Thursday, February 25, 2021

Covid-19: curves and maps to understand why “the trend is no longer good”


SCIENCE – On February 17, the government spokesperson recalled that if the Covid-19 epidemic seemed to be stagnating, or even decreasing, it was important not to proclaim victory and that nothing could change the situation.

In fact, in recent days, several indicators making it possible to follow the evolution of the coronavirus in France have started to rise again. Olivier Véran and Gabriel Attal both recalled this weekend that the trend is “not good” at the national level.

Speeches that take place while restrictive measures should be announced for the Alpes-Maritimes on Monday, February 22. To understand what is happening and the concerns of the executive, The HuffPost offers you to take stock of the Covid-19 epidemic in curves and maps.

Global curves of Covid-19 in France

If we look at the main coronavirus monitoring indicators scrutinized by the government, we see that the downward trend has stopped.

Here is a description of the main indicators monitored:

  • Incidence rate: this is the number of cases detected per 100,000 inhabitants. It is very useful, because it gives an inventory of the epidemic in almost real time (a few days delay for the appearance of symptoms, or even before their appearance for contact cases). But it is dependent on the screening skills.
  • Positivity rate: it is the number of positive tests compared to the total tests carried out. It makes it possible to “control” the incidence rate. If there are a lot of cases in a territory (incidence rate), but this is only due to highly developed screening, the positivity rate will be low. Conversely, if it increases, it means that a greater proportion of people tested are positive, but above all that the infected people who are not tested, who fall through the cracks, are potentially more numerous.
  • Resuscitation bed occupancy rate by Covid-19 patients: It is a figure scrutinized, because it allows to know if the hospitals are able to manage the influx of patients. It is very useful because there is little risk of bias: it does not depend on screening and bed occupations are well reported to the authorities. Its disadvantage: there is a significant delay between contamination and going into intensive care, of about two to three weeks.
  • Death in hospital: Like resuscitations, it is a fairly reliable indicator, but with a significant delay.

We see on these indicators that hospitalizations stagnate again and that the incidence and positivity increase.

Maps and curves by department

All of France is not affected in the same way, as we can see if we look at the evolution of the incidence rate by department. The map below shows that for the most recent figures (February 18), the north and part of the west of France are seeing the number of coronavirus cases start to rise again.

While still a few days ago, the map was overwhelmingly green (you can go back in time by pressing the “Play” button or by moving the black dot above the map).

The incidence rate alone is a useful barometer, but it can sometimes be misleading. The positivity rate makes it possible to limit bias. This is why we have also developed a map of France based on the incidence rate and positivity. Each department is colored according to the evolution of these indicators. The first map (“trends” button) shows the evolution over time of the incidence rate and positivity. Clearly, to know if the situation is improving or deteriorating in each department.

As these rates depend on increases in screening, we have chosen to highlight only the decreases and increases in the two rates for more than a week.

The second map (“global index” button) shows the state of a department in relation to the vigilance and alert thresholds developed by the government during the deconfinement last May. Compared to the second wave, a large part of the departments are no longer beyond the two alert thresholds. But we see that the situation remains complicated in a majority of departments.

We see the same trend emerging, with a north-western quarter of France turning dark blue (increase over 7 days in incidence and positivity).

Incidence and positivity rate curves by department

Some territories are more affected than others. The curves below, which show the evolution over time of the incidence rate and positivity by department, allow us to better understand this.

If the situation in the Alpes-Maritimes is problematic, with an alarming incidence rate (over 550), the evolution is for the moment uncertain, with a very recent drop. We will have to wait a few days to see if this is confirmed, helped by new measures.

Other departments with lower incidence have a worrying trajectory. The Nord and Pas-de-Calais are seeing their curve increase exponentially. Certain departments of Brittany and Île-de-France also saw their curve sharply upward.

Causes unknown, variant suspected

How to explain this sudden acceleration? Hard to say. The fact that it seems regionalized for the moment could raise concerns that the part of the English variant is partly responsible.

We know that it is more contagious (between 35% and 70%), even if the mechanisms are uncertain. In fact, the recent drop could have been false good news. Let us imagine that with the current measures, including curfew, the reproduction rate of the classic virus, the famous “effective R”, is 0.8. This means that an infected person infects an average of 0.8. So the epidemic is declining.

But with a variant, say, 50% more contagious (estimates fluctuate between 35% and 75%), things change. If the classic coronavirus, with current measurements, has an R of 0.8, that of the 501Y.V1 coronavirus (English variant) would be around 1.2. In this theoretical scenario, once the variant cases are dominant, the epidemic progresses again. Exponentially.

And the fact is that the share of variants in Brittany, Île-de-France and Hauts-de-France is significant. However, things are not that simple. If we superimpose the latest estimates from Public Health France on the proportion of variants (the darker the department, the more variants) with the last map of the evolution of the incidence (the more green it is). (the lower the rate, the redder it is, the more it increases), we see similarities, but also divergences.

As always, the evolution of the Covid-19 epidemic depends on multiple factors and is not easy to analyze, let alone predict. The cold wave that affected France for ten days, and in particular its northern part, could also have played a role in this beginning of the epidemic resurgence. In which case, the current warmth could be good news.

In any event, these indicators should be monitored carefully. In particular, it will be necessary to check the evolution of these indicators at the national and departmental level. It will also be necessary to see if the hospital figures follow the trend, with a lag. The next few days will be crucial in determining how France will spend the winter.

See also on The HuffPost: understanding the mutations of the coronavirus in 2 minutes



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