Static and Evolving Norovirus Genotypes: Implications for Epidemiology and Immunity
Fig 6
Model of norovirus evolution and infection.
GII.4 noroviruses acquire phenotypic changes in their major capsid protein over time (evolving genotype), while non-GII.4 viruses retain a highly conserved capsid protein for decades (static genotype). Clustering of the different noroviruses has shown the presence of twelve groups (provisionally called immunotypes), with only one immunotype containing an evolving genotype (GII.4). Immunotypes represented by static genotypes can only re-infect individuals naïve to that particular immunotype, while the GII.4 evolving genotype can re-infect individuals by periodically replacing its variants. This model predicts that children are constantly exposed and infected with strains from each of the different immunotypes (until a broad immunity develops), while older individuals are more likely to become ill from evolving genotypes. This model would explain the epidemiological differences reported in the distribution of norovirus genotypes in children and adults [17, 21, 37, 38].