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SARS-CoV-2 titers in wastewater are higher than expected from clinically confirmed cases

Our take —

This study utilized wastewater in Massachusetts for SARS-CoV-2 surveillance. In the absence of sufficient testing, utilization of wastewater to quantify the number of possible positive SARS-CoV-2 infections in a catchment area is a potential additional benchmark for public health officials and decision-makers to understand the scale of cases, and in this case indicated an epidemic much larger than reported cases. While critical attention to methodological assumptions is necessary, wastewater surveillance may be able to play a role in outbreak detection and epidemic monitoring

Study design

Other

Study population and setting

Sewage samples collected between March 18 and March 25, 2020 from a major urban treatment facility in Massachusetts were tested for: 1) the presence of; and 2) the quantity of SARS-CoV-2. Samples (n=10) were tested using RT-qPCR, and biobanked samples of wastewater collected some time prior to the first case of SARS-CoV-2 in the United States were used as negative controls (n=7). Genetic sequencing was performed to confirm the RT-qPCR results, and viral titer was quantified. The investigators then compared the amount of virus detected in the wastewater to the amount they would have expected given the number of confirmed SARS-CoV-2 cases in the same catchment area.

Summary of Main Findings

Samples collected during the period of March 18- March 25, 2020 tested positive using RT-qPCR, and were confirmed with a 97-98% match to SARS-CoV-2 through genetic sequencing. All samples collected during the period prior to the first SARS-CoV-2 cases in the United States tested negative. In looking at the quantity of virus in the tested samples, it was found that there were approximately 100 viral particles per mL of sewage. The number of cases that would be expected, based on this finding and a number of assumptions such as typical stool size, the concentration of stool in wastewater, and the amount of virus shed in the stool of a positive person, is much higher than the number of known positive cases in the same area.

Study Strengths

The assumptions involved in the final conclusion that confirmed positive cases underestimate the true number of positive cases were clearly presented. A range of interpretations, including one that involved the most conservative interpretation, still suggested an underestimation of the total number of cases.

Limitations

Further studies are needed to quantify viral shedding in the stool of positive patients over the course of disease, in both asymptomatic and symptomatic patients.

Value added

This approach could be used to provide decision-makers with data to understand the scale of the epidemic in their area and can be utilized as an early detection surveillance system to detect new outbreaks.