Interview: What wastewater samples tell us about COVID-19 spread in Sacramento

While COVID-19 case numbers and hospitalizations are relatively low right now in California, the case rate is picking up slightly due to omicron subvariants. But are the number of positive tests really the best way to determine the level of virus in the community?

Many believe that testing wastewater is a better option that’s more accurate and more immediate.

Christoph Dobson, the general manager at regional San, the wastewater agency for more than 1.5 million people in Sacramento County, spoke with CapRadio’s Randol White. 

What are the samples telling us right now in Sacramento County as far as virus numbers go? 

We had the omicron surge over the winter and then there was kind of this trough probably through the month of March where the numbers were very low. And then starting in April, there's been a mild uptick in wastewater concentrations. 

Some people are calling it more like a swell than a spike.

Right. It's about two times what we were in the trough, so we've kind of doubled the concentrations in the wastewater. But you have to put that in perspective. When omicron was spiking, it was 12 times higher than we are today. 

Sacramento is just one of many sample points. Are there any statewide or national trends that are showing up? 

Yeah, it's similar throughout the country, although the West region has the lowest concentrations, so it seems to be just a milder effect in the West. 

Let's get a bit into the sampling process. This is not a new method, but with home testing now so popular, do you believe it might be the best way to judge community spread? 

Yeah, definitely. Public health experts are starting to get a lot more interested in our data now. Basically, the sewage doesn't lie. Everyone has to go to the bathroom and it all ends up somehow a part of our sample. 

Wastewater treatment is a process that extends from the toilet all the way to your facilities. At what point is the sewage sampled? 

Well, we're doing a couple of different studies, so there are actually two different sample locations. One is the Stanford study. They're running the Sewer Coronavirus Alert Network. They call it SCAN. That's about a dozen agencies in the Bay Area and Central Valley. And we're doing daily sampling for them and they sample it in the primary sludge. So this is a little ways into the process, but it's more concentrated organic material that's collected. 

And then the other study that we're doing is with a private company called Bio Bot. It's a nationwide study of about 100-plus agencies. And for that one, we're doing what we call influent sampling. It's twice weekly. And influent essentially is just the raw sewage just upstream of our treatment plant. 

Timeline wise, from the time I flush my toilet at home until the time it becomes part of that sludge that the Stanford studies looking at, what are we looking at? 

It's probably a day or so. 

Since this works so well with tracking COVID-19, how else might it be used? Are there other sorts of pathogens or viruses that we could track effectively with this method? 

It's actually really interesting. Sewage surveillance, believe it or not, has been around since the 1940s. They were tracking polio back then. We focus on COVID now, but it can be used for other infectious diseases. And actually, Stanford is currently tracking the flu virus and other respiratory viruses. And then another area that we're not involved in is to track the relative prevalence of some kind of a drug use in a community.