Background. Although the pattern of viral shedding over time has been documented in volunteer challenge studies, understanding of the relationship between clinical symptomatology and viral shedding in naturally acquired influenza infections in humans remains limited.
Methods. In a community-based study in Hong Kong from 2008 to 2014, we followed up initially healthy individuals and identified 224 secondary cases of natural influenza virus infection in the household setting. We examined the dynamic relationship between patterns of clinical symptomatology and viral shedding as quantified using reverse transcription polymerase chain reaction and viral culture in 127 cases with a clinical picture of acute respiratory infection.
Results. Viral shedding in influenza A virus infections peaked on the first 1–2 days of clinical illness, and decreased gradually to undetectable levels by day 6–7, matching closely with the dynamics of clinical illness. Viral shedding in influenza B virus infections rose up to 2 days prior to symptom onset and persisted for 6–7 days after onset with a bimodal pattern.
Conclusions. Our results suggest that while clinical illness profiles may serve as a proxy for clinical infectiousness in influenza A virus infections, patients may potentially be infectious even before symptom onset or after clinical improvement in influenza B virus infections.