Despite fewer reported cases, COVID up in sewage tests

Signs indicate more virus prevalence, but vaccinated people may not know they're infected

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Testing of wastewater at the Englewood-Littleton sewage plant is finding record-high signs of COVID prevalence in the south metro populace, while the number of people receiving positive COVID tests has dropped — a situation that may show vaccinated people are not getting sick enough to feel the need to test, a Colorado researcher says.

Since the onset of the COVID-19 pandemic, South Platte Renew, a wastewater treatment plant in Englewood that is the third largest in Colorado, has been on the front lines of helping health officials make sense of the state of the virus. 

The plant collects daily samples of sewage produced by about 300,000 people, according to Pieter Van Ry, the site's director, covering populations in Arapahoe, Douglas and Jefferson counties. Samples are sent to various labs both in-state and outside Colorado that test for the virus's genome, an indicator of how prevalent COVID may be in a given community. 

In recent weeks, tests have shown higher amounts of COVID being detected in South Platte's wastewater than at any other point of the pandemic. But reported cases for the area are down by about half their record peak from last winter. The reason has to do with the difference in how the wastewater and clinical tests are conducted.

“We use wastewater surveillance to supplement our human clinical testing,” said Rachel Jervis, an epidemiologist at the Colorado Department of Public Health and Environment. “It's really complementary information to give us more data about COVID in a particular community.” 

About 50% of people who catch COVID will “shed (the) COVID virus in their stool,” regardless if they have symptoms, according to Jervis, meaning it can be an early indicator for how much the virus is spreading in an area. 

But unlike a clinical test such as a nasal swab or saliva sample, wastewater testing can't show how many people are currently infected. 

“We're really just looking at the volume of the genetic material of COVID,” Jervis said. “We're not able to link it to an individual person and so we can't use it to align the case data.”

Some of South Platte's waste samples find their way to Carol Wilusz, a professor and researcher at Colorado State University, whose lab has been testing wastewater for COVID twice a week since August 2020.

Wilusz and her team begin by filtering the sample in order to isolate the viral material before extracting its RNA data. Then, through an enzymatic process, they are able to turn the RNA into DNA. After this, they test the DNA using a polymerase chain reaction (PCR) to detect the virus's genetic material and how much of it there is. This produces a copy number, similar to a viral load, that can be used to gauge how present COVID is in the wastewater. The data is then sent to CDPHE and, along with data from other labs and testing facilities, is published on their dashboard. 

An advantage of wastewater testing, Wilusz said, is that it is involuntary as opposed to clinical testing, which may show a skewed picture based on who is choosing, and not choosing, to get tested.

“Testing of wastewater is completely unbiased, it doesn't matter if you're a Democrat or a Republican or rich or poor,” she said. “Regardless of who or where you are, you're getting tested in an unbiased fashion.”

Since the end of October, viral copies in wastewater collected by South Platte have hovered at around 300,000 per liter, shooting past last winter's COVID peak, when copies per liter were 263,000.

This comes as clinical cases from tests in the area have sharply decreased to well under 100,000 per day, a case count comparable to summer 2020. 

Wilusz said it appears that COVID is more prevalent in the area than it was last year but thanks to climbing vaccination rates and the return of masking policies, fewer people may be getting sick and feeling the need to test. 

Still, Jervis, the epidemiologist at CDPHE, urged more people to get tested routinely and said clinical testing remains a crucial tool in the fight against COVID. 

“While we use wastewater data to understand trends, wastewater data can't be used for any individual to make behavioral choices,” she said. “And so the only way to know whether you could have COVID and could be exposing your family and friends is if you get tested.”

Click here and scroll down to select "South Platte" to view data on wastewater and clinical tests on CDPHE's dashboard. 

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