Viral shedding and clinical illness in naturally acquired influenza virus infections

Lau, Lincoln L. H.; Cowling, Benjamin J.; Fang, Vicky J.; Chan, Kwok-Hung; Lau, Eric H. Y.; Lipsitch, Marc; Cheng, Calvin K. Y.; Houck, Peter M.; Uyeki, Timothy M.; Malik Peiris, J. S.; Leung, Gabriel M.

BACKGROUND: Volunteer challenge studies have provided detailed data on viral shedding from the respiratory tract before and through the course of experimental influenza virus infection. There are no comparable quantitative data on naturally-acquired infections. METHODS: In a community-based study in Hong Kong in 2008, we followed up initially well individuals to quantify trends in viral shedding based on culture and reverse transcription polymerase chain reaction (RT-PCR) through the course of illness associated with seasonal influenza A and B virus infection. RESULTS: Trends in symptom scores more closely matched changes in molecular viral loads measured by RT-PCR for influenza A than influenza B. For influenza A virus infections, replicating viral loads measured by culture declined to undetectable levels earlier after illness onset than molecular viral loads. Most viral shedding occurred during the first 2–3 days after illness onset and we estimated that 1–8% of infectiousness occurs prior to illness onset. Only 14% of infections with detectable shedding by RT-PCR were asymptomatic, and viral shedding was low in these cases. CONCLUSIONS: Our results suggest that ‘silent spreaders’ (i.e. individuals who are infectious while asymptomatic or pre-symptomatic) may be less important in the spread of influenza epidemics than previously thought.