Status and Challenges for Vaccination against Avian H9N2 Influenza Virus in China

Abstract

In China, H9N2 avian influenza virus (AIV) has become widely prevalent in poultry, causing huge economic losses after secondary infection with other pathogens. Importantly, H9N2 AIV continuously infects humans, and its six internal genes frequently reassort with other influenza viruses to generate novel influenza viruses that infect humans, threatening public health. Inactivated whole-virus vaccines have been used to control H9N2 AIV in China for more than 20 years, and they can alleviate clinical symptoms after immunization, greatly reducing economic losses. However, H9N2 AIVs can still be isolated from immunized chickens and have recently become the main epidemic subtype. A more effective vaccine prevention strategy might be able to address the current situation. Herein, we analyze the current status and vaccination strategy against H9N2 AIV and summarize the progress in vaccine development to provide insight for better H9N2 prevention and control.

Keywords: H9N2 AIV; antigenicity; influenza virus; vaccine.

Figure 1

Genetic evolution of H9N2 virus in China. All available nucleotide sequences of the HA genes of H9N2 influenza virus in China as of 31 December 2021 from GISAID (www.gisaid.org) were downloaded. Duplicated sequences and sequences with 99% identity were further removed, and a total of 1923 sequences were finally gutted. Following this, the MAFFT tool was used to perform a global multiple alignment of the sequences, and the MUSCLE tool was used to perform a partial alignment to correct part of the alignment errors and manually correct some of the frameshift errors. The H9N2 virus HA gene sequences were used to construct a phylogenetic tree using IQtree software based on the maximum likelihood method. Different colors in the phylogenetic tree represent virus strains isolated during different periods; the darker the color is, the more recent the isolation time. Adobe Illustrator 2021 and Interactive Tree Of Life (iTOL) were used to annotate the strains with antigen groups and vaccines.