Risk factors for severe disease from pandemic (H1N1) 2009 virus infection reported to date are considered similar to those risk factors identified for complications from seasonal influenza. These include the following groups:

• Infants and young children, in particular <2 years
• Pregnant women
• Persons of any age with chronic pulmonary disease (e.g. asthma, COPD)
• Persons of any age with chronic cardiac disease (e.g. congestive cardiac failure)
• Persons with metabolic disorders (e.g. diabetes)
• Persons with chronic renal disease, chronic hepatic disease, certain neurological conditions (including neuromuscular, neurocognitive and seizure disorders, but not including autism spectrum disorders),
• Hemoglobinopathies, or immunosuppression, whether due to primary immunosuppressive conditions, such as HIV infection, or secondary conditions, such as immunosuppressive medication or malignancy
• Children receiving chronic aspirin therapy
• Persons aged 65 years and older

A higher risk of severe complications from pandemic (H1N1) 2009 virus infection has also been observed in individuals who are obese (particularly in those who are morbidly obese) and among disadvantaged and indigenous populations.

The Guidelines panel had the following additional comments concerning persons at higher risk of developing complicated or severe influenza disease, which should be taken into account in applying these guidelines:

• The higher risk during pregnancy should be applied to a two-week post-partum period¹⁶
• There are limited data from the pandemic on the extent to which HIV-infected patients are at higher risk of complicated or severe illness, though there are some data from seasonal influenza indicating a higher risk and limited data relating to mortality from pandemic influenza¹⁷. The decision to administer influenza antiviral medicines to such patients will depend on local priorities and availability of such antivirals.