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Review 1: "How early into the outbreak can surveillance of SARS-CoV-2 in wastewater tell us?"

This study explores wastewater surveillance for monitoring COVID-19 outbreaks and identifies the case prevalence required for detecting infection in a hospital setting. The claims are somewhat supported by the data presented, but confounding variables limit policy applications.

Published onOct 02, 2020
Review 1: "How early into the outbreak can surveillance of SARS-CoV-2 in wastewater tell us?"
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How early into the outbreak can surveillance of SARS-CoV-2 in wastewater tell us?
Description

There is increasing interest to use wastewater-based surveillance of SARS-CoV-2 as an early warning of the outbreak within a community. Despite successful detection of SARS-CoV-2 in wastewaters sampled from multiple locations, there is still no clear idea on the minimal number of cases needed in a community to result in a positive detection of the virus in wastewaters. To address this knowledge gap, we sampled wastewaters from a septic tank and biological activated sludge tank located on-site of a hospital. The hospital is providing treatment for SARS-CoV-2 infected patients, with the number of hospitalized patients per day known. It was observed that > 253 positive cases out of 10,000 persons are required prior to detecting SARS-CoV-2 in wastewater. There was a weak correlation between N1 and N2 gene abundances in wastewater with the number of hospitalized cases. This correlation was however not observed for N3 gene. The occurrence frequency of SARS-CoV-2 is at least 5 times lower in the partially treated wastewater than in the septic tank. Furthermore, abundance of N1 and N3 genes in the activated sludge tank were 50 and 70% of the levels detected in septic tank, suggesting poor persistence of the SARS-CoV-2 gene fragments in wastewater.

RR:C19 Evidence Scale rating by reviewer:

  • Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.

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General overview:

In this study, the author aims to address the minimal number of cases needed in a community to result in a positive detection of SARS-CoV-2 in the wastewaters of a hospital treating infected patients, and the correlation between N gene abundance in wastewater with the number of hospitalized cases. In their results, they found that > 253 positive cases for every 10,000 people are required to detect SARS-CoV-2 in the wastewater. A weak correlation between N1 and N2 gene abundances in wastewater and the number of hospitalized cases has also been observed. The claims are generally supported by the data and the methods used.

Comments:

1. Usually in the hospital the infected cases are confirmed, and a subset are found to exhibit symptoms. Compared to the individuals exhibiting symptoms, the ones that are asymptomatic have a lower viral load. This should be considered and discussed when the author uses a hospital model to make conclusions for the community.

2. Most of the viral particles or genomes should be attracted to the biological activated sludge in the bottom of the tank. Please explain the reason why about 8 out of 52 samples (15.4%) collected from the biological activated sludge tank were tested positive.

3. Referring to lines 60-61, monitoring for SARS-CoV-2 in wastewater has been done in China and therefore the listed countries should include “China.”

4. In line 100, the full names for “LOD” should be mention first.

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