Marin officials probe COVID spike in San Rafael wastewater

Marin County public health leaders said this week they are puzzled by a COVID-19 data spike linked to a San Rafael sewage treatment plant.

The University of California at Berkeley’s Nelson Lab has been monitoring 14 wastewater plants across the state since the summer of 2020, including nine in four Bay Area counties: Marin, San Francisco, Contra Costa and Alameda.

The amount of virus found in the wastewater at most of these plants has diminished over recent weeks as the omicron surge subsides. But the plant in San Rafael’s predominately Latino Canal neighborhood showed a sharp rise when a reading was taken on Jan. 18.

“I don’t know what to make of that particular finding,” Dr. Matt Willis, the Marin County public health officer, said Monday. “I’m always a little hesitant to hang my hat on one number. At the same time, we’re super sensitive to the reality that this is a particularly vulnerable community and don’t want to dismiss it.”

Monitoring wastewater is increasing seen as the most effective of way of gauging the level of transmission in communities.

Spikes in infection levels show up in wastewater long before PCR testing data becomes available, due to delays in people getting tested and processing times. In addition, with more people using rapid antigen tests, positive results often aren’t reported to public health departments. Wastewater data also capture asymptomatic people who might be infected without knowing it.

Willis said since the first week of January, wastewater measurements taken at Marin’s other plants have shown the amount of COVID-19 virus in the samples as remaining about the same or dropping.

“Our case rates among our Latinx residents are tracking in parallel to our case rates among our non-Latinx residents,” Willis said.

Willis said it is possible, however, that there are more Latino residents infected with the virus than testing data indicate. Fewer Latino residents might be choosing to get tested either because they’re asymptomatic or they don’t want to take the time.

Willis said most residents of the Canal area have access to free county testing sites within walking distance of their residences.

He said it is feasible that infection rates among Marin Latino residents are still rising because that community has not been as quick to seek out booster shots as it was to receive initial inoculations.

Overall, 68% of Marin residents of all races and ethnicities who are eligible for a booster have received one, compared to 44% of those in Marin who identify as Latino. And just 37% of people eligible for a booster who live in one of the census tracts located in the Canal have received a booster.

“We do have lower booster rates among the Latinx community; that is something to recognize,” Willis said. “We are working strategically to increase booster rates with our partners there.”

Willis said the fact that the federal government initially said that booster shots were only necessary for older, more vulnerable people might have given Latino residents the idea they aren’t necessary.

“We were very successful in reaching our Latinx residents for the first round of vaccines,” Willis said. Among Marin’s Latino population, 98% have received at least one vaccine dose and 87% are fully vaccinated.

There is also a possibility that the high Jan. 18 number resulted due to a technical glitch. Willis said the UC Berkeley lab has decided to relocate the site where it takes the Canal readings. He said in order for the readings to be accurate, they have to be taken at a location where the wastewater stream is moving smoothly.

Kara Nelson, a UC Berkeley professor of environmental engineering who helps oversee the program, wrote in an email Monday, “We don’t have any additional data for that location yet due to some challenges last week with sample collection so we don’t know what the trend is since the 18th. We hope to have more information later this week.”