The recent pandemic of the 2009 pandemic influenza A (H1N1) infrequently caused severe disease. We describe 2 cases of 2009 H1N1 influenza with rapid progression resulting in respiratory failure and need for prolonged intensive care support. Real-time polymerase chain reaction amplification for influenza A (using a Centers for Disease Control and Prevention protocol) and the 2009 H1N1 influenza (using an in-house protocol) was performed on serial respiratory and serum specimens from both patients collected over 3 weeks. Both patients repeatedly demonstrated 2009 H1N1 influenza in respiratory specimens. Evidence of influenza A viremia was also detected in both cases, although it was confirmed as 2009 H1N1 influenza in only one. The presence of viremia in cases of severe 2009 H1N1 influenza has potential prognostic and therapeutic implications. Detection of viremia may be useful as a predictive marker for severe disease. Antiviral agents with low serum levels may be ineffective if administered to patients with influenza viremia.
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