Four months into monitoring the presence of COVID-19 in sewage across Minnesota, researchers at the University of Minnesota Medical School’s Duluth Campus say the levels of viral particles in the wastewater reflects increasing cases across the state and has even given them early signs of outbreaks.
Since people infected with COVID-19 shed the virus in their feces, samples taken of raw sewage at an area’s wastewater treatment plant give researchers a snapshot of the amount of virus in that community. Medical school assistant professors Glenn Simmons Jr. and Richard Melvin are collecting samples from almost two dozen wastewater plants in the state with the hopes of figuring out a formula to determine how many cases of COVID-19 an area has based on the amount of viral particles in its wastewater, as the News Tribune reported in May.
They’re not quite there yet, but what they’re finding has been useful.
In June, before the state linked four Mankato and Minneapolis bars to an outbreak of COVID-19 cases, wastewater samples from those areas showed signs that cases were increasing there.
“We kind of started seeing things that indicated that something was going on down there before that story actually broke,” Simmons said in a telephone interview with the News Tribune on Thursday. “Even right now, things in a lot of places are trending up.”
Also in June, samples began reflecting larger viral loads, which Simmons said could be due to the end of the statewide restrictions. The stay-at-home order ended in mid-May while indoor bars and restaurants reopened in early June.
“There’s a coinciding of certain policies that have been enacted and the changes we saw in the viral amounts in the wastewater in those areas,” Simmons said.
Sure enough, in late June, Minnesota's daily COVID-19 confirmed case count started to climb back up — a trend that continued through July.
While people usually get tested for COVID-19 after they are already symptomatic, they likely shed the virus in stool before ever showing symptoms. Public health officials might be able to determine how many people in an area actually have the virus based on wastewater testing.
But that will require more work. With the help of statisticians and epidemiologists analyzing results, they’re getting there.
“We’re still working really, really hard to see if we can get this to the point where we can interpret how many people these numbers are representing,” Simmons said. “It’s not as clear-cut as we had hoped.”
And it can be complicated by factors like rainwater diluting the sample or the mass movement of people, like vacationers. In the case of samples from Duluth, the number of viral particles in the wastewater hasn’t changed much, despite a decrease in cases in late spring and then a significant increase in cases over the last month.
“Tourism, I suspect, is also playing a particular role in it,” Simmons said.
Simmons figures that although the number of cases locally went down in late spring, that coincided with the start of the city’s tourism season in early summer.
“It’s like we’re just almost replacing the drop from the so-called local population with those people who are a little bit more transient,” Simmons said.
Meanwhile, cases continue to rise rapidly in Duluth and have more than doubled in the last three weeks – from 208 total cases on July 28 to 451 cases on Aug. 13, according to St. Louis County data.
Samples of Duluth wastewater are taken from the Western Lake Superior Sanitary District in Duluth, which treats sewage for more than 125,000 residents in Duluth, Cloquet, Hermantown and other nearby communities.
The medical school researchers are taking wastewater samples from 22 sites across Minnesota with another eight added soon, and even more after that.
“We’re looking to basically cover as much of the state as possible,” Simmons said. “At the very least, we’d like to at least double the number of sites that we’re working with.”
Across the U.S., more communities are monitoring sewage for COVID-19 as a potential early-warning system. This week, state health officials in Colorado announced they were collecting and monitoring samples across the state.
Simmons said an even more localized approach, by monitoring wastewater from nursing homes, prisons and schools, would be better. Tackling a larger monitoring system in this pandemic could help prepare us for the next pandemic, he said.
“We’re working very hard to make sure that we have something in place before the next thing comes,” Simmons said. “Because there will be a next thing.”