What is the pandemic potential of avian influenza A(H5N1)?

On March 25, 2024, a national authority in Viet Nam notified WHO of a case of human infection with highly pathogenic avian influenza (HPAI) A(H5N1) virus (clade unreported). Then on April 1, a second but unrelated human infection with HPAI A(H5N1) virus (clade 2.3.4.4b) was reported in Texas, USA. The two cases occurred under different circumstances with different outcomes. The case in Viet Nam followed exposure to infected birds and resulted in death. By contrast, the case in Texas was in a worker on a commercial dairy farm who presented with only mild illness and was treated outside hospital. Genomic analysis at the US Centers for Disease Control and Prevention (CDC) revealed that the virus from the patient in Texas was closely related to viruses detected in cattle and dead birds from the same farm. This case occurred in the midst of an outbreak of HPAI A(H5N1) in dairy farms across multiple US states. No evidence of human-to-human transmission could be detected in either case, and the CDC and WHO deemed the risk to human health as low. While transmission of this virus from an infected bird to a person is not unheard of, the spread of the virus to cows and the subsequent human case represents one of several concerning developments in recent years for this virus.