2009 influenza A (H1N1): a clinical review.

Since the onset of the 2009 influenza A (H1N1) pandemic, the virus has caused significant morbidity and mortality. Most cases of 2009 H1N1 have presented as mild febrile illnesses with cough, sore throat, and occasional gastrointestinal symptoms. Dyspnea has been more commonly associated with the onset of severe pulmonary disease. Unlike seasonal influenza, the prevalence of 2009 H1N1 is greatest among children and young adults, although older patients and those with comorbidities are more likely to experience worse clinical outcomes. Among the most severely affected, critical illness evolves within 4 to 6 days from symptom onset, and approximately 70% of these patients require mechanical ventilation ranging in duration from days to weeks. Compared with prior influenza seasons, the need for rescue oxygenation therapy with nitric oxide, prone ventilation, high-frequency oscillation, and extracorporeal membrane oxygenation has increased. Specific medical care with neuraminidase inhibitors and antibiotics for secondary bacterial pneumonia are the mainstays of therapy. With optimal care, mortality rates range from 5% to 7% among those hospitalized and reach approximately 20% among those admitted to the intensive care unit.