A-Z Index × Submit A-Z Index × Submit A-Z Index Search Dropdown × Submit Facebook Twitter LinkedIn Syndicate Emerging Infectious Disease journal ISSN: 1080-6059 Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.
Swine influenza A viruses (IAV-S) caused the 2009 H1N1 pandemic and pose a future zoonotic and pandemic threat. Vietnam represents a critical hotspot for IAV-S emergence within East and Southeast Asia, with dense swine and human populations and intensive livestock trade. We conducted genomic surveillance of IAV-S in Vietnam during 2020–2024, extending previous surveillance from 2013–2019. We identified multiple co-circulating H1 and H3 clades, including pandemic H1N1, Eurasian avian-like, and European lineages, by conducting phylogenetic analysis of 56 IAV-S isolates (21 H1N1, 31 H1N2, and 4 H3N2). Three H1 clades persisted exclusively in Vietnam, circulating up to 12 years. Phylogeographic analysis revealed multiple independent introduction events from North America, Europe, China, Thailand, and Cambodia. We detected extensive reassortment that frequently involved pandemic H1N1 virus internal genes. We identified several lineage-specific mutations associated with mammalian adaptation. Our findings underscore the ongoing IAV-S evolution and need for sustained surveillance in Vietnam.
Swine influenza A viruses (IAV-S) contribute to zoonotic and pandemic risk because of their genetic diversity, rapid evolution, and interspecies transmission ( 1 ). Swine are evolutionary intermediaries, enabling reassortment between avian, human, and endemic swine influenza viruses and generating genotypes with pandemic potential ( 2 ). Circulation in swine might also enable the adaptation of avian virus hemagglutinins to bind to α2,6-linked sialic acids receptors found in the mammalian upper respiratory tract. This unique role was dramatically illustrated by the 2009 H1N1 pandemic, caused by a reassortant swine virus containing gene segments from classical swine, Eurasian avian-like swine, and human seasonal virus lineages ( 3 ).
Globally, influenza A virus subtypes H1N1, H1N2, and H3N2 are endemic in swine populations, displaying extensive genetic and antigenic diversity across different geographic regions. A phylogenetic nomenclature system for IAV-S defines 3 lineages for IAV-S H1, the classical swine lineage (1A, including the 2009 H1N1 pandemic virus [pH1N1]), the pre-2009 human seasonal lineage (1B), and the Eurasian avian-like lineage (1C). IAV-S H3N2 are defined by the introduction from human seasonal virus and circulation decade (1970, 1990, 2000, and 2010) ( 4 ). The evolutionary dynamics of IAV-Ss are influenced by multiple human seasonal influenza spillovers into swine, followed by sustained within-host transmission and rapid adaptation ( 5 ). The diversification of IAV-Ss is amplified by the intensive global swine production and international trade, which enables the movement of live swine and viruses across continents, heightening the risk for zoonotic emergence ( 6 ).
East and Southeast Asia is a critical hotspot for the emergence and evolution of IAV-Ss. High, dense human and swine populations with poultry production, including live-bird markets, enabled frequent reassortment and interspecies transmission in the region ( 7 ). Vietnam is the second largest swine producer in Asia other than China ( https://ww…