2021-11-18-世界卫生组织-Assessment_of_risk_associated_with_influenza_A_H5N6_virus_3页_214kb

Summary of the Risk Assessment for Highly Pathogenic Avian Influenza A(H5N6) Virus

Core Content

This document provides an assessment of the current risk posed by the highly pathogenic avian influenza (HPAI) A(H5N6) virus to humans, based on data up to November 2021. It outlines the spread of the virus in birds, the number of human infections, and the implications for public health and pandemic risk.

Main Points

• Virus Spread in Birds:
  The HPAI A(H5N6) viruses belonging to the HA genetic lineage 2.3.4.4b have been spreading in birds in China and neighboring countries since late 2020. These viruses have been found in environmental surfaces and live poultry markets in China, indicating their presence in the human-animal interface.

• Human Infections:
  As of 2021, 26 human infections with A(H5N6) were reported to WHO, with 20 cases having onset after June 2021. The majority of cases (25) were reported from China, and 1 from Lao PDR. The median age of infected individuals was 55 years, and most had exposure to poultry. The single case from Lao PDR was in a child under 18 years of age. Human-to-human transmission cannot be ruled out, although it remains rare.

• Genetic Characteristics:
  All A(H5N6) viruses from human cases in China with onset after February 2021 were classified as HPAI and belonged to HA clade 2.3.4.4b. Some gene segments were related to previously identified HPAI A(H5N1) and A(H5N8) viruses. Molecular markers associated with mammalian adaptation, such as E627K and D701D/N in the PB2 gene, and Q226L in the HA1 gene, were identified in some cases. However, these markers were not present in environmental or poultry viruses and likely arose from intra-host mutations.

• Pandemic Risk:
  The overall pandemic risk from HPAI A(H5N6) is considered not significantly changed compared to previous years. While the zoonotic threat remains elevated due to the virus's circulation in birds, there is no evidence of increased intrinsic zoonotic potential of clade 2.3.4.4b A(H5N6) viruses compared to other HPAI A(H5) subtypes.

• Candidate Vaccine Viruses (CVVs):
  CVVs for clade 2.3.4.4b A(H5) have been developed, including a recommended A(H5N8) CVV derived from A/Astrakhan/3212/2020. The HA of these viruses is closely related to the 2021 A(H5N6) human viruses, with at most 4 amino acid differences. Antigenic characterization and reagent development are ongoing to assess cross-reactivity and potential mismatches between N8 and N6 viruses.

• Antiviral Susceptibility:
  No genetic markers associated with reduced susceptibility to clinically relevant antiviral drugs (e.g., neuraminidase or polymerase inhibitors) were found in recent A(H5N6) viruses, indicating that current antiviral treatments remain effective.

• Recommended Actions:

  • Countries, especially those with National Influenza Centers and GISRS laboratories, should remain vigilant for zoonotic infections.
  • Human samples that are influenza A-positive but negative for seasonal or other subtypes should be sent to WHO Collaborating Centers or H5 Reference Laboratories for further analysis.
  • Antigenic characterization and reagent development for A(H5N6) viruses are being prioritized.
  • Additional data on the prevalence and genetic/antigenic features of A(H5N6) viruses in birds and other animals are needed.
  • Measures to reduce human exposure to potentially infected birds should be implemented to prevent further zoonotic infections.
  • WHO continues to collaborate with FAO, OIE, and OFFLU to monitor the avian influenza situation and the evolution of A(H5) viruses.

Key Information

• Geographic Distribution:
  The A(H5N6) viruses in China are part of the HA clade 2.3.4.4b, which has been found in birds across Europe, Africa, and Asia. These viruses have been linked to human infections in the Russian Federation and Nigeria, although not all isolates could be sequenced.

• Zoonotic Potential:
  Although some genetic markers suggest potential for mammalian adaptation, these changes were not observed in poultry or environmental isolates and are likely due to intra-host mutations. Therefore, the zoonotic potential of the virus is not significantly higher than previously identified HPAI A(H5) subtypes.

• Public Health Implications:
  The risk to humans is closely tied to the continued circulation of the virus in poultry and the presence of infected birds in environments where humans interact with them. Reducing viral spread in birds and implementing preventive measures at the human-animal interface are critical to mitigating risk.

• Collaboration and Surveillance:
  WHO is working closely with FAO, OIE, and OFFLU to monitor the situation and provide updated risk assessments. This includes the development of specific reagents and antigenic characterization of A(H5N6) viruses.

Conclusion

The HPAI A(H5N6) virus remains a zoonotic threat, primarily due to its presence in poultry and live bird markets. While the pandemic risk has not significantly increased compared to previous years, continued surveillance, research, and preventive measures are essential to manage and mitigate the risk to human populations.