Norovirus (NoV) is the second most frequen causative agent of acute gastroenteritis (AGE) in young children after rotavirus [1
]. NoV AGE in young children is likely to result from primary NoV infection. An early seroprevalence study in Finland found that 49% of infants were infected with NoV by the age of 2 years [2
]; some of those NoV infections were clinically manifest, and others were subclinical. In a prospective study of rotavirus vaccine recipients in Finland during 1993–1995, NoV AGE was observed in 20% of the children in a 1-year follow-up study [1
]. In this study, genogroup GII accounted for 90% of the cases of NoV AGE [1
Since 1995, NoV GII-4 genotype has emerged worldwide, displaced other NoV GII genotypes, and caused an increase in overall incidence of NoV infection [3
]. The reason for the surge of GII-4 activity is unknown, and 2 questions arise: is GII-4 inherently more virulent than other NoVs, and if so, what is the underlying mechanism? The polymerase region has been identified to be a marker of major NoV-associated gastroenteritis outbreaks (i.e. virulence) [3
]; however, changes in the capsid region also affect the antigenicity, host specificity, host cell binding, and virus entry properties and, thereofore, may affect virulence [4
A confounding factor in studies of virulence of NoV genotypes is pre-existing immunity, which might ameliorate the clinical course of gastroenteritis caused by old NoV genotypes but not by a newly emerged type, such as GII-4. We therefore conducted a comparative study on the clinical severity of AGE caused by NoV GII-4, compared with other genotypes, in young children. We propose that our study cases of NoV AGE in infants likely represent primary NoV infection in previously naive children. Our study material therefore provides a unique opportunity to examine the relationship between the clinical features of our study cases and the genotypes of causative NoVs and, in the case of GII-4, variants of GII-4 without the influence of pre-existing immunity on the clinical severity of episodes.