Covid-19: how to read the epidemic this summer 2022?

A dominant variant, less violent than some previous, albeit more portable, resuscitation beds becoming vacant week by week, fewer new hospitalizations: we have all but forgotten an epidemic of Covid-19, also driven from our minds by our national electoral woes Bomb crash in Ukraine.

However, the epidemic is still there and developing quietly. But this summer of 2022 is not the summer of 2020 or 2021. Admittedly, the number of hospital admissions is comparable (10,500 on June 16, 11,500 on the same day in 2021, 13,900 on this Friday)but many other parameters have changed: distribution of the vaccine, less testing, reopening of our social life… So how should we look at the epidemic today?

Tracking the “R”, the reproductive rate of the virus

The slightest resort to screening is “a really very important selection bias and it is therefore difficult to compare different time periods in terms of the number of tests,” says Professor Pierre-Marie Preux, epidemiologist and director of the Inserm unit of tropical neuroepidemiology from Limoges. He estimates that ” The most important parameter is the effective R “(Reproduction rate of the virus, editor’s note) … which has just started to rise again and exceeds 1 again (One affected person infects more than one other person), so the epidemic is increasing again”.

But this health crisis has taught us that it is actually a series of parameters that should be taken into account. “These are the cases and not the severe cases that are treated in the hospital or intensive care unit, which depend on the severity of the variant, the vaccination rate and the effectiveness of the vaccine on the variant in question,” recalls Professor Preux. Before agreeing that, as in every phase since 2020, science doesn’t know everything: “With the BA-5 (the currently dominant variant, ed.) we have no real idea of ​​​​the first two parameters. What is worrying, however, is that we know this The effectiveness of the vaccine decreases over time and the time since the last dose (whatever it was) is starting to get a little long…”

In 40 years, the University of Limoges has become an international reference in tropical neurology

“The epidemic progresses as positive cases and positivity rates increase together”

Spotted by the public as the founder of CovidTracker, data scientist Guillaume Rozier recalls the mathematical limit of the “R”: “Although it is the reference indicator for epidemiologists, it is not data that results directly from a measurement, but a calculation , which can be made from confirmed cases, emergency room visits, hospital records. For him, “it’s the same to review the cases and the screening.” From what angle? There, “The less we seek, the less we find”.

Who is Guillaume Rozier, founder of CovidTracker?

Guillaume Rozier also asks us to consider several parameters at the same time in order to assess whether or not there will be a resurgence of the epidemic. Two more precisely: “You have to look at both the number of positive cases and the test positivity rate, which puts those cases in relation to the total number of tests.”, which clears the Volume of Tests effect. “If these two indicators rise, we know that the epidemic is starting again and that its consequences will be measurable using hospital data between one and two weeks later,” he says. And vice versa in case of a decline. But “if the two dates do not point in the same direction, we cannot conclude on the dynamics of the epidemic”.

It is up to observers to adjust as they have since the beginning of this health crisis.

Julian Bigay

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