Mathematical modeling and adequate environmental sampling plans are essential for the public health assessment of COVID-19 pandemics: development of a monitoring indicator for SARS-CoV-2 in wastewater

Since many infected people experience no or few symptoms, the SARS-CoV-2 epidemic is frequently monitored through massive virus testing of the population, an approach that may be biased and may be difficult to sustain in low-income countries. Since SARS-CoV-2 RNA can be detected in stool samples, quantifying SARS-CoV-2 genome by RT-qPCR in WWTPs has been proposed as an alternative tool to monitor virus circulation among human populations. However, measuring SARS-CoV-2 viral load in WWTPs can be affected by many experimental and environmental factors. To circumvent these limits, we propose here a novel indicator WWI that partly reduces and corrects the noise associated with the SARS-CoV-2 genome quantification in wastewater. This method has been successfully applied in the context of Obepine, a French national network that has been quantifying SARS-CoV-2 genome in a representative sample of French WWTPs since March 5th 2020. On August 26th, 2021, 168 WWTPs were monitored twice a week in the metropolitan and overseas territories of France. We detail the process of elaboration of this indicator, show that it is strongly correlated to the incidence rate and that the optimal time lag between these two signals is only a few days, making our indicator an efficient complement or even a credible alternative to the incidence rate. This alternative approach may be especially important to evaluate SARS-CoV-2 dynamics in human populations when the testing rate is low.