• C Raina MacIntyre, Holly Seale, Tham Chi Dung, Nguyen Tran Hien, Phan Thi Nga, Abrar Ahmad Chughtai, Bayzidur Rahman, Dominic E Dwyer, and Quanyi Wang.
• Faculty of Medicine, School of Public Health and Community Medicine, University of New South Wales, Sydney, Australia.
• BMJ Open. 2015 Jan 1;5(4):e006577.

ObjectiveThe aim of this study was to compare the efficacy of cloth masks to medical masks in hospital healthcare workers (HCWs). The null hypothesis is that there is no difference between medical masks and cloth masks.Setting14 secondary-level/tertiary-level hospitals in Hanoi, Vietnam.Participants1607 hospital HCWs aged ≥18 years working full-time in selected high-risk wards.InterventionHospital wards were randomised to: medical masks, cloth masks or a control group (usual practice, which included mask wearing). Participants used the mask on every shift for 4 consecutive weeks.Main Outcome MeasureClinical respiratory illness (CRI), influenza-like illness (ILI) and laboratory-confirmed respiratory virus infection.ResultsThe rates of all infection outcomes were highest in the cloth mask arm, with the rate of ILI statistically significantly higher in the cloth mask arm (relative risk (RR)=13.00, 95% CI 1.69 to 100.07) compared with the medical mask arm. Cloth masks also had significantly higher rates of ILI compared with the control arm. An analysis by mask use showed ILI (RR=6.64, 95% CI 1.45 to 28.65) and laboratory-confirmed virus (RR=1.72, 95% CI 1.01 to 2.94) were significantly higher in the cloth masks group compared with the medical masks group. Penetration of cloth masks by particles was almost 97% and medical masks 44%.ConclusionsThis study is the first RCT of cloth masks, and the results caution against the use of cloth masks. This is an important finding to inform occupational health and safety. Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection. Further research is needed to inform the widespread use of cloth masks globally. However, as a precautionary measure, cloth masks should not be recommended for HCWs, particularly in high-risk situations, and guidelines need to be updated.Trial Registration NumberAustralian New Zealand Clinical Trials Registry: ACTRN12610000887077.Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.

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