Background: The nature of influenza viral shedding during naturally acquired infection is not well understood.
Methods: A cohort study was conducted in Hutterite colonies in Alberta, Canada. Flocked nasal swabs were collected during 3 influenza seasons (2007-2008 to 2009-2010) from both symptomatic and asymptomatic individuals infected with influenza. Samples were tested by real-time reverse-transcription polymerase chain reaction for influenza A and influenza B, and the viral load (VL) was determined for influenza A positive samples.
Results: Eight hundred thirty-nine participants were included in the cohort; 25% (208) tested positive for influenza viruses. They experienced 238 episodes of viral shedding, of which 23 (10%) were not accompanied by symptoms. For seasonal and pandemic H1N1, VL peaked at or before onset of acute respiratory infection. For H3N2, VL peaked 2 days after the onset of acute respiratory infection, which corresponded to peaks in systemic and respiratory symptom scores. Although the duration of shedding was shorter for asymptomatic participants, the peak level of VL shedding was similar to that of symptomatic participants. Viral loads for children and adults revealed similar patterns.
Conclusions: Molecular viral shedding values follow symptom scores, but timing of peak VL varies by subtype. Asymptomatic infections are infrequent.