A wastewater treatment plant in the village of Saint-Maurice (Isère).
(illustration) -
ROMAIN DOUCELIN / SIPA
Is it relevant to use wastewater analysis to observe the epidemic evolution of Covid-19?
Asked about this on LCI on Sunday, the infectious disease specialist Karine Lacombe was quite nuanced on the precision of these data: “It is an indicator which is very interesting but it is an indicator which is still crude.
It does not make it possible to know exactly where the epidemic has slowed down, where the epidemic has resumed.
"
However, wastewater makes it possible to locate, within a city, areas where the virus is actively circulating.
Sunday January 31, to justify the government's decision not to set up a third containment intended to slow the Covid-19 epidemic, Olivier Véran cited several indicators now well known: incidence rate (the number of new daily cases ), number of intensive care admissions… But also, more unexpectedly, “traces of viruses in wastewater”, which are “down” in Ile-de-France.
Although this method has been used successfully for nearly a year in different areas of the territory, it does not necessarily count among the most highlighted epidemic analysis data.
Asked later in the day on LCI about the relevance of this indicator, the head of the infectious disease department of Saint-Antoine hospital, Karine Lacombe, was moreover quite reserved: “It is an indicator that is very interesting but it is an indicator which is still rude.
It does not make it possible to know exactly where the epidemic has slowed down, where the epidemic has resumed.
"
"This is one of the indicators that we put in the balance, such as the number of positive [cases] every day, the number of people who arrive in intensive care ... We see that there, we are rather above the threshold of 3,000 that we did not want to exceed […].
Is this indicator less important than that of wastewater?
We ask ourselves the question, ”she concluded.
What provoke the disbelief of several Internet users, some relaying this excerpt on Twitter alongside a map of Marseille produced by the city's marine firefighters, distinguishing several urban areas according to whether their wastewater shows a "low level". "," Moderate "," high "or" very high "of coronavirus.
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“I think Karine Lacombe's statement testifies to a poor knowledge of wastewater analysis.
It is about a method of global analysis rather than "coarse" ", estimates Vincent Maréchal, professor of virology at Sorbonne University and co-founder of the Epidemiological Observatory in wastewater (Obépine) - of which he had recently detailed the operation at
20 minutes
.
“What we measure in wastewater is the virus excreted by sick or asymptomatic people.
The question that should rather be asked is: "Is wastewater predictive or not of what we can see through other indicators of virus circulation, such as the incidence rate? ? "
It has been demonstrated, on several occasions, that this was the case, for example at the end of June 2020, since the gradual increase of viruses in the wastewater of Ile-de-France announced the second wave, then confirmed by the number hospitalizations ”, explains the specialist.
Likewise, studies carried out at the Île d'Yeu wastewater treatment plant, on a much smaller scale, have made it possible to establish an “oscillation of the virus corresponding to the flow of tourists”.
"There is a real correlation between the viral load detected in wastewater and what is happening in the population", adds Vincent Maréchal.
The various wastewater analysis curves available on the Obépine network site, carried out for many large cities, thus match the incidence rate over the months, as can be seen for example by focusing on the city. of Nancy (Meurthe-et-Moselle), in the Grand Est.
More "judicious" zone by zone targeting when the virus is circulating "little"
It is also possible to establish more precise data on certain areas or in very specific neighborhoods, even if the national network presents great disparities at this level, as Vincent Maréchal underlines: “The more one enters the networks the more precisely the viral loads can be localized, but it depends on the structures.
These analyzes are extremely dependent on the organization of the wastewater network, some are easy to access, others much less.
"
According to the specialist, the fact of focusing on very specific areas is not necessarily the most relevant when the virus is circulating very actively: “I agree with Karine Lacombe's analysis with regard to Marseille.
Marine firefighters have been communicating since January on the very active circulation of the virus by detailing it area by area while long-term surveys show that such circulation has been going on for a while throughout the city.
When the virus is actively circulating, there is little point in looking precisely neighborhood by neighborhood or area by area.
It is more judicious when there is little circulation since it makes it possible to identify the places concerned and to intervene, by multiplying the tests or by encouraging isolation.
"
Contacted by
20 Minutes
, the Marseille firefighters, responsible for this analysis, had not responded to our requests before the publication of the article.
At the beginning of January, however, they explained to us that they were studying the wastewater at the wastewater treatment plant located under the Vélodrome stadium to draw an overall trend, then completed by a complete weekly mapping of the city (distinguishing 37 sectors in total).
"Wastewater remains one indicator among others, which should not replace others and even less those of the population: they are two different arms to use, while doctors perhaps have too much of a tendency to rely on on symptomatic monitoring of patients, ”concludes Vincent Maréchal.
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