A recent study of influenza transmission in household contacts in France found a SAR of 24.1%, and a within-household correlation of ρ=0.29 [28] . We will assume a reduced SAR of 20% to allow for the possibility of some transmission occurring prior to randomization and the likely inclusion of some index cases without influenza. A relative risk reduction of at least 30% is generally accepted to be clinically and epidemiologically important. We note that the efficacy of masks and hand-washing were estimated to give relative risk reductions of 90% and 75% respectively during a nosocomial outbreak of SARS, and while we doubt such high efficacy in the household setting we anticipate relative risk reductions of perhaps around 30%-50%, although there is no literature to guide us on such estimates (and hence the need for this trial!). The average household size in Hong Kong excluding houses with single or double occupancy is 3.8 (source: Hong Kong Thematic Household Survey 2002), therefore the average number of household contacts per index case would be m=2.8.