weeks, the cards of France for the coronavirus have had two colors. The west seems relatively preserved by the epidemic and the east shows a circulation of the virus sometimes ten times stronger than on the other side of the country. Renaud Piarroux, epidemiologist at Assistance Publique – Hôpitaux de Paris (AP-HP) and author of the book “The wave. The epidemic seen from the field” (CNRS editions), enlightens us on the reasons for such a territorial divide.
La Dépêche du Midi: why is the virus unevenly distributed across the country?
Renaud Piarroux: In epidemics, there is a very strong geographic determinism linked to urbanization, population density, behavior or even the climate. Initially, at the time of the first wave, the epidemic was marked by specific events which precipitated its explosion. For example, the virus hit the Grand Est because there was an evangelical meeting [à Mulhouse, en février 2020, NDLR]. But if these people had met in Grenoble, the epidemic could as well have exploded in Rhône-Alpes. These are unpredictable, random events. The second wave, it responds more to geographical characteristics conducive to the spread of the virus, which has become widespread.
Does the cold explain why the epidemic situation has worsened in certain regions?
There have been several studies on the subject and we are now sure that when it is colder, around 5 to 10 ° C, the transmission of the virus is facilitated. One of the hypotheses to explain this is that the cold has an impact on the aerodynamics of the droplets carrying the virus. When the weather is dry and hot, the droplets responsible for transmitting the virus dry out. When it is very cold, they fall more quickly. But when it’s 5 ° C, they stay in the air. There is also the behavior of the population. In colder areas, we regroup under shelters, we ventilate the rooms less …
Can the proximity of the most affected regions to other European countries explain this difference between west and east?
It’s a bit like the chicken and the egg problem. The movement of people still plays a role in epidemics, but the pattern is not: an explosion of cases in Belgium, which ended up passing through France and arriving in Lille. It should be seen as the same focus which developed at the same time, with comparable weather and traffic conditions, as between the Alpes-Maritimes and Savoy which are adjacent to the most affected regions of Italy: Lombardy , Piedmont and the Aosta Valley. In Occitania, which is neighboring Spain, as the climate is warmer and the virus is relatively contained on the Spanish side with strict measures, there is not particularly an outbreak at the border.
The most impacted areas during the second wave are roughly the same as those of the first wave. However, one would have thought that collective immunity in these regions would have preserved them more …
Collective immunity exists, but it is insufficient. If it was enough, we would have a reverse map between the two waves, with heavy traffic in Bordeaux, Nantes or Toulouse. For the moment, the geographic factors of vulnerability act more strongly than immunity. In the first wave, the epidemic storm was stopped by containment, so herd immunity did not develop. We are now at around 20% immunity in the most affected regions, far from the 70% of the immune population necessary to stop the spread of the virus.
So it’s partial group immunity, in a way. The problem that arises in places where the virus circulates little, such as in Occitanie or Aquitaine, is that this protection is even more partial. In the Grand Est, immunity is higher and continues to increase. But other territories remain very vulnerable.
Are the more spared western regions, such as Occitania, therefore likely to be more affected?
At its lowest, the incidence rate was around 75 to 80 new positive cases per 100,000 inhabitants in early December. We started to see an increase in mid-December and we were at 104 just before Christmas. There is less urgency to take measures in Occitania than in the Grand Est, but we must remain vigilant because, despite the rather mild climate, there may be aggravating factors. Already, winter: the coldest and wettest month is not December, so we must be vigilant. Then there is this variant virus discovered in the UK, which is said to be 50 to 75% more contagious. If it spreads in France, that will change the situation. An increase of about 60% in transmission is huge! Without being alarmist, we must remain extremely vigilant.
Will the vaccine be the solution to stem the circulation in the territory?
We will have a million people vaccinated in January and February, it is a start but it is not yet. Initially, we will not act on the distribution but limit the dangerousness for the elderly. We will save lives in nursing homes thanks to vaccination, but these are not the people who are spreading the epidemic. In the spring, more people will be able to get vaccinated and the epidemic situation will improve. But you have to go through the winter first.
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