Viral shedding and transmission potential of asymptomatic and paucisymptomatic influenza virus infections in the community

Abstract

Influenza virus infections are associated with a wide spectrum of disease. However, few studies have investigated in detail the epidemiology and virology of asymptomatic and mild illness with influenza virus infections. METHODS: In a community-based study in Hong Kong from 2008 to 2014, we followed up initially healthy individuals who were household contacts of symptomatic persons with laboratory-confirmed influenza, to identify secondary infections. Information from daily symptom diaries was used to classify infections as symptomatic (with at least two signs/symptoms of: fever ≥37.8°C, headache, myalgia, cough, sore throat, runny nose and sputum), pauci-symptomatic (with one symptom only), or asymptomatic (reporting none of these symptoms). We compared the patterns of influenza viral shedding between these groups. RESULTS: We identified 235 virologically-confirmed secondary cases of influenza virus infection in the household setting, including 31 (13%) pauci-symptomatic and 25 (11%) asymptomatic cases. The duration of viral RNA shedding was shorter and declined more rapidly in pauci-symptomatic and asymptomatic cases compared with symptomatic cases. The mean levels of influenza viral RNA shedding in asymptomatic and pauci-symptomatic cases were approximately 1 to 2 log10 copies lower than in symptomatic cases. CONCLUSIONS: The presence of influenza viral shedding in influenza patients with very few or no symptoms reflects their potential for transmitting the virus to close contacts. These findings suggest that further research is needed to investigate the contribution of persons with asymptomatic or clinically mild influenza virus infections to influenza virus transmission in household, institutional, and community settings.