Certain patients with seasonal influenza virus infection or pandemic influenza (H1N1) 2009 virus infection are recognized to be at higher risk of developing severe or complicated illness. The Guidelines Panel did not review the evidence for the definition of these higher risk groups, but adopted, as the basis for treatment decisions in the context of these guidelines, the description developed through the WHO Consultation on Clinical Aspects of Pandemic (H1N1) 2009 Influenza⁴ as listed in Part I, Annex 1.

However, an important consideration in the management of influenza virus infections is that influenza virus infection in any patient can result in severe or complicated illness. This is particularly true for pandemic (H1N1) 2009 virus infection, in which about 1/3 of severely ill patients admitted to intensive care units were previously healthy persons not belonging to any known higher risk group.