The COVID-19 pandemic showcased wastewater testing as an early-warning system for tracking viral spread.

But the tests can’t tell if the waste comes from locals or visitors. How does that affect results in tourist destinations like Phoenix and Las Vegas?

The Las Vegas metro area’s 2.3 million residents are joined each week by nearly 1 million visitors.

The authors of the JAMA Network Open paper wanted to know how that influx affects wastewater flow and viral load.

Data beginning with the 10-week shutdown in March 2020 and ending in February 2022 showed around 60% of COVID detected in late 2021 came from visitors.

Moreover, the omicron BA.1 variant was 40% to 100% more prevalent in that group.

Even so, wastewater testing still provided an early warning of Omicron spread in the area.