Our data are subject to some limitations. To make this report available in a timely manner, we censored hospital outcome data. Ascertainment of cases of H1N1 2009 admitted to ICUs in 2010 as well as in 2009 may not have been complete, and we cannot exclude the possibility that a small number of cases were not reported to the registry, and false negative diagnostic tests may well have underestimated the true burden of H1N1 2009 in our patients. Among the patients with confirmed influenza A, there were 29 in whom the influenza was not sub-typed in 2010 and 132 in 2009. Thresholds for undertaking testing both in hospital and in the community were not standardised. It is not possible to reach a conclusion regarding the efficacy of vaccination as the vaccination status of many patients was unknown. Finally, while we report a similar severity of the illness to 2009, we do not have information about anti-viral treatment, potential viral mutation and resistance to anti-viral drugs. Finally, we did not evaluate the role played by a corticosteroid therapy on the outcome of patients with ARDS, and we were not able to cope with the controversy about the effect of this treatment [30] .