Slow Clearance of Norovirus following Infection with Emerging Variants of Genotype GII.4 Strains.

The emergence of new norovirus genotype GII.4 strains is associated with widespread norovirus epidemics. Extended periods of viral shedding can contribute to the epidemic potential of norovirus. To describe the duration of viral shedding in infections with novel emerging GII.4 strains versus infections with previously circulating strains, we performed a prospective cohort study of patients hospitalized with norovirus gastroenteritis during separate winter seasons. Rectal swab samples were obtained at the time of inclusion and weekly during follow-ups. The subgenotype strain was determined from capsid sequences. The outcome was defined by the detection of virus for >14 days (slow clearance) or by the detection of negative samples within 14 days (rapid clearance). Two major epidemic GII.4 strains emerged during the study period, GII.4 New Orleans 2009, in 2010, and GII.4 Sydney 2012, in 2012. From these two seasons, sequences were available from 24 cases where the duration of shedding could be determined. The median age of the patients was 83 years and 50% were women. The majority of patients were infected with virus that clustered with the respective season's epidemic strain (n = 19), whereas 5 patients had previously circulating strains (3 were Den Haag 2006b, in 2010, and 2 were New Orleans 2009, in 2012). Among the patients infected with an epidemic strain, the proportion who shed virus for >14 days was significantly higher (16/19 [84%] versus 1/5 [20%], P = 0.01). In summary, a slow clearance of norovirus from stool was more common in infections with novel epidemic GII.4 strains. This suggests that the average duration of shedding may be longer during seasons when new GII.4 strains have emerged.