COVID-19 is climbing sharply in the Bay Area's sewers just as students prepare to head back to classrooms: virus concentrations in the region's wastewater were 136% higher in July than in June, and nearly 54% of monitored sites are now rated "high," according to the Stanford- and Emory-affiliated network WastewaterSCAN, whose figures were first reported by the San Francisco Chronicle.

Wastewater testing has become the most reliable barometer of COVID's spread now that most people test at home or not at all. The current summer bump has already pushed past last winter's peak, though it remains below the high the region hit in summer 2025 — and the timing, as roughly a million Bay Area students prepare to return to shared classrooms and buses in the coming weeks, is what public-health watchers are eyeing. Unlike earlier surges, this one isn't being driven by a single dominant variant.

The clearest picture of where COVID-19 is spreading in the Bay Area no longer comes from clinics or test-positivity counts. It comes from what gets flushed.

WastewaterSCAN, a monitoring network run with Stanford and Emory University researchers, reported that SARS-CoV-2 concentrations in Bay Area wastewater rose 136% from June to July, with nearly 54% of the sites it monitors across the region now classified as "high" — figures first surfaced by the San Francisco Chronicle's Aidin Vaziri, which The Dissent could not independently re-pull from the network's JavaScript-rendered dashboard.

"We are seeing 'high' levels in San Francisco, San Jose, Sunnyvale, San Rafael, Vallejo and Napa," Amanda Bidwell, WastewaterSCAN's scientific program manager, told the Chronicle — a spread that stretches across the region's urban core and into the North Bay.

The appeal of sewage as a tripwire is that it doesn't depend on sick people showing up anywhere to be counted.

The U.S. Centers for Disease Control and Prevention, which maintains its own national wastewater network, notes that COVID-19 wastewater trends can now diverge from hospitalization data because the virus is "causing severe disease less frequently than earlier in the pandemic." In other words: more spread doesn't automatically mean more crowded emergency rooms — but it does mean more virus circulating in the places people gather. That distinction is precisely why the sewers now serve as an early-warning system, catching a rise before it shows up in case counts.

One of those gathering places is about to be school. Bay Area districts including San Francisco Unified are set to reopen for the 2026-27 academic year in the coming weeks, funneling students back into classrooms, cafeterias and buses at the exact moment the wastewater signal is climbing. WastewaterSCAN says the summer increase may not have peaked.

A few details make this wave distinct from past ones. First, it isn't confined to Northern California — the Chronicle reported that the network is registering elevated SARS-CoV-2 levels across its national footprint. Second, and more unusual, no single strain is running the show.

"It is important to note we are seeing a mixture of variants circulating, rather than just one dominant variant, like we have seen in the past," Bidwell told the Chronicle. A more genetically mixed wave can complicate the tidy "new variant, new surge" narrative that defined earlier chapters of the pandemic.

For Bay Area families, the practical read is straightforward. The region is in a genuine summer uptick — above the winter peak, below the summer-2025 high — and the sewers are flagging it before the case counts and hospital dashboards will. The CDC updated its respiratory-virus activity data as recently as Aug. 7 and is continuing to publish COVID-19 trend estimates through the summer.

None of this is a reason to panic; severe outcomes remain far below pandemic-era peaks. But the wastewater is doing what it's designed to do — warning early, quietly, from underground — right as the region's schools prepare to open their doors to tens of thousands of kids.