Viral Shedding and Transmission Potential of Asymptomatic and Paucisymptomatic Influenza Virus Infections in the Community

Abstract

Background: Influenza virus infections are associated with a wide spectrum of disease. However, few studies have investigated in detail the epidemiological and virological characteristics 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 (≥2 signs/symptoms, including fever ≥37.8°C, headache, myalgia, cough, sore throat, runny nose and sputum), paucisymptomatic (1 symptom only), or asymptomatic (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%) paucisymptomatic and 25 (11%) asymptomatic cases. The duration of viral RNA shedding was shorter and declined more rapidly in paucisymptomatic and asymptomatic than in symptomatic cases. The mean levels of influenza viral RNA shedding in asymptomatic and paucisymptomatic cases were approximately 1-2 log10 copies lower than in symptomatic cases.

Conclusions: The presence of influenza viral shedding in patients with influenza who have 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.

Keywords: asymptomatic; epidemiology.; influenza virus; public health; viral shedding.

Figure 1.

Proportion of patients with acute respiratory illness (ARI) (solid line) and paucisymptomatic patients (dashed line) remaining symptomatic in naturally acquired influenza virus infections by day since first symptom onset (day 0).

Figure 2.

Patterns of viral shedding in naturally acquired influenza virus infections by day since first symptom onset (day 0) in patients with acute respiratory illness (ARI) (upper panel) and paucisymptomatic patients (lower panel). RT-PCR, reverse-transcription polymerase chain reaction.

Figure 3.

Rate of viral load decline as measured by reverse-transcription polymerase chain reaction (RT-PCR) in paucisymptomatic patients (blue line) and patients with acute respiratory illness (ARI) (black line).

Figure 4.

Estimated duration of viral shedding from the start of virus shedding (0 on the x-axis), accounting for interval censoring in the data on detection of virus by reverse-transcription polymerase chain reaction at 3-day intervals, for patients with acute respiratory illness (ARI) (solid line), paucisymptomatic patients (dashed line), and asymptomatic patients (dotted line).