Household transmission studies of children and adults reveal that being female (female relative risk [RR]: 1.87, 95% confidence interval [CI]: 1.17-2.73) is a significant factor associated with higher secondary attack rates of influenza-like illness, with attack rates being higher among children and young adults than older adults (>55 years of age) [11] . Reported male-female differences in the incidence of infection vary with age in several countries, with a higher incidence of infection with 2009 H1N1 in young women than young men of comparable age [12] [13] [14] [15] . While pregnancy has been clearly linked with increased disease severity, the vast majority of infected females of reproductive ages are not pregnant, suggesting that additional factors are contributing to the increased incidence of infection. In contrast, in Asia, the majority of reported H1N1 cases have been male (57.1%) [16, 17] . In China, males (male odds ratio [OR]: 1.94, 95% CI: 1.07-2.66) also shed the 2009 H1N1 virus in pharyngeal and nasopharyngeal samples for a longer duration than females [18] suggesting that the transmission potential may be higher in males. Other countries reported no male-female differences in the number of cases of 2009 H1N1, but did not analyse the data stratified by both age and sex [19] [20] [21] [22] [23] [24] [25] .