State of California—Health and Human Services Agency
California Department of Public Health

Health Advisory

TO: Healthcare Providers and Clinical and Commercial Laboratories

CDPH Influenza A Subtype Testing Guidance: Continued Surveillance During the Summer Months

6/11/2025

Key Messages

In order to maintain awareness of influenza A subtypes circulating in California, healthcare providers should:  

Continue testing for influenza throughout the summer, particularly in patients with respiratory illness and/or conjunctivitis and avian influenza A(H5N1) exposure risk factors.

Please report symptomatic patients with H5N1 exposure risk factors to the local health department of the patient’s residence.
Influenza A positive samples from patients with exposure risk factors for H5N1 infection should be subtyped at a public health laboratory, unless subtyping has already been performed, and results show a seasonal subtype (i.e. H1 or H3).

Continue testing for influenza in hospitalized patients with severe respiratory illness regardless of whether they have exposure risk factors for H5N1 infection.

Hospitals and clinical laboratories should ensure that influenza A positive samples from such patients are subtyped either in the clinical laboratory or sent to a commercial or public health laboratory for subtyping.

Background

As of June 11, 2025, there have been 38 laboratory confirmed human cases of avian influenza A(H5N1) in California. Although sporadic human cases have occurred, primarily in people with occupational exposures, influenza A(H5N1) continues to pose a low risk to the general public, and no person-to-person spread has been detected.

However, rapid detection and characterization of novel influenza A viruses (i.e., influenza A virus subtypes that are different from currently circulating H1 and H3 human influenza virus types) remain critical components of state and national efforts to prevent additional human cases of H5N1.

Recommendations

As the 2024-2025 influenza season ends, the California Department of Public Health (CDPH) urges healthcare providers to maintain awareness of potential influenza A(H5N1) infection. In alignment with recommendations from the Centers for Disease Control and Prevention (CDC) Strategy for Summer 2025 Influenza Surveillance, CDPH recommends the following:

Healthcare providers should: 

Continue to test for influenza in patients with respiratory illness and exposure risk factors for avian influenza (A)H5N1, and if positive for influenza A, arrange for subtyping at a public health laboratory.

Continue to test for influenza in hospitalized patients with severe respiratory illness (particularly those in the ICU) regardless of exposure history and arrange for all influenza A positive specimens to be sent to a clinical, commercial, or local public health laboratory for subtyping.

Report suspected cases of novel influenza, particularly avian influenza A(H5N1), to local health department of the patient’s residence based on symptoms and relevant exposure, OR if an influenza A test was positive, and subtyping was attempted but did not identify a seasonal influenza A subtype (this includes unsubtypeable results). 
In patients with combination of respiratory illness and conjunctivitis, please test for influenza even in the absence of avian risk factor.

Clinical and commercial laboratories should:

Report any samples that are influenza A positive and for which subtyping was attempted but did not identify a seasonal influenza A subtype to the local health department of the patient’s residence and urgently direct these samples to a local public health laboratory for additional testing.

From June through September 2025, submit influenza A positive samples that were not subtyped in clinical or commercial laboratories to local public health laboratories for subtyping.

Continued influenza testing and subtyping all influenza A positives over the summer is key to maintaining visibility of the influenza A subtypes circulating in the community, and for enhanced detection of novel influenza cases. 