Sewage Samples Show COVID-19 Spreading Fast in Some French Cities

By Geert De Clercq

February 12, 2021

PARIS (Reuters) - Sewage samples from a new nationwide COVID-19 monitoring system show that in some French cities traces of coronavirus are spiking above levels seen during the second wave of the epidemic in the autumn.

France's new "Obepine" network continuously samples city sewage in nearly 50 waste water stations and publishes charts that indicate the quantity of genetic material from the SARS-CoV-2 virus, which causes COVID-19.

For cities where data are available from spring 2020, the indicator charts show a strong correlation with charts of the number of positive cases and can give early warning signals.

"In Lille, Marseille and Strasbourg we see a strong uptick, while in the Paris region the situation seems more under control," said Vincent Marechal, a Sorbonne university virology professor and co-founder of the Obepine network.

He added that sampling shows major disparities between cities, which could mean measures may have to be adapted locally. For now, the government has a nationwide curfew in place after experimenting with more regional solutions.

The indicator readings are derived from RT-qPCR that identifies the viral "E gene" in wastewater samples, and translates virus levels to a scale of 0 to 150 that incorporates historic levels. ( For that reason, the indicator can generate negative numbers when virus is present, but at lower levels.

In central Paris, COVID-19 indicator readings fell from over 150 in April 2020 to -90 in June-July, but rose back to 150 in early November and have stayed at that level since then.

In Marseille however, readings peaked at 125 in early November, fell back to 100 following the second lockdown, but spiked to 175 from January. Nice, Toulouse and Strasbourg show similar patterns.

As asymptomatic people still shed the virus in their feces, the system can give warnings before hospitals admissions rise.

"When virus circulation is slow, the system can give up to three weeks advance notice, probably because of young people who are infected but do not get sick," said Laurent Moulin, who heads the Eau de Paris water company laboratory.

He said in June-July 2020, when after a strict lockdown seemed to have vanquished the epidemic, wastewater sampling gave early warning about a possible resurgence.