Pandemic influenza A (H1N1) virus emerged in Mexico during
the spring of 2009 and spread rapidly worldwide [1], resulting
in the first influenza pandemic of the twenty-first century. The
epidemiological features, clinical spectrum of illness and riskfactors for severe disease of pandemic influenza were broadly
consistent across all countries [2–4]. Cases of infection
occurred mostly in children and young adults. The majority of
patients had self-limited mild-to-moderate uncomplicated disease.
However, some patients developed severe illness and
some died; most of these were adults between the ages of 20
and 50 years, with or without underlying medical conditions
[2–6]. Importantly, most patients requiring intensive-care unit
(ICU) admission had respiratory failure due mainly to primary
influenza pneumonia [5,7,8]. Bacterial co-infection, although
relatively infrequent, has also been associated with poor prognosis
[5,9]. These patterns differed significantly from those