Interruption of transmission may allow containment of major outbreaks, like pandemic influenza. Opportunistic data collected during the SARS epidemic in Asia suggested that population-wide use of face masks may significantly decrease transmission of not only SARS but also influenza [3, 4, 5, 6, 7] . As part of pandemic preparedness, many are contemplating the contribution wide-spread use of masks could have [8, 9] . As this has major implications for resource allocation and for communication, there is great need for data to guide such decisions and make them evidence-based.