Continued Circulation of Highly Pathogenic H5 Influenza Viruses in Vietnamese Live Bird Markets in 2018-2021

We isolated 77 highly pathogenic avian influenza viruses during routine surveillance in live poultry markets in northern provinces of Vietnam from 2018 to 2021. These viruses are of the H5N6 subtype and belong to HA clades 2.3.4.4g and 2.3.4.4h. Interestingly, we did not detect viruses of clade 2.3.4.4b, which in recent years have dominated in different parts of the world. The viruses isolated in this current study do not encode major determinants of mammalian adaptation (e.g., PB2-E627K or PB1-D701N) but possess amino acid substitutions that may affect viral receptor-binding, replication, or the responses to human antiviral factors. Several of the highly pathogenic H5N6 virus samples contained other influenza viruses, providing an opportunity for reassortment. Collectively, our study demonstrates that the highly pathogenic H5 viruses circulating in Vietnam in 2018-2021 were different from those in other parts of the world, and that the Vietnamese H5 viruses continue to evolve through mutations and reassortment.

Automated summary

From 2018 to 2021, we identified 77 highly pathogenic avian influenza viruses in live poultry markets in northern provinces of Vietnam. These viruses belong to the H5N6 subtype and HA clades 2.3.4.4g and 2.3.4.4h. Interestingly, we did not detect viruses from clade 2.3.4.4b, which have been dominant in other parts of the world. The isolated viruses in Vietnam showed amino acid substitutions that may affect viral characteristics and could potentially reassort with other influenza viruses.