Highly pathogenic avian influenza (HPAI) viruses such as H5N1, H5N8, H5N6 and H5N2 have been circulating worldwide, posing serious threats to both poultry farming and public health. Notably, the H5N1, H5N6 and H7N9 HPAI viruses have caused significant mortality in China. Among these, the H5N1 HPAI virus is of great concern due to its impact on global public health (Zhu et al., 2022). Since the first occurrence of H5N1 HPAI virus human infection in 1997, it has manifested a sporadic epidemic pattern in numerous regions across the globe. Thereafter, 925 cases of human infection with different clades of H5N1 HPAI viruses have been documented, with a cumulative case fatality rate exceeding 50%, highlighting the severe public health threat posed by the H5N1 HPAI viruses (https://www.cdc.gov/bird-flu/php/technical-report/h5n1-06052024.html?CDC_AAref_Val=https://www.cdc.gov/flu/avianflu/spotlights/2023-2024/h5n1-technical-report-06052024.htm). Recently, newly H5N1 HPAI viruses were detected on dairy cattle and poultry farms in the United States (US), and spread to herds across fifteen states, resulting in at least nine confirmed human infections during April and September 2024. Subsequently, these viruses also caused many human infections in Asia (e.g. Cambodia and China). This epidemic marks the first confirmed case of the H5N1 HPAI virus being transmitted between humans and other mammalian species, deviating from prior cases involving avian-to-human transmission.
H5 HPAI viruses have undergone extensive genetic diversification, evolving into 10 distinct clades, numbered 0 to 9. Emerging in 2003, clade 2 strains have become the predominant strain in circulation and have further evolved into various subclades, including 2.2.1, 2.2.1.1, 2.2.1.2, 2.3.2.1, and 2.3.4.4, among others. The clade 2.3.4.4 was further divided into eight subclades, namely clades 2.3.4.4a to 2.3.4.4h. Particularly, the novel H5N1 HPAI viruses of 2.3.4.4b clade evolved from the previously prevalent lineage of H5Nx virus and have been in circulation from 2021 to the present (Fig. 1A). Consequently, these reassortments of H5 strains have the potential to alter the genetic landscape of avian influenza strains, increasing the diversity and unpredictability of emerging strains, which may accelerate the evolution of clade 2.3.4.4b viruses for pandemic potential (Siegers et al., 2024).