Summary

Background: A pandemic (H1N1) influenza A virus was identified in 2009.

Objectives: To investigate predictors for pandemic (H1N1) 2009 virus infection among hospitalized patients with a flu-like illness and to identify parameters suggesting a severe clinical course.

Methods: We analyzed a cohort of all patients hospitalized during a 2 month period with a flu-like syndrome who were tested for pandemic (H1N1) 2009 infection. Demographic, clinical and laboratory, along with outcome parameters, were recorded and compared between pandemic (H1N1) 2009 virus-positive and negative hospitalized patients.

Results: Of the 179 examined hospitalized patients suspected of having pandemic (H1N1) 2009 infection 65 (36%) were found positive. These patients tended to be younger and had significantly fewer comorbidities. In addition, they had a significantly higher frequency of fever (94%), cough (86%) and myalgia (29%). Furthermore, age < 65 years and cough were independent predictors for pandemic (H1N1) 2009 virus positivity in a multivariate regression analysis. Notably, 14 of the 65 positive patients (21.5%) had acute respiratory insufficiency requiring treatment in the intensive care unit. These patients were neither older nor previously sicker than patients with non-severe disease, but were distinguished by augmented inflammatory markers, significant lymphopenia associated with disease severity, and overall mortality of 21.4%.

Conclusions: Pandemic (H1N1) 2009 virus-positive hospitalized patients tend to be younger and have fewer comorbidities as compared to compatible negative patients. A significant number of relatively young and previously healthy positive patients might develop severe disease associated with a robust inflammatory reaction and significant lymphopenia.