Pandemic (H1N1) 2009 virus causes severe illness, including pneumonia, which leads to hospitalization and even death. To characterize the kinetic changes in viral load and identify factors of influence, we analyzed variables that could potentially influence the viral shedding time in a hospital-based cohort of 1,052 patients. Viral load was inversely correlated with number of days after the onset of fever and was maintained at a high level over the first 3 days. Patients with pneumonia had higher viral loads than those with bronchitis or upper respiratory tract infection. Median viral shedding time after the onset of symptoms was 9 days. Patients <13 years of age had a longer median viral shedding time than those >or=13 years of age (11 days vs. 7 days). These results suggest that younger children may require a longer isolation period and that patients with pneumonia may require treatment that is more aggressive than standard therapy for pandemic (H1N1) 2009 virus.