Not reporting test results? Scientists still track COVID via your toilet waste


ewage from millions of Angelenos flows into an LA wastewater treatment plant in Carson, where scientists like Karla Ellison collect samples for coronavirus testing. Photo by Robin Estrin.

Millions of Angelenos might be surprised to know that they’re participating in a science experiment every time they use the toilet.

Each time we flush, our waste rushes through the sewer pipes beneath our homes and city into a wastewater facility where scientists collect gallons of the brown muck in clear jugs and test it for the coronavirus.

They pass along the data about how much of the virus is concentrated in our sewage to the LA County Department of Public Health, where officials use it to identify trends in the spread of the virus.

How are scientists measuring COVID in our wastewater?

We store the virus in our gastrointestinal tracts when we’re sick, and we release it into the sewer when we use the bathroom, which, turns out, is really great for science.

Some scientists in LA County started testing our wastewater for the virus in 2020, and public health officials have been analyzing that data as part of their early alert system. It helps tell them where we’re headed with this virus.

So data about COVID in our wastewater can tell us how many people are sick?

Not exactly. There’s no one-to-one correlation between the concentration of the virus in our wastewater and the number of people who are sick. 

Dr. Ferrer called the data a “crude measurement” for community spread. 

“It doesn't exactly tell you who is infected, but it obviously gives you a sense in given communities what that level of infection might be,” says Ferrer. “What we use it for is to establish trends.”

Throughout the pandemic, data from the Joint Water Pollution Control Plant in Carson shows how much COVID was detected in sewage, along with new reported cases. Photo courtesy of Los Angeles County Sanitation District.

This chart shows wastewater data and hospitalizations in 2022. Photo courtesy of Los Angeles County Sanitation District.

Throughout the pandemic, there’s been a clear correlation between spikes in the concentration of coronavirus in our sewage and spikes in cases and hospitalizations.

“When you start seeing higher concentration levels in the wastewater, you should anticipate that shortly after, as you could see here, you are going to see increases in hospitalizations,” says Dr. Ferrer. “That trend has followed consistently.”

How does wastewater data complement other metrics we have for gauging community transmission, like reported testing results?

Now that at-home tests are widely available, it’s impossible to know how many people are testing positive for COVID-19, or even how many tests people are taking. In fact, there are about as many people reporting test results now as there were in July of 2020, when we were still facing testing supply shortages and backlogs. Plus, a lot of people carrying the virus aren’t testing at all because they don’t have symptoms. 

Health officials can look at wastewater data and tell when there might be a lot of sick people in our community, even when those people don’t know about it. 

Officials see wastewater data as a complement to other tools they have for measuring spread.

What are the limitations of looking at wastewater data?

The data doesn’t capture everyone’s waste. In some parts of the county, like Malibu and rural parts of the San Fernando Valley, some people’s sewage pipes lead to septic tanks.

And then there’s a larger issue. Naoko Munakata, a wastewater supervisor at the LA County Sanitation District, says that when the pandemic started, different water treatment facilities in Los Angeles and around the state hurried to set up testing capabilities separately. That means now there are several competing ways to test wastewater. 

“Everyone was kind of doing their own thing. And as a result, people developed a lot of different methods,” Munakata says. “There's no standardized method.”

That prevents scientists from being able to combine data that’s coming in from all these wastewater plants across the county into a single number.

Health officials can only use this data to talk about trends, whether the amount of virus in our community is going up or going down.

An automated process retrieves small samples from the Carson facility’s sewer pipes and stores them in these several-gallon jugs. At the end of each day, a composite of our sewage is sent out for coronavirus testing. Photo by Robin Estrin.

If scientists can test for coronavirus in sewage, couldn’t they test for other viruses as well?

Yes, actually, scientists can test our sewage for all sorts of viruses, including influenza. But those tests are really only useful if there’s a relatively high amount of virus spreading through a community, and like COVID-19, we shed that virus through our GI tract.

Take monkeypox, for example. The county health department hasn’t taken on monkeypox testing across the county because there’s not enough of the virus in our community to register in our wastewater. 

“If transmission continues, and monkeypox cases continue to grow, I worry that we may get to a point where wastewater surveillance will be useful,” says Dr. Prabhu Gounder, the physician who oversees all wastewater surveillance at the LA County Department of Public Health.