• Written By:
    Comprehensive Ophthalmology

    Review of: Respiratory virus shedding in exhaled breath and efficacy of face masks

    Leung N, Chu D, Shiu E, et al. Nature, April 2020

    This study examined the efficacy of surgical masks to alleviate the transmission of coronavirus, influenza viruses and rhinoviruses.

    Study design

    Researchers recruited 246 participants with acute respiratory virus illnesses who were managed at the outpatient clinic of a private hospital in Hong Kong between March 2013 and May 2016. The staff screened all individuals for respiratory symptoms regardless of the purpose of the visit. Half of the patients were randomized to wear a face mask during the first exhaled breath collection. Nasal swabs were tested for 12 common respiratory viruses: coronaviruses (NL63, OC43, 229E and HKU1), influenza A and B, and respiratory syncytial virus, parainfluenza virus (types 1–4), adenovirus, human metapneumovirus and enterovirus/rhinovirus.

    The primary outcome of the study was the efficacy of face masks in preventing virus dissemination in exhaled breath or cough. The authors compared the viral shedding in respiratory droplet and aerosol samples with and without a mask.


    Reverse transcription PCR confirmed that 123 (50%) of patients were infected by at least 1 respiratory virus. Of these participants, 90% were infected by human (seasonal) coronavirus (n = 17), influenza virus (n = 43) or rhinovirus (n = 54). Individuals with coronavirus tended to cough the most (17 coughs/30-min exhaled breath collection). Viral RNA for all 3 viruses could be detected in respiratory droplets (26-40%) and aerosols (35-56%). For coronavirus, researchers could detect OC43 and HKU1 from both respiratory droplets and aerosols; NL63 could also be detected in aerosols.

    Coronavirus could be detected in both the respiratory (30%) and aerosol (40%) samples of individuals not wearing a mask, but was not identified in any of samples from patients wearing masks. For the influenza virus, masks reduced virus detection in respiratory droplets from 26% to 4% but did not mitigate detection in aerosols (35%).


    A large proportion of participants had undetectable viral shedding in exhaled breath. No forced coughing was performed. The authors did not confirm the infectivity of the coronavirus or rhinovirus detected in exhaled breath or attempt to culture the viruses.

    Clinical significance

    These findings indicate that surgical masks can reduce the emission of influenza virus particles into the environment through respiratory droplets, but not in aerosols. The results also demonstrated the efficacy of surgical masks to reduce coronavirus detection and viral copies in large respiratory droplets and in aerosols. These results have critical implications for control of COVID-19, suggesting that surgical face masks could be used by ill people to reduce transmission. It is important to remember that infected people can be symptomatic or asymptomatic.