By 1 January 2025, 37 human cases of highly pathogenic avian influenza
A(H5N1) had been detected in California. None was in San Francisco.
On 9 January 2025, a case was identified in a **school-aged child in San
Francisco**.
How it was found
Not by anyone suspecting bird flu. It was found through **enhanced
surveillance — subtyping a sample of influenza A–positive specimens each
week.**
A child with influenza A is an ordinary event. Subtyping a fraction of those
specimens routinely, including from people with no known exposure, is what
turned one of them into an H5N1 detection.
What the investigation found
No source of exposure was identified.
**No laboratory evidence of human-to-human transmission among close
contacts.**
Both of those are uncomfortable findings to publish, and both are the right
ones to publish. A case with no traceable exposure means the route is unknown
— not that there was none.
The two lessons
**Enhanced surveillance and timely subtyping of a subset of influenza
A–positive specimens, including specimens from persons without known
A(H5N1) exposure, are important to detect avian influenza A virus
infections.**
Testing only people with a known exposure finds only the cases you already
expected. The value of routine sampling is precisely the case nobody would
have looked for.
**Public health investigations are critical to monitoring for human-to-human
transmission.**
That is the question everything about H5N1 turns on, and it can only be
answered by investigating every case found — including the ones that turn out,
as this one did, to go nowhere.
*Source: Tobolowsky FA, Morris E, Castro L, et al. Highly Pathogenic Avian
Influenza A(H5N1) Virus Infection in a Child with No Known Exposure — San
Francisco, California, December 2024–January 2025. MMWR, Centers for Disease
Control and Prevention.*
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