Transmission potential of influenza in respiratory droplets

Abstract

Influenza virus is a respiratory infection of public health importance, because it is associated with a substantial burden of morbidity and mortality in Hong Kong and elsewhere. The traditional paradigm suggests that influenza virus transmits between humans primarily by respiratory droplets and secondarily by contact, while there is controversy over the role of aerosol transmission. Interventions to control influenza epidemics often require identification of infected persons, which is challenging without laboratory testing, while asymptomatic infections may have a role in community spread.

I conducted a series of studies on the potential contribution of aerosols to the transmission of influenza, the efficacy of surgical face masks against aerosols, the potential for a new rapid test to be used for the identification of cases of influenza virus infection, and the potential importance of asymptomatic infections in the spread of influenza epidemics. In one study, I used a Gesundheit-II (G-II) device to collect exhaled breath particles from outpatients in two size fractions, ≥5μm (coarse) and <5μm (fine) and maintained viability of any viruses collected in the fine fraction. In a second study, I used a cyclone NIOSH air sampler to detect influenza virus in fine particles in patient rooms. In the third study, I used data from three separate studies of outpatients to characterize the clinical diagnostic performance of the Sofia Influenza A+B Fluorescent Immunoassay. Finally, I conducted a systematic review and meta-analysis of published estimates of the proportion of influenza virus infections that are asymptomatic.

In the first study, I showed that influenza A virus, including A(H1N1)pdm09 and A(H3N2), were found in both fine and coarse fractions of exhaled breath particles. I also showed that wearing a surgical face mask led to a significant and substantial reduction in influenza A virus shedding in the coarse fraction of exhaled breath. In the second study I detected influenza virus RNA in aerosols at low concentrations half of the time in patient rooms with patients with confirmed influenza virus infection. I estimated that the sensitivity and specificity of the Sofia rapid test for detection of influenza A and B were 77% (95% CI 71-82%) and 91% (95% CI 89-92%) respectively. In the meta-analysis I found that estimates of the asymptomatic fraction were affected by study design, with estimates from outbreak investigations having a pooled mean of 16% (95% CI: 13%, 19%), while estimates from cohort studies adjusting for incidence of non-influenza illnesses fell in the range 65%-85%.

Influenza virus could be detected in fine particles in the exhaled breath of outpatients with influenza, and in the rooms of inpatients with confirmed influenza, supporting a role of aerosol transmission in the spread of influenza in the community. The Sofia rapid test had higher sensitivity than other point-of-care tests, but poorer specificity leading to a low positive predictive value outside of peaks in influenza activity. Uncertainty remains about the frequency of asymptomatic infections in the community which depends on how infections are identified, and their importance in transmission.