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Young patients may be at risk for more severe influenza A (H1N1) infection

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Critical illness due to influenza A (H1N1) may occur rapidly and frequently in young adult individuals, according to findings from concurrent studies conducted in Canada and Mexico.

The study in Canada involved 168 critically ill patients with influenza A (H1N1) in 38 adult and pediatric ICUs who were prospectively observed between April 16 and August 12, 2009.

Though the average age of patients with influenza A (H1N1) was 32.3, there were 50 cases of critical infection among children. Other factors associated with critical illness included severe hypoxemia (100% of patients,) the need for mechanical ventilation (81.0% of patients), use of rescue therapies such as lung rescue therapy (28%) and multisystem organ failure.

The average Sequential Organ Failure Assessment score among all patients was 6.8 on day 1, indicating that shock and nonpulmonary acute organ dysfunction was common, according to the researchers.

The mortality rate among critically ill patients was 14.3% (95% CI, 9.5%-20.7%) at the 28-day mark and 17.3% (95% CI, 12.0%-24.0%) at the 90-day mark.

The observational study conducted in Mexico involved 58 critically ill patients. It was conducted from March 24 to June 1, 2009.

The median age of patients in this study was 44 (range, 10-83) years.

All but one patient presented with respiratory symptoms, and all but two patients received mechanical ventilation to treat severe acute respiratory distress syndrome and refractory hypoxemia (median day 1 ratio of PaO₂ to fraction of inspired oxygen, 83 [IQR, 59-145] mm Hg).

The average time from hospital admission to ICU admission was 1 day (IQR, 0-3 days).

The mortality rate was 41.4% (95% CI, 28.9%-55.0%) by the 60-day point. Factors associated with mortality included initial severity of illness, worse hypoxemia, higher creatine kinase levels, higher creatinine levels and ongoing organ dysfunction.

Kumar A et al. JAMA. 2009;302(17).

Dominguez-Cherit G et al. JAMA. 2009;302(17).