(Bloomberg) -- U.S. public health officials are expanding their monitoring of Covid-19 in sewage, which has become a crucial early warning for surges of new cases.
The Centers for Disease Control and Prevention this week began sharing virus wastewater trends on its public-facing Covid data website. And the agency is in the midst of expanding the number of places from which raw sewage gets monitored for rising or falling waves of disease, adding hundreds of new sites in the coming months.
The U.S. struggle to track Covid in real-time has been one of the biggest frustrations of the pandemic. Early on, testing capabilities were only a fraction of what was needed. At-home tests, now more plentiful, mostly don't get reported to health authorities. And even when local health departments and health care providers do get data, consolidating it for real-time analysis has been a challenge.
But with wastewater, the sewage – and the data it contains – keeps flowing.
Paying attention to that data can alert health officials to prepare medical surge teams, send out mobile testing units and to arrange for adequate supplies. It's also a useful tool for health officials to help confirm what they're seeing from other sources.
“The advantage wastewater surveillance has is that it's not dependent on human behavior, beyond using the bathroom,” said Amy Kirby, program lead for CDC's National Wastewater Surveillance System. “As the dynamics of the pandemic change, it remains an accurate measure.”
It is not the first time disease hunters have turned to sewage waste to track coronavirus cases. During the SARS outbreak that started in 2003, researchers traced a cluster of cases in a Hong Kong apartment building to drainage pipes from bathrooms. There is debate about whether those cases spread through sewage itself or some other path involving pipes.
When SARS-CoV-2 arrived in the U.S., American researchers began monitoring wastewater for the presence of the virus, worried about sewer-based spread. Fortunately, there's been no evidence of that with this outbreak.
But it led to a useful monitoring tool: People with Covid-19 shed the virus in their stool early in the course of an infection, said Kirby, sometimes before they test positive with a nasal swab. That can provide an early warning when a wave of infections begins.
State and local health departments have been using, and publishing, the data since relatively early in the pandemic. The CDC has been monitoring it as well with Kirby's program watching for SARS-CoV-2 signs in wastewater since 2020. Many cities track and publish the data on their own: Boston, Miami and dozens of others all make at least some data available.
To help get more places watching their wastewater, the CDC has convened working groups with state and local health officials who already use wastewater to track Covid levels. And they're offering guidance and information sharing to help bring new sites online. As of Friday, the agency has begun posting wastewater data from 255 towns, cities, municipalities and other places.
The CDC has also contracted with a company called LuminUltra to collect wastewater data from 500 sites; about 200 are online so far. Kirby said the agency has identified hundreds more sites that it wants to enroll.
Trends, Not Totals
The data have limits. They can tell health officials whether the trend of cases in the community is increasing or decreasing, often days ahead of nasal-swab test results. But it can't tell who's sick, or how many cases there are.
About 80% of U.S. households are connected to a sewer system, according to the agency, limiting its use in some rural regions that rely on well-water and septic systems. Wastewater tests only capture information from what's known as a sewershed. That's the network of pipes and drains that make up a sewer network. Those often don't match perfectly with county boundaries, especially in parts of the U.S. such as the Northeast where systems can be far older and more convoluted.
“It's a hyperlocal type of data; it's most useful in the communities it's being collected,” Kirby said.
In Missouri, there are more than 100 monitoring sites across the state. Since May 2020, the data have helped the health department predict where cases are heading.
To pull the sewage, a machine called an auto-sampler takes small volumes of sewage every 15 minutes for 24 hours. (And yes, it smells.) A one-liter sample is shipped by courier to a lab that spins down the liquid into a solid pellet. Then it's analyzed for signs of virus. In total, it takes about 3-4 days from sewer to result, though running genetic analysis to identify variants can take longer.
It's an evolving science. The state is working on ways to use the data to estimate how many people might be infected, not just identifying a rising or falling wave. The system has also had to compensate for events like heavy rain, which can dilute samples and skew the data.
But it's proven helpful. During one of Missouri's last waves, it let the state predict where the outbreak was headed, said Jeff Wenzel, chief of the Bureau of Environmental Epidemiology of the Missouri Department of Health and Senior Services.
“We were able to anticipate the trend and the time to get to a peak and come back down,” he said in an interview.
The data's as useful as the samples are unglamorous.
“It's just a dingy fluid,” Wenzel said.
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