Development of Norovirus Vaccines

Norovirus Genome

Noroviruses encode a ~7.5 kb positive-sense, single-stranded RNA genome with three open reading frames (ORFs), which encode both the structural and the non-structural genes (Fig. 1) ^[5]. ORF1 encodes a ~200 kDa polyprotein, which is autoprocessed by a virally encoded protease to yield the non-structural viral replicase proteins essential for viral replication (Fig. 1) ^[5]. ORF2 encodes the 57 kDa major structural capsid protein VP1. ORF3 encodes a 22 kDa minor basic structural protein that has been hypothesized to function in packaging the genome into virions (Fig. 1). Norovirus protein functions are shown in Table 1 ^[6].

The Norovirus genus contains more than 40 different virus strains, which are divided into five genogroups based on sequence similarity. Genogroup I (GI), GII, and GIV viruses are primarily human pathogens, but there is a porcinespecific virus within the GII genogroup. GIII and GV viruses infect bovine and murine species, respectively. Each genogroup is further subdivided into genoclusters, based on sequence similarity and phylogenetic analysis. Comparison of the fulllength capsid sequence demonstrated that the five different genogroups varied by up to 60%, up to 57% among the human NoVs. In addition, sequences within a genocluster can vary as much as 20–30% in the capsid gene ^[5].

Fig. 1. The norovirus genomic structure and capsid domains.

Table 1 Noroviruses proteins and their functions

*These functions have been proposed, but have yet to be demonstrated fully.

Norovirus Vaccines

Vaccines against norovirus diseases have been shown to be of high significance. Among the vaccine candidates that are currently under clinical studies, recombinant VP1-based virus-like particles (VLPs) that mimic major antigenic features of noroviruses are the common ones, with proven safety, immunogenicity, and protective efficacy, supporting a high success likelihood of a useful norovirus vaccine ^[7,8].

The TAK-214 is an adjuvanted VLP-based bivalent vaccine containing two types of VLPs, one from the GI.1 Norwalk virus (NV) strain and the other one from a consensus GII.4 sequence (GII.4c) derived from three GII.4 variants. Several phase 1 clinical studies were conducted, proving good safety and tolerability. In a phase 2a trial, two doses of intranasally delivered vaccine using chitosan and monophosphoryl lipid A (MPLA) as adjuvants were demonstrated to protect healthy adults against challenge of homologous GI.1 NV. The first in-human phase 2b field efficacy study of the TAK-214 vaccine was conducted over two winter seasons in a U.S. Navy training facility with over 4600 participants. the vaccine efficacy against GII.2 noroviruses was 57.4% (95% CI, 7.0 to 80.5; p = 0.0321), indicating that the TAK-214 vaccine elicits a broad immune response and confers cross-genotype protection ^{[9, 10]}.

Another norovirus vaccine candidate in clinical study is a recombinant VP1-based bivalent vaccine (VXA-NVV-104). This vaccine contains recombinant adenovirus-based vectors ^[11] carrying genes encoding noroviral VP1s to express VP1 antigens locally in the epithelial cells within the intestine of vaccine recipients to induce mucosal immunity. The adenovirus vector also contains specific RNA-encoding sequences to generate double-stranded RNAs as an adjuvant for enhanced immunogenicity of the VP1 antigens. Both VP1-encoding genes of the GI.1 NV strain and the GII.4 Sydney variant are included as a bivalent vaccine for broad efficacy. Therefore, this tablet vaccine is designed to be administered orally to elicit the mucosal immune response locally within the intestine to prevent norovirus infection ^{[8, 11]}.

The third norovirus vaccine candidate under clinical study is a bivalent VLP-based vaccine (Hansenulapolymorpha) that was developed by the National Vaccine and Serum Institute (NVSI) of China. It is composed of two recombinant VLPs representing the GI.1 and the GII.4 genotypes, respectively, with aluminum salt as the adjuvant ^[8]. And the forth norovirus vaccine is the Longkoma Norovirus Vaccine. This is a quadrivalent vaccine consisting of four aluminum salt adjuvanted VP1 proteins, each representing GI.1, GII.3, GII.4, or GII.17 genotypes, produced by means of a yeast expression system ^[8].