Surgical face masks may be effective in preventing the transmission of seasonal coronaviruses and influenza virus from symptomatic individuals, based on a study conducted by researchers at the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, LKS Faculty of Medicine of the University of Hong Kong (HKUMed) and the University of Maryland. In the study, masks were shown to reduce the detection of influenza virus in respiratory droplets and seasonal coronaviruses in droplets and aerosols. Further research is needed to determine whether masks can specifically prevent the transmission of SARS-CoV-2, which is closely related to seasonal coronaviruses. The findings are now published in the journal Nature Medicine [link to publication].

Previous research had shown that respiratory viral infections, including those caused by coronaviruses, spread between humans mostly through close contact. However, there is no direct evidence of how influenza and coronaviruses spread between individuals who are in close proximity – whether by direct contact, large respiratory droplets, or through breathing another person’s exhaled breath. Respiratory viruses can also survive in the environment and potentially spread through indirect contact. Larger respiratory droplets, which fall near the source, as well as aerosols of smaller fine particles can both contain virus and cause short-range transmission. In addition, aerosols can remain in the air for longer and potentially transmit infection over longer distances, particularly if they are generated at higher concentration or if there is little ventilation.

Non-pharmaceutical measures, such as social distancing, handwashing, ventilation, and the use of face masks, could be important measures to prevent viral transmission. Although the use of face masks has been suggested as a strategy to slow down the transmission of influenza virus, little is known about the relative importance of this strategy in the transmission of other respiratory viruses, including seasonal coronaviruses.

Dr Nancy Leung Hiu-lan, Research Assistant Professor of School of Public Health, HKUMed and the research team recruited 246 people with suspected respiratory viral infections to breathe into a machine — the Gesundheit II — to compare the relative amount of virus in exhaled breath with or without a surgical face mask. The research team provided the same type of surgical mask to all the participants and ensured it was worn correctly.

“In 111 people infected by either coronavirus, influenza virus or rhinovirus, masks reduced detectable virus in respiratory droplets and aerosols for seasonal coronaviruses, and in respiratory droplets for influenza virus. In contrast, masks did not reduce the emission of rhinoviruses.” said Dr Leung.

SARS-CoV-2 and seasonal coronaviruses are closely related and may be of similar particle size. Professor Benjamin Cowling, Division Head of Epidemiology and Biostatistics, School of Public Health, HKUMed, and Co-Director of the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, expressed, “the ability of surgical masks to reduce seasonal coronavirus in respiratory droplets and aerosols implies that such masks can contribute to slowing the spread of SARS-CoV-2 when worn by infected people.”

About the research team
The research was conducted by a team led by Professor Benjamin Cowling, Division Head of Epidemiology and Biostatistics, School of Public Health, HKUMed, and Co-Director of the WHO Collaborating Centre for Infectious Disease Epidemiology and Control.

The project was supported by the General Research Fund of the University Grants Committee [Grant No. 765811], the Health and Medical Research Fund [Grant No. 13120592] and a commissioned grant from the Food and Health Bureau and the Theme-based Research Scheme [Project No. T11-705/14-N] of the Research Grants Council of the Hong Kong SAR Government.

About the WHO Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, HKUMed
The School of Public Health, LKS Faculty of Medicine of The University of Hong Kong (HKUSPH) has a long and distinguished history in public health education and high impact research. With world leading research in infectious diseases as well as on non-communicable diseases of both local and global importance, the School has made significant contributions through its research and advocacy to improve the health of populations and individuals, both locally and globally. The School is a leading research and teaching hub in public health on influenza and other emerging viruses, control of non-communicable and infectious diseases, tobacco control, air pollution, psycho-oncology, behavioural sciences, exercise science, life-course epidemiology, and health economics, health services planning and management. Work done by HKUSPH researchers has informed international (e.g. the World Health Organization and the Food and Agriculture Organization), national and local public health policies.

The School of Public Health hosts the WHO Collaborating Centre (WHO CC) for Infectious Disease Epidemiology and Control. With a view to protecting the public's health in Hong Kong and across the region, the WHO CC aims to coordinating research on the control and prevention of infectious diseases and providing local and regional education and training in infectious disease epidemiology and control. Members of the WHO CC are involved in the response to COVID-19 and conducted a range of scientific research projects. The team has created a website to share the latest scientific findings and the implications for evidence-based public health policies on the ongoing COVID-19 outbreak [link to website: https://covid19.sph.hku.hk/].