The cold weather in North America is stretching out norovirus season. Chan and colleagues report in Emerging Infectious Diseases that a 2012 offseason outbreak of the Sydney strain of norovirus affected infants and young children more than other populations.
In late 2012, a new norovirus strain of genogroup II, genotype 4 originating in Sydney, Australia (GII.4 Sydney 2012), became the predominant norovirus strain and caused a severe norovirus season globally. In Hong Kong, China, this strain caused an off-season communitywide surge in acute gastroenteritis during summer (July–October) 2012. We report hospital admission of persons with this novel strain, which was skewed toward infants and young children for whom fecal viral load of this strain were higher than for patients in other age groups.
Our findings agree with evidence that infants and young children are likely to have the highest rate of infections in health care and community settings. In our study, infants and young children had the highest fecal viral load, compared with other age groups. Similarly lower Ct in children was reported in a historical cohort of norovirus gastroenteritis in the United Kingdom (1993–1996). The higher viral load may relate to delayed viral clearance related to immune naivety. Higher fecal viral shedding also supports a recent mathematical model suggesting that children aged <5 years are more infectious than older children and adults.