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Technical Report: Highly Pathogenic Avian Influenza A(H5N1) Viruses

Purpose

This report provides an update to the December 29, 2023, report to include seven new human cases (5 in Cambodia, 1 in the United States and 1 in Vietnam) and recent activity in wild birds, poultry and other animals, including the multi-state veterinary outbreak in U.S. dairy cattle, and updated information on monitoring for H5N1 virus infections in the United States. Even given these updates, CDC believes the overall risk to human health associated with the ongoing outbreaks of highly pathogenic avian influenza A(H5N1) viruses has not changed and remains low to the U.S. general public at this time.

Executive Summary

A small number of sporadic human cases of highly pathogenic avian influenza (HPAI) A(H5N1) have been identified worldwide since 2022, amidst a panzootic of these viruses in wild birds and poultry. Nearly all human cases reported globally since 2022 were associated with poultry exposures, and no cases of human-to-human transmission of HPAI A(H5N1) virus have been identified. One human case of HPAI A(H5N1) virus infection in a farm worker reported in April 2024 in the United States and was attributed to exposure to presumptively infected dairy cattle. One previous human case was reported in the United States in 2022. In a few cases, the source of exposure to HPAI A(H5N1) virus has been unknown. To date, HPAI A(H5N1) viruses currently circulating most commonly in birds and poultry, with spillover to mammals and humans do not have the ability to easily bind to receptors that predominate in the human upper respiratory tract. This is a major reason why the current risk to the public from HPAI A(H5N1) viruses remains low. However, because of the potential for influenza viruses to rapidly evolve and the wide global prevalence of HPAI A(H5N1) viruses in wild birds and poultry outbreaks and following the identification and spread among dairy cattle in the United States, additional sporadic human infections are anticipated. Continued comprehensive surveillance of these viruses in wild birds, poultry, mammals, and people worldwide, and frequent reassessments are critical to determine the public health risk, along with ongoing preparedness efforts.

Key findings

  • CDC is actively working on the domestic situation with clade 2.3.4.4b HPAI A(H5N1) viruses in wild birds, with outbreaks in poultry and backyard flocks, and infections of other animals, including dairy cattle. These activities include conducting surveillance among people with relevant exposures and preparing for the possibility that contemporary HPAI A(H5N1) viruses gain the ability for increased transmissibility to and among people.
  • CDC, along with state and local public health partners, continues to monitor people in the United States who have been exposed to infected birds, poultry, or other animals for 10 days after exposure. To date, more than 8,800 people in 52 jurisdictions have been monitored since 2022, and only two human cases have been identified.
  • H5 candidate vaccine viruses (CVV) produced by CDC are expected to provide good protection against current clade 2.3.4.4b HPAI A(H5N1) viruses detected in birds and mammals, including dairy cattle. These H5 CVVs are available and have been shared with vaccine manufacturers.
  • Because influenza viruses are constantly changing, CDC performs ongoing analyses of HPAI A(H5N1) viruses to identify genetic changes that might allow for spread more easily to and between people, more serious illness in people, reduce susceptibility to antivirals, affect the sensitivity of diagnostic assays, or reduce neutralization of the virus by vaccine induced antibodies. To date, few genetic changes of public health concern have been identified in HPAI A(H5N1) viruses circulating in wild birds and poultry worldwide and detected in dairy cattle in the United States.
  • Currently, HPAI A(H5N1) viruses circulating in birds and U.S. dairy cattle are believed to pose a low risk to the general public in the United States; however, people who have job-related or recreational exposures to infected birds or infected mammals are at higher risk of infection and should take appropriate precautions outlined in CDC guidance.
  • Comprehensive surveillance and readiness efforts are ongoing, and CDC continually takes preparedness measures to be ready in case the risk to people from HPAI A(H5N1) virus or from other novel influenza A viruses changes.

Explore more

The latest H5N1 technical report and all previous technical reports are available from the H5N1 technical report webpage.

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Content Source:
National Center for Immunization and Respiratory Diseases (NCIRD)