High prevalence of prolonged norovirus shedding and illness among hospitalized patients: a model for in vivo molecular evolution.

During a 2-year survey in an academic hospital, 8 (8.4%) of all norovirus (NoV)-positive patients showed prolonged norovirus illness and shedding (duration, 21-182 days). All patients had underlying illnesses, resulting in some level of immunodeficiency in 5. Four patients were admitted to the hospital with gastroenteritis, 2 acquired norovirus while hospitalized, and 2 were outpatients. Genotypes GII.4 and GIIb-GII.3 were found. Reinfection occurred in 3 patients. Full capsid sequences were determined from strains detected in sequentially collected stool specimens to study evolution. The greatest number of amino acid mutations in a given patient was 11; they were detected in NoV isolates recovered over a 119-day period and were mapped to positions at or near putative antigenic sites. In the patient with most severe immune dysfunction, only 5 amino acids mutated over 182 days, suggesting immune-driven selection. The severe impact on patients and hospitals and the potential role of prolonged shedders as a reservoir for viral antigenic variants lead us to stress the importance of confinement of outbreaks of NoV infection that occur in hospitals.