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Surgical masks significantly reduce coronavirus detection in respiratory aerosol

Results of a Hong Kong study recently published in Nature Medicine indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

Most existing evidence on the filtering efficacy of face masks and respirators comes from in vitro experiments with nonbiological particles, which may not be applicable to droplets carrying infectious respiratory viruses. "We aimed to explore the importance of respiratory droplet and aerosol routes of transmission with a particular focus on coronaviruses, influenza viruses and rhinoviruses, by quantifying the amount of respiratory virus in exhaled breath and by determining the potential efficacy of surgical face masks in preventing respiratory virus transmission," wrote the researchers from the University of Hong Kong. [Nat Med 2020, doi: 10.1038/s41591-020-0843-2]

Of the 246 enrolled individuals with a diagnosed acute respiratory infection, half were randomized to not wearing and another half to wearing a face mask during exhaled breath collection. In the group not wearing a mask, 30 percent of coronavirus-infected patients, 26 percent of influenza-infected patients and 28 percent of rhinovirus-infected patients had viral RNA in respiratory droplets, while viral RNA could be identified in the aerosols of 40 percent of coronavirus-infected patients, 35 percent of influenza-infected patients and 56 percent of rhinovirus-infected patients.

In contrast, no virus was identified in the respiratory droplets or aerosols collected from coronavirus-infected participants wearing face masks. "This difference was significant in aerosols and showed a trend toward reduced detection in respiratory droplets," noted the researchers.

Wearing a face mask significantly reduced the detection of influenza virus in respiratory droplets (from 26 percent to 4 percent), but no significant reduction was achieved in detecting influenza, coronavirus or rhinovirus RNA in aerosols.

"Among samples collected from participants without a face mask, we found that the majority of those with influenza virus or coronavirus infection did not shed detectable virus in respiratory droplets or aerosols and among those who did, the viral load in both tended to be low," wrote the researchers. "Given the high collection efficiency of our bioaerosol collecting device and the 30-minute period of exhaled breath collection for each participant, this might imply that prolonged close contact would be required for transmission to occur."

"Our results also indicate that there could be considerable heterogeneity in contagiousness of individuals with coronavirus or influenza virus infection," they added.

Overall, 24 percent of participants had a fever of ≥37.8 °C, with influenza patients being more than twice as likely as coronavirus or rhinovirus patients to have a detectable fever. Coronavirus-infected participants coughed the most, with an average of 17 coughs during the 30-minute exhaled breath collection. However, the researchers identified viral RNA in a small number of participants who did not cough at all during the 30-minute exhaled breath collection. "[This] would suggest droplet and aerosol routes of transmission are possible from individuals with no obvious signs or symptoms," they wrote.

"[Our findings] demonstrated the efficacy of surgical masks at reducing coronavirus detection and viral copies in large respiratory droplets and in aerosols. This has important implications for control of coronavirus disease 2019 [COVID-19], suggesting that surgical face masks could be used by ill people to reduce onward transmission," concluded the researchers.
[MIMS May 2020]