Wastewater virus levels shed light on intensity of Sacramento’s current COVID surge
For more evidence that coronavirus activity has been rising fast in the Sacramento area, look below us.
Levels of the virus that causes COVID-19 detected in human wastewater have grown substantially since about the start of June, according to sewershed data analyzed by a Stanford-based research team.
The data collection effort, called the Sewer Coronavirus Alert Network (SCAN), examines samples collected daily from several wastewater treatment plants in California, including Sacramento and Davis.
“What we do is we use settled solids that are collected from wastewater treatment plants, and then we look for SARS-CoV-2 viral genomes in those wastewater solids to then get a sense of how much COVID transmission is happening at a community level,” explained Marlene Wolfe, one of the researchers leading the Stanford project.
“People sort of naturally contribute to this biological sample that represents the entire community when they just do everyday activities like using the toilet.”
This gives wastewater testing a few advantages over traditional forms of testing. It can detect increased viral activity more quickly than clinical testing, Wolfe said, sometimes several days to a week ahead depending on whether lab resources are in high demand.
“Often people are infectious before they are actually sick with COVID,” Wolfe said. “The infectiousness of a person is also reflected in shedding in their stool, so people are likely shedding SARS-CoV-2 in their stool towards the beginning of their infection, possibly even before their COVID test.”
Wolfe said, though, that the wastewater program is intended to work “hand-in-hand with clinical data,” not as a replacement, “to help us corroborate what’s going on.”
The research team shares daily updates to the local health offices it partners with. Charts of the wastewater data are also available publicly, on an online data dashboard.
When viral levels in wastewater started spiking a few weeks ago, health officials expressed some concern, Wolfe said.
“From sort of everyone who we’re monitoring right now, we had this question a couple weeks ago of, ‘Hey, we’re starting to see some higher levels in the wastewater. What do you think? Do you think that’s real? Do you think that’s going to turn into something?’”
It did, in fact, turn into something. In Sacramento County, the daily case rate has risen from 3.8 per 100,000 to 11.2 in just three weeks and so far has shown no sign of slowing down.
“Unfortunately, we can definitely say that this increase in wastewater has also been associated with an increase in cases,” Wolfe said. “And so we’re going to be watching the wastewater closely and hoping to see a downturn in the wastewater in the near future.”
Recent updates to those charts appear to show at least some level of increase in SARS-CoV-2 levels detected recently across all eight of the treatment plants it monitors, five of which are in the Bay Area. Some of the most striking, ongoing increases lately have been observed at plants in Sacramento, Davis, Gilroy and Oceanside.
In those regions, wastewater virus levels have risen to their highest points in at least three months. That matches up with the story in Sacramento County, where local health officials recently reported the daily case rate rising to its highest point since early April.
“The wastewater data gives this advantage of giving us a peek ahead and a little bit of an indicator of what may be coming for us in the next week,” Wolfe said. “But it’s also really important when a new trend starts that we are checking that against the clinical data.”
Healthy Davis Together, a joint initiative between the city and UC Davis, announced in a Monday statement that local wastewater sampling showed SARS-CoV-2 levels starting to “tick up at the City’s Wastewater Treatment Plant with corresponding fluctuations in a few regions in the city.” The statement cited SCAN.
“At the same time, clinical COVID-19 testing data indicate a rise in the highly contagious Delta variant among Davis residents who test positive for COVID,” city officials wrote in a Healthy Davis Together news release.
“These two combined data streams indicate the risk of infection is still present and serve as an important reminder to remain alert and take appropriate action, when needed, to prevent the spread.”
There are some limitations and uncertainties with the wastewater monitoring, Wolfe acknowledged, which is why it remains important for it to be used in tandem with clinical testing.
For one, SCAN is broad, community-level surveying. If there is an uptick in cases, wastewater alone can’t give any insight on specific hotspots or outbreaks within each city, since those samples all come from the same treatment plant. Nor can it reveal any demographic trends or assist with contact tracing.
There’s also some uncertainty involving COVID-19 variants.
“If you have a new variant like Delta, does that change the amount of SARS-CoV-2 that people are shedding in their feces? That’s possible,” Wolfe said.
The research team is still studying the impact of variants on the data, but Wolfe said the “relationship between overall cases and wastewater has held strong” even during the rise of previous variants, like Alpha.
“We can say pretty clearly, if you have higher SARS-CoV-2 in wastewater in this area vs. that area, you’re gonna have higher COVID cases,” Wolfe said.
This story was originally published July 19, 2021 at 5:00 AM.
