Highly Pathogenic Avian Influenza Virus (H5N1) Clade 2.3.4.4b Introduced by Wild Birds, China, 2021

Abstract

Highly pathogenic avian influenza (HPAI) subtype H5N1 clade 2.3.4.4b virus has spread globally, causing unprecedented large-scale avian influenza outbreaks since 2020. In 2021, we isolated 17 highly pathogenic avian influenza H5N1 viruses from wild birds in China. To determine virus origin, we genetically analyzed 1,529 clade 2.3.4.4b H5N1 viruses reported globally since October 2020 and found that they formed 35 genotypes. The 17 viruses belonged to genotypes G07, which originated from eastern Asia, and G10, which originated from Russia. The viruses were moderately pathogenic in mice but were highly lethal in ducks. The viruses were in the same antigenic cluster as the current vaccine strain (H5-Re14) used in China. In chickens, the H5/H7 trivalent vaccine provided complete protection against clade 2.3.4.4b H5N1 virus challenge. Our data indicate that vaccination is an effective strategy for preventing and controlling the globally prevalent clade 2.3.4.4b H5N1 virus.

Keywords: China; H5N1; antigenic property; clade 2.3.4.4b; influenza; influenza virus; phylogeny; protective efficacy; virulence; viruses; wild birds.

Figure 1

Formation of 2 representatives of avian influenza (H5N1) virus from China. Viral particles are represented by colored ovals containing horizontal bars representing the 8 gene segments (from top to bottom: PB2, PB1, PA, HA, NP, NA, M, and NS). Gene segments of the reassortants are colored according to their corresponding source virus. HA, hemagglutinin; LPAIV, low pathogenicity avian influenza virus; M, matrix; NA, neuraminidase; NP, nucleoprotein; NS, nonstructural protein; PA, polymerase acidic; PB, polymerase basic.

Figure 2

Replication and virulence of representative highly pathogenic avian influenza virus (H5N1) viruses in mice and ducks. A) Virus titers in organs of mice that were humanely killed on postinfection day 3 with 10⁶ EID₅₀ of the test viruses. B, C) MLD₅₀ of the indicated viruses. D) Virus titers in organs of ducks that were killed on day 3 post inoculation with 10⁶ EID₅₀ in 0.1 mL of the indicated viruses. E) Virus shedding from ducks on the indicated days after inoculation. F) Death pattern of ducks in the infected and control groups. EID₅₀, 50% egg infectious dose; MLD₅₀, 50% lethal dose for mice.

Figure 3

Antigenic difference of avian influenza (H5N1) virus isolates with H5 vaccine strains and protective efficacy of H5 vaccine against the challenge of H5N1 virus. A) Antigenic cartography of H5N1 viruses. The map was generated by using Antigenic Cartography software (https://acmacs-web.antigenic-cartography.org); 1 unit (grid) represents a 2-fold change in the HI assay results (Appendix 2 Table 7). B) HI antibody titers of vaccinated chickens against the vaccine seed virus and the challenge strain. C) Virus shedding from chickens on postchallenge days 3 and 5. D) Survival patterns of chickens in the vaccinated and control groups after challenge with the indicated H5N1 virus. EID₅₀, 50% egg infectious dose; HI, hemagglutination inhibition.