San Francisco is weathering an unusual spring surge of overlapping infections. Federal wastewater monitoring shows elevated signals for influenza A, SARS-CoV-2, and RSV across California through late April 2026, while local sampling sites in the city are also picking up norovirus, rotavirus, and human metapneumovirus (HMPV). The collision of respiratory and gastrointestinal pathogens has clinicians and public-health researchers working to explain why so many viruses are peaking at once.
What surveillance data show
The CDC’s wastewater respiratory dashboard categorizes viral activity on a scale from “very low” to “very high.” As of late April 2026, California’s readings for influenza A and SARS-CoV-2 remain above seasonal baselines, and RSV activity, which typically fades by early spring, has lingered longer than expected. The dashboard gives public-health teams a near-real-time window into community transmission because viral genetic material appears in sewage days before patients show up at clinics.
San Francisco contributes directly to that picture. The WastewaterSCAN research program operates sampling sites at the city’s Oceanside and Southeast treatment plants. (The program’s laboratory protocols are publicly documented, though results are published separately on the WastewaterSCAN dashboard rather than in the protocol document itself.) Unlike the CDC’s respiratory-only panel, WastewaterSCAN tests for a broader set of pathogens, including norovirus, rotavirus, and HMPV. That wider lens is how researchers first spotted the simultaneous rise in stomach and lung viruses from the same sewage samples, a pattern standard clinical testing often misses because most people with a mild stomach bug never visit a doctor.
Clinical laboratory data add a second layer of confirmation. The CDC’s NREVSS dashboard tracks lab-confirmed detections for a broad set of respiratory viruses nationwide. When wastewater signals and lab positivity rates trend upward together, epidemiologists treat the combined evidence as a strong indicator that infections are genuinely rising rather than reflecting changes in who gets tested.
Why so many viruses at once
Doctors and researchers point to several overlapping factors, though none has been proven as the single driver.
Immunity gaps from the pandemic era. School closures, reduced social mixing, and delayed routine childhood vaccinations between 2020 and 2022 left many young children with less natural and vaccine-derived immunity than they would have built in a typical pre-pandemic period. Those children are now encountering multiple pathogens in a compressed window as they enter daycare and elementary school. A 2022 CDC analysis documented significant drops in routine pediatric vaccination during the pandemic, and immunization experts say catch-up efforts have been uneven.
Seasonal overlap extended by a mild, wet spring. Northern California’s cool, damp weather through April 2026 has kept people indoors longer than usual, sustaining conditions that favor airborne and close-contact transmission for both respiratory and enteric viruses.
Shifts in vaccine guidance. In early 2025, the Trump administration published an HHS fact sheet that describes rotavirus vaccination under “shared clinical decision-making,” a designation that frames the shot as a conversation between clinician and parent rather than a blanket recommendation. Some pediatricians worry the language change could reduce uptake over time by making the vaccine seem optional. No data have yet shown a measurable decline in rotavirus vaccination rates in San Francisco or California as a direct result, but the policy shift has drawn concern from the American Academy of Pediatrics and other medical groups.
Norovirus strain dynamics. The CDC’s CaliciNet laboratory network genotypes norovirus outbreak strains nationally. CaliciNet data can reveal whether a particularly transmissible genotype is fueling the current wave, but as of May 2026 no publicly released CaliciNet finding has identified a specific genotype driving San Francisco’s elevated readings. Without that molecular detail, it remains unclear whether the city’s norovirus signal reflects a broader national strain pattern or a more localized cluster tied to schools, shelters, or food-service settings.
What remains unclear
Important gaps persist. The San Francisco Department of Public Health has not published city-level clinical positivity rates alongside the wastewater findings, so it is not yet possible to say precisely how many residents are testing positive for each pathogen. The NREVSS dashboard offers national and regional aggregates but does not break data down by city. Without that local clinical layer, wastewater readings function as an early warning, not a confirmed case count.
The relative contribution of waning immunity versus new viral variants is also unresolved. Wastewater and lab surveillance can confirm that influenza, RSV, and SARS-CoV-2 are all active, but they cannot explain why one virus dominates in a given week or why some households report back-to-back infections with different pathogens. Detailed viral sequencing and clinical studies would be needed to untangle those questions, and that analysis typically lags weeks behind real-time surveillance.
HMPV, which causes cold-like symptoms in most adults but can lead to serious illness in infants and older adults, has drawn attention on social media but remains less well-studied than influenza or RSV. Its appearance in San Francisco wastewater adds to the overall viral burden without yet triggering a separate public-health advisory.
What San Francisco residents can do now
With multiple viruses circulating at once, physicians say layered precautions offer the best protection. Staying home when sick, improving indoor ventilation, wearing a well-fitting N95 or KN95 mask in crowded indoor spaces, and keeping up to date on recommended vaccines all reduce both personal risk and community spread.
Parents with young children should confirm that routine immunizations, including rotavirus, are on schedule and discuss any questions with their pediatrician rather than skipping doses based on shifting policy language. For older adults and people with compromised immune systems, the overlapping viral season raises the stakes on avoiding crowded, poorly ventilated settings when local wastewater signals are elevated.
Public-health officials say they will continue to update wastewater dashboards and clinical surveillance through May 2026, and that residents should watch for advisories from the San Francisco Department of Public Health if any single pathogen begins straining hospital capacity.
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*This article was researched with the help of AI, with human editors creating the final content.
