We collected data using electronic case report forms. The study-coordinating centre was the Australian and New Zealand Intensive Care-Research Centre, Monash University, Melbourne, Australia [10] . H1N1 2009 infection is subject to mandatory reporting in both Australia and New Zealand and wherever possible diagnoses were confirmed with the relevant public health authorities. In addition, to confirm the completeness of case ascertainment, we contacted all ICUs that had no reported cases at the end of each study period. Cases transferred between ICUs were counted as a single ICU case. We made no assumptions for missing data and all proportions were calculated as percentages of available data.