FILE: Customers wearing face masks ride the elevator at San Francisco Centre in downtown San Francisco. 

A new COVID-19 strain is now driving the majority of infections in the Bay Area as disease levels remain high at the start of the school year, according to Stanford research. 

The XFG “Stratus” strain now dominates in parts of San Francisco, Sacramento and San Jose, according to the Stanford WastewaterSCAN program, which tracks COVID-19 through sewage samples at four sites across those three cities. The new strain makes up more than half of all variants detected at three of the four locations. 

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“We have routinely seen XFG as the predominant variant since July 2025,” Amanda Bidwell, the scientific program manager for WastewaterSCAN, told SFGATE in a statement. 

Data from the week of Aug. 16 shows the strain firmly outpacing other variants: “We are seeing the XFG variant … make up approximately 82% of variants followed by NB.1.8.1 [Nimbus] at approximately 5.2%,” Bidwell explained.

Dr. Peter Chin-Hong, an infectious disease expert at UCSF, told SFGATE that Stratus is behaving “very similar” to earlier COVID lineages like Nimbus, which spread during summer and, in some cases, caused severe “razor-blade” sore throat symptoms. 

“There’s a little bit more sore throats with some people,” Chin Hong said. “Particularly in those who are older, who didn’t get vaccinated last year.” He added that Stratus has shown “a wider range of symptoms,” including congestion and gastrointestinal issues, though most cases........

© SFGate