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06 april 2020

Respiratory virus shedding in exhaled breath and efficacy of face masks - Nature Medicine


Identification of coronavirus and influenza viruses in exhaled breath an coughs

Key findings

  • Surgical facemasks significantly reduce detection of coronavirus RNA in aerosols with a trend towards reduced reduction of coronavirus RNA in respiratory droplets.
  • Surgical masks can efficaciously reduce the emission of influenza virus particles into the environment in respiratory droplets, but not in aerosols.
  • This has important implications for control of COVID-19, suggesting that surgical face masks could be used by ill people to reduce onward transmission.


  • Little evidence on the efficacy of face masks in reducing viral release from an individual with respiratory infections.
  • 246 participants; infections by ≥1 respiratory virus (confirmed by RT-PCR) in 123 (50%): 111 (90%) were infected by coronavirus (n = 17), influenza virus (n = 43) or rhinovirus (n = 54).
  • 122 (50%) participants randomized to not wearing a face mask and 124 (50%) participants randomized to wearing a face mask during 30-min exhaled breath collection. The profiles of the participants randomized to with-mask versus without-mask groups were similar.
  • On average, viral shedding was higher in nasal swabs than in throat swabs for coronavirus (median 8.1 log10 virus copies vs. 3.9).
  • Among samples collected without a face mask, the majority of participants with influenza virus and coronavirus infection did not shed detectable virus in respiratory droplets or aerosols, whereas for rhinovirus we detected virus in aerosols in 19 of 34 (56%) participants (compared to 4 of 10 (40%) for influenza and 8 of 23 (35%) for coronavirus).
  • For those who did shed virus in respiratory droplets and aerosols, viral load in both tended to be low (Fig. 1). Given that each exhaled breath collection was conducted for 30 min, this might imply that prolonged close contact would be required for transmission to occur.
  • In the subset for coronavirus (n = 4) that did not cough during exhaled breath collection, we did not detect any virus in respiratory droplets or aerosols from any participants.

 a–c, Virus copies per sample collected in nasal swab (red), throat swab (blue) and respiratory droplets collected for 30min while not wearing (dark green) or wearing (light green) a surgical face mask, and aerosols collected for 30min while not wearing (brown) or wearing (orange) a face mask, collected from individuals with acute respiratory symptoms who were positive for coronavirus (a), influenza virus (b) and rhinovirus (c), as determined by RT–PCR in any samples. P values for mask intervention as predictor of log10 virus copies per sample in an unadjusted univariate Tobit regression model which allowed for censoring at the lower limit of detection of the RT–PCR assay are shown, with significant differences in bold. For nasal swabs and throat swabs, all infected individuals were included (coronavirus, n=17; influenza virus, n=43; rhinovirus, n=54). For respiratory droplets and aerosols, numbers of infected individuals who provided exhaled breath samples while not wearing or wearing a surgical face mask, respectively were: coronavirus (n=10 and 11), influenza virus (n=23 and 28) and rhinovirus (n=36 and 32). A subset of participants provided exhaled breath samples for both mask interventions (coronavirus, n=4; influenza virus, n=8; rhinovirus, n=14). The box plots indicate the median with the interquartile range (lower and upper hinge) and ±1.5×interquartile range from the first and third quartile (lower and upper whiskers).