While SARS-CoV-2 wastewater surveillance is a valuable way to measure disease prevalence and predict surges, a new study conducted in Las Vegas suggests that the tool may overestimate the true viral burden in areas with large influxes of tourists or commuters.
University of Nevada researchers worked with the Southern Nevada Water Authority on the study, which was published yesterday in JAMA Network Open.
The team analyzed and genomically sequenced samples from seven Las Vegas water treatment plant locations that serve southern Nevada and one manhole on the south Las Vegas Strip from March 2020 to February 2022. Treatment plants 2 to 7 serve location populations, while plant 1 serves a mix of locals and visitors, and the manhole represents mainly visitor waste.
The aim was to determine the relative influence of the nearly 1 million weekly visitors to Las Vegas, a city of 2.3 million residents, on levels of the SARS-CoV-2 Omicron BA.1 variant in wastewater from Dec 13 to 20, 2021.
Over 60% of viral load from visitors
Omicron BA.1 was detected in the Las Vegas Strip manhole on Dec 7, about 1 week before it was found at the treatment plant locations and clinically confirmed, and rapidly displaced the Delta variant. On Dec 13, Omicron made up an average of 48.0% of all SARS-CoV-2 genomes from manhole samples and 4.1% of the genomes at treatment plans 2 and 3, rising to 82.0% and 48.0%, respectively, on Dec 20.
The researchers estimated that visitors contributed over 60% of the SARS-CoV-2 load to Strip sewers and that the prevalence of Omicron among the visitors was 40% to 60% on Dec 13, climbing to 80% to 100% on Dec 20.
By late 2021, the estimated undercount of COVID-19 cases for facility 1 was 14.0 per 100,000 people, compared with 4.2 and 4.5 for facilities 2 and 3, respectively. Omicron-specific mutations overtook Delta at the Las Vegas Strip manhole and facility 1 faster than at facilities 2 to 7.
The prevalence of Omicron among the visitors was 40% to 60% on Dec 13, climbing to 80% to 100% on Dec 20.
"We hypothesized that with tourism returning to prepandemic levels, visitors started contributing a disproportionate fraction of SARS-CoV-2 RNA to facility 1, and in early December 2021, these visitors were more likely to be infected with the Omicron VOC than the local population," the authors wrote.
After subtracting facility 1's viral load by 60% (ie, estimate for visitor contributions), the relative viral loads for facilities 1 to 3 were more aligned with each other and with the total sewershed. The estimated COVID-19 case undercount for facility 1 (average, 5.4 per 100,000 people) was also closer to that of facilities 2 and 3.
"Without this adjustment, the wastewater-derived infection estimate for this sewershed would be nearly 3-fold higher, leading to a highly inaccurate overall infection estimate for southern Nevada," the researchers wrote.
After adjustment, the mean (SD) estimated undercount for Facilities 1 to 3 was 4.6, higher than the 3.4 calculated earlier in the pandemic. "This might be attributable to fewer individuals with infection seeking clinical testing, in part due to greater availability of at-home rapid antigen tests," the researchers wrote.
Using the adjusted viral load for facility 1 and a method for estimating COVID-19 incidence throughout southern Nevada, an estimated 476,000 local residents, or about 20% of the population, had COVID-19 in January 2022. But only 94,427 infections were confirmed for the seven sewersheds in this period, for an estimated undercount of 5.0 per 100,000 people.
Sewage surveillance complements clinical tools
When Nevada officials issued stay-at-home orders in response to the pandemic in March 2020, closing all Las Vegas Strip hotels and casinos, wastewater flows into the sewers fell by roughly 15%, the researchers noted. "Studies show that mobile populations can have a significant impact on wastewater flow rate and contaminant loading, particularly during special events and on holidays and weekends," they wrote.
Wastewater surveillance is an important complement to clinical tools that can provide time-sensitive data for decision- and policy-makers, according to the authors: "This study represents a novel approach for quantifying the confounding effects of mobile populations on wastewater surveillance data, thereby allowing for modification of an existing WBE [wastewater-based epidemiology] framework for estimating COVID-19 incidence in southern Nevada."
