Background: Observational epidemiologic data suggest that transmission of viral respiratory infection was significantly reduced during the SARS epidemic with the use of face masks as well as other infection control measures. However, there are no prospective randomised control trials on face masks in prevention of viral respiratory infections Aims: To determine the efficacy of surgical masks and P2 masks in households on the interruption of transmission of respiratory viruses.

Methods: Prospective cluster randomized trial comparing surgical masks, non-fit-tested P2 (respirator) masks with no masks in interruption of viral transmission between household members. Families of children presenting to emergency department with influenza like illness (ILI) were randomised to one of the three groups and followed up for development of respiratory illness in other family members. Nasopharyngeal swabs of index patients and contacts that developed ILI were tested with a multiplex respiratory viral PCR for influenza A and B, parainfluenza, RSV, picornavirus, enterovirus, rhinovirus, adenovirus, coronaviruses human metapneumovirus.

Results: We recruited 286 adults with exposure to respiratory infections in the Australian winters of 2006 and 2007 - 94 adults were randomized to surgical masks, 90 to P2 masks and 102 to the control group. Using intention to treat analysis, we found no significant difference in the relative risk of respiratory illness in the mask groups compared to control group. However, compliance with mask use was less than 50%. In an adjusted analysis of compliant subjects, masks as a group had protective efficacy in excess of 80% against clinical influenza-like illness. The efficacy against proven viral infection and between P2 masks (57%) and surgical masks (33%) was non-significant.

Conclusions: This is the first RCT on mask use to be conducted and provides data to inform pandemic planning. We found compliance to be low, but compliance is affected by perception of risk. In a pandemic, we would expect compliance to improve. In compliant users, masks were highly efficacious. A larger study is required to enumerate the difference in efficacy (if any) between surgical and non-fit tested P2 masks.