## 1 Introduction  
Asian highly pathogenic avian influenza (HPAI) A (H5N1) virus is potentially able to cause a global pandemic due to its relatively large spread via avian hosts and its potential to directly infect humans [1]. In Vietnam, over 2750 outbreaks of H5N1 occurring in poultry were reported to the World Organisation for Animal Health (OIE) from the end of 2003 to 03 July 2019, among which one outbreak killed and disposed of 1120 birds in 2019 [2]. Influenza haemagglutinin (HA), a homotrimic type I transmembrane protein, is one of the major surface glycoproteins of H5N [3]. The viral HA protein plays a critical role in the infectivity and pathogenicity of influenza virus by binding specifically to sialic acid on glycoconjugates of host membrane proteins and enabling virus entry into host cells [3]. Many studies have shown that protection provided by influenza vaccines is mediated by mainly an anti-HA neutralizing antibody [4]. Therefore, HA has been reported as a major target antigen for influenza vaccine development [5]. However, H5N1 HA antigenic variation occurs frequently. A/H5N1 evolved into 10 distinct clades (0-9) that are divided into many subclades [6]. In Vietnam, six clades (clades 1, 2, 3, 5, 7, and 8) exist, among which clade 1 (subclades 1.1, 1.1.1, and 1.1.2) and clade 2 (subclades 2.3.2.1, 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c) are the most common [6]. In particular, one or more strains could circulate at the same time, in the same area, or both. Current strain-specific vaccines (for example, NIBRG-14 or RE-6) have limited effectiveness in the protection of poultry. The timely update of seasonal vaccines, which is necessary to ensure protection, requires significant time and effort. Recent studies of a universal influenza vaccine showed its potential efficacy through enhanced breadth and durability compared to those of alternate seasonal influenza vaccines [7; 8; 9; 10; 11; 12]. In 2008, [PERSON] and [PERSON] demonstrated that a polyvalent influenza virus-like particle (VLP) vaccine in which haemagglutinin represented different clades of A/H5N1 was a potential approach to elicit an immunity response against isolates from emerging clades and subclades of A/H5N1 [7]. Following studies reported that a vaccine containing haemagglutinin designed by a methodology called computationally optimized broadly reactive antigen (COBRA) elicited a broad immunity response against multiple H5N1 isolates from different clades [8; 9; 10; 11]. The COBRA HA VLP vaccines elicited higher antibody titres than both a polyvalent HA VLP vaccine and a homologous vaccine [8; 9; 10]. In 2018, [PERSON] and co-workers demonstrated that the highly conserved influenza virus haemagglutinin stalk domain is one of the promising targets for a universal vaccine. They generated a formulated modified mRNA encoding full-length HA in lipid nanoparticles (LNPs) and subsequently vaccinated rabbits, mice, and ferrets. Immunization with HA mRNA-LNPs induced HA stalk-specific antibody responses against the HA stalk domain of homologous, heterologous, and heterosubtypic influenza viruses in these animals [13]. Therefore, universal influenza vaccines protecting poultry against many H5N1 variants will become a new potential trend in vaccine development. Based on this trend, we designed two novel artificial HA proteins (including H5.c1 representing all of the A/H5N1 subclades 1.1, 1.1.1, and 1.1.2 and H5.c2 representing all of the A/H5N1 subclades 2.3.2.1, 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c). Each amino acid site of the artificial HA was determined from a comparison of all of the HA sequences of A/H5N1 recorded in poultry from 2005 to 2015 in Vietnam, selecting for either the most common or most recent epitope. The aim of our research was to generate artificial HA proteins that will elicit broadly antigenic responses to protect poultry against many A/H5N1 strains of clades 1.1, 1.1.1, and 1.1.2 or 2.3.2.1, 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c that have recently circulated.