Factors associated with severe illness in pandemic 2009 influenza a (H1N1) infection: implications for triage in primary and secondary care

Pandemic (H1N1) 2009 influenza virus (pH1N1/09) infection spread rapidly around the globe, leading to a phase 6 pandemic level of alert declared in June 2009. The WHO declared the end of the pandemic in August 2010. Although for the majority of infected patients, it manifest as a mild, self-limiting illness, a proportion appeared to follow an adverse clinical course, requiring higher level care and aggressive management strategies. Experience with previous pandemics suggests that H1N1 will continue to circulate for many years. The aim of this review is to evaluate data from published case series reporting patients with pH1N1/09 influenza to identify clinical markers of severe disease. Comorbid illnesses including chronic lung disease, obesity and pregnancy have been shown to confer increased risk of severe infection. Admission vital signs, laboratory investigations and chest radiographic features can guide admitting clinicians to stratify patients' risk of severe disease, however, the currently available severity scoring tools have only a limited role in risk assessment. Knowledge of high risk parameters remains important for clinicians triaging patients with suspected pH1N1/09 influenza and to inform strategies for future pandemics.