Low Prevalence of H5N1 Influenza Virus in 2024-2025 Respiratory Season

Strategic Insights -

A sustained outbreak of H5N1 influenza virus among wild fowl and domestic livestock has led to significant concerns about the potential for zoonotic infections in humans in North America. Despite this, the University of Washington researchers have reported that there were no H5 infections detected in 740 samples collected between March 2024 and April 2025 from patients with confirmed influenza A virus infection in a large academic medical system in Seattle, WA. This research highlights the importance of rapid PCR subtyping of H5 influenza virus to inform hospital infection prevention and public health measures to control the spread of the virus.

Key Takeaways:

  • The United States Centers for Disease Control and Prevention has recommended rapid H5 subtyping for all hospitalized cases with influenza A virus infection to enable prompt initiation of antiviral treatment and infection prevention.
  • The researchers developed a qualitative multiplex RT-qPCR assay to subtype H5 influenza virus in nasal, nasopharyngeal, and conjunctival specimens with a limit of detection of 250 copies/mL.
  • No cross-reactivity was observed with other common respiratory viruses, including seasonal H3N2 and H1N1 influenza A viruses.
  • The researchers retrospectively subtyped 590 influenza A virus-positive clinical specimens and detected no H5 positives.
  • After clinical implementation, the researchers performed 150 clinically ordered H5 subtyping tests and again detected no positives.
  • The study enhances clinical pandemic preparedness activities and highlights the exceedingly low prevalence of H5N1 influenza virus during the 2024-2025 respiratory season.
  • The H5N1 influenza virus has led to the culling of almost 200 million birds, infected cow herds across 17 states, and resulted in 70 human infections as of July 2025.

Statistics:

  • 715 million: Number of nasal, nasopharyngeal, and conjunctival specimens collected between March 2024 and April 2025 from patients with confirmed influenza A virus infection in a large academic medical system in Seattle, WA.
  • 740: Number of samples collected between March 2024 and April 2025 from patients with confirmed influenza A virus infection in a large academic medical system in Seattle, WA.
  • 250 copies/mL: Limit of detection of the qualitative multiplex RT-qPCR assay used to subtype H5 influenza virus.
  • 590: Number of influenza A virus-positive clinical specimens retrospectively subtyped and detected no H5 positives.
  • 150: Number of clinically ordered H5 subtyping tests performed after clinical implementation and detected no positives.
  • 70: Number of human infections caused by H5N1 influenza virus as of July 2025.
  • 17: Number of states where cow herds were infected by H5N1 influenza virus.
  • 200 million: Number of birds culled due to H5N1 influenza virus outbreak.

Sources:

  • NewsRx. New Influenza A Virus Study Results from University of Washington Described (Validation of H5 influenza virus subtyping RT-qPCR assay and low prevalence of H5 detection in 2024-2025 influenza virus season). Life Science Weekly. November 4, 2025; p 3287.
  • Journal of Clinical Microbiology. Validation of H5 influenza virus subtyping RT-qPCR assay and low prevalence of H5 detection in 2024-2025 influenza virus season. 2025.
  • Amer Soc Microbiology, 1752 N St NW, Washington, DC 20036-2904, USA. Journal of Clinical Microbiology can be contacted at: Amer Soc Microbiology, 1752 N St NW, Washington, DC 20036-2904, USA.