One trend that appears consistent across more than 60% of the datasets evaluated is that more females are hospitalized with critical illness than males ( Figure 1 ). The first cases in the USA were in California (April-May 2009), where the a majority of hospitalized cases (21/26) were women, five of whom were pregnant [26] . Initial analyses of data from critically ill patients in the USA during the first wave reported no male-female difference [27] , but subsequent state-specific reports from the first and second waves illustrated differences between the sexes [28] [29] [30] . In Canada, a significant majority of critically ill patients have been young women (female RR: 1.3, 95% CI: 1.0-1.6) [5, 31] . Other countries also report that rates of hospitalization have been higher among females than males, with a majority of the females being of reproductive age (15-49 years of age) [14, [32] [33] [34] [35] [36] . Analyses of cohorts of patients in Mexico and Australia/ New Zealand revealed a trend for more females than males being hospitalized [6, 37] . Evaluation of these differences in some countries is confounded by age, as many studies do not report male-female differences according to age group [6, 27, 37] .