COVID Is Back in the Bay Area: Is It the XFG "Stratus" Variant?

The San Francisco Bay Area, California, is experiencing an uptick in wastewater concentration of the SARS-CoV-2 virus, following patterns of resurgence observed in previous years.¹

Prior research has affirmed spring and summer waves of SARS-CoV-2 in addition to the recurring waves observed during winter months.² However, the increase in wastewater concentration in the Bay Area appeared a little later than expected, according to Stanford University's WastewaterSCAN team.¹ The team usually sees the summer wave begin in late May or June, with a team member describing this year as having a slower start.

As of July, WastewaterSCAN reported the concentration of the virus was up 136% compared with June levels. As of August 4, median levels were observed to be even higher than during the Bay Area's peak winter season. However, when put into context, current levels are significantly lower than the wave observed in 2025.

Bay Area COVID Wastewater Levels Are Climbing Again This Summer

As of August 8, the majority of states were categorized as having "very low" wastewater viral activity levels—the amount of virus detected in wastewater relative to historical baseline levels—according to the CDC's most recent update.³ Yet states like California, Washington, Oregon, Nevada, Alabama, and Georgia were categorized as having a "low" wastewater viral activity level, defined as greater than 2.6 and up to 4.9.

The WastewaterSCAN team's findings align with national data, with a team member noting that COVID levels "still appear low" compared to historic data and past infection waves, in an interview with KQED.¹ The team also observed that the current wave is a mixture of variants, echoing national trends, rather than 1 dominant variant circulating in Bay Area wastewater.

The most common variant is XFG, also known as the Stratus variant, accounting for 82% of variants detected in Bay Area wastewater. Those infected with XFG have reported a scratchy or hoarse throat that can easily be mistaken for allergies, an infectious disease expert at Stanford said in an interview with KQED. Cough, congestion, fatigue, and fever have also been reported alongside the sore throat.

Another variant in circulation is NB.1.8.1, or the Nimbus variant, nicknamed "razor blade throat" after the intense sore throat people have reported after infection.

Symptoms, Contagiousness, and Whether You Need Another COVID Shot

The Stratus and Nimbus variants are descendants of the Omicron variant and are more transmissible than previous variants, particularly the more recent Stratus variant.⁴

The Stratus variant's spike protein has 4 mutations, making it slightly more transmissible than previous variants, an infectious disease expert at the University of California, San Francisco (UCSF), said in an interview with KQED. However, there is no evidence that Stratus causes more severe disease than earlier variants, the expert noted, even as rising case counts mean more vulnerable people are likely to be hospitalized simply because more people overall are getting infected.

Whether to mask up in crowded indoor settings depends on how high local wastewater levels climb, according to a Stanford infectious disease physician, who recommended checking county-level wastewater data before deciding. A well-fitting N95 or KN95 mask can meaningfully lower the risk of infection in crowded spaces such as public transit or grocery stores.¹

Incubation times have also shortened since the early years of the pandemic. In 2020, the average incubation period ran 5 to 7 days; three days has since become a more typical window between exposure and symptom onset, according to a UCSF infectious disease expert.⁴ Because of this, health experts recommend testing as early as 2 days after a known exposure if symptoms are present and testing again roughly 48 hours later if an initial result comes back negative, since early antigen tests can still miss an active infection.

For health care professionals, when in conversation with patients, it is important to reassure them that the 2025-2026 COVID-19 vaccine was formulated to target the Omicron variant and its descendants, including the currently dominant XFG variant.¹ Addressing common patient questions—especially around vaccine efficacy, isolation guidance, and adherence—is important to public health and to curbing serious COVID-19 in vulnerable patients.

References

1. Severn C. Yes, that ‘summer cold’ really might be COVID (again). What to know about Bay Area viral levels. KQED. August 4, 2026. Accessed August 17, 2026. https://www.kqed.org/news/12091325/summer-covid-test-2026-wastewater-variants-isolation-contagious-xfg-san-francisco-bay-area-california

2. McCrear S. COVID-19 may follow a different seasonal pattern than influenza, RSV. AJMC^®. June 29, 2026. Accessed August 17, 2026. https://www.ajmc.com/view/covid-19-may-follow-a-different-seasonal-pattern-than-influenza-rsv

3. National Wastewater Data for Respiratory Viruses. Centers for Disease Control and Prevention. Updated August 14, 2026. Accessed August 17, 2026. https://www.cdc.gov/wastewater/respiratory-viruses/national.html?cove-tab=1

4. Severn C. COVID is everywhere (again) in the Bay Area. What to know about the latest XFG “Stratus” variant. KQED. August 8, 2025. Accessed August 17, 2026. https://www.kqed.org/news/12045979/xfg-stratus-nimbus-nb181-covid-variant-symptoms-razor-blade-throat-incubation-period-testing#xfg-stratus-nimbus-covid-symptoms-sore-throat