San Francisco health staff told the Health Commission on Jan. 6 that highly pathogenic avian influenza A (H5N1) is circulating widely among animals but that, to date, the risk to the general public in the city remains low.

Dr. George Hahn, director of the Communicable Disease Section, said the current H5N1 strain reached North America in 2021 and has since caused outbreaks in poultry and, beginning in 2024, in U.S. dairy herds. "To date, the virus has not been able to spread easily from person to person," Hahn said. He added that most confirmed human infections in the United States have been mild, and that antiviral medications appear effective against the strain now circulating.

Why it matters: California has seen widespread animal outbreaks, and state and federal agencies have taken steps such as a state of emergency declaration in California to speed response. San Francisco health officials emphasized local surveillance and planning so the city can respond quickly if the virus changes.

Key points from the presentation

- Animal detections: Hahn said more than 10,000 wild birds and more than 128 million poultry have been affected nationally since late 2021; he reported at least 650 dairy herds in California had been affected since summer 2024, representing about three-quarters of affected U.S. cattle herds.

- Human cases: As of Dec. 31, 2024, Hahn said there were 66 confirmed human cases in the United States and 37 in California; he reported none in San Francisco.

- Transmission and clinical picture: Hahn told commissioners that human infections have primarily occurred in dairy and poultry workers after close contact with infected animals and that conjunctivitis (redness of the eyes) has been a predominant symptom in many cases. He said drinking raw (unpasteurized) milk from infected animals is a potential transmission route but that pasteurized dairy products are safe.

- Surveillance in San Francisco: The Department of Public Health (DPH) is subtyping local influenza specimens at its public health laboratory, sending at least 20 influenza A specimens per week for subtyping from Zuckerberg San Francisco General Hospital, and testing wastewater for influenza subtypes. Hahn said more than 300 local specimens had been subtyped since summer 2024, with none shown to be H5N1.

- Preparedness and planning: DPH has stood up an incident management/planning team using National Incident Management System structures and has drafted mitigation strategies for three scenarios: an isolated human case in San Francisco, sustained person-to-person transmission, and a more severe pandemic-like situation. Hahn said the city plans tabletop exercises to test plans and identify gaps.

Quotes and clarifications

- "The risk to the general public currently remains low," Hahn said, while adding that influenza viruses mutate and require ongoing monitoring. "It could happen next week. It could happen next month, next year, or never."

- Dr. Susan Phillips, the health officer, and other DPH leaders said the department is coordinating clinical, laboratory, environmental health and communications resources and working with hospitals and partners to prepare for multiple scenarios.

Unanswered questions and ongoing work

DPH said it is coordinating with the state agriculture department on monitoring of farm workers and that testing and case-finding practices can vary by farm operation. Officials also noted national constraints on H5N1 vaccines and said federal allocation decisions will affect deployment if vaccination becomes appropriate.

Ending

Officials said San Francisco will continue active surveillance, clinician outreach and planning exercises. Hahn closed by noting that while the current strain has not shown sustained human-to-human spread, the city is preparing for multiple potential scenarios and will update the commission as the situation evolves.