Vietnam: A novel reassortant influenza A(H5N1) virus
In the Greater Mekong Subregion (GMS), a novel reassortant influenza A(H5N1) virus has emerged since mid-2022, causing infections in both humans and poultry. Recently, this virus triggered human outbreaks in Cambodia at the beginning of this year. It carries surface proteins from clade 2.3.2.1c, which has been circulating locally, while its internal genes derive from a more recent clade 2.3.4.4b virus. The infiltration and widespread spread of this reassortant influenza A(H5N1) virus in the GMS pose significant threats to animal and human health, given the historical impact of Highly Pathogenic Avian Influenza (HPAI) outbreaks in the area. Moreover, this reassortment event underscores not just the adaptability of the virus but also the constant risk of the emergence of new, potentially more harmful strains.
In Vietnam, HPAI A(H5N1) viruses continue to persist among poultry, though with sporadic outbreaks. Implementing poultry vaccination against HPAI A(H5) viruses has been pivotal in Vietnam's efforts to control HPAI. Analysis of the genetic makeup of HPAI A(H5N1) viruses collected from live poultry markets and outbreaks between January 2022 and December 2023 revealed ongoing circulation of both clades 2.3.2.1c and 2.3.4.4b in Vietnam. Clade 2.3.2.1c primarily circulated in southern Vietnam, while clade 2.3.4.4b was detected nationwide.
In 2023, a few HPAI A(H5N1) viruses identified in live poultry markets in the southern provinces displayed genetic reassortment between clade 2.3.2.1c and 2.3.4.4b genes, yet no PB2-E627K substitution was detected.
