The objective of this study was to identify case characteristics and clinical course of the disease in patients hospitalized with 2009 pandemic influenza A (H1N1) infection during the first wave of the pandemic and to identify risk factors associated with the complicated course of illness.

Material and methods: A retrospective study of adult cases of the laboratory-confirmed 2009 pandemic influenza A (H1N1) virus admitted to three hospitals in Kaunas between November 1, 2009, and March 15, 2010, was carried out. The main outcome measures were clinical characteristics, risk factors for complicated disease, treatment, and clinical course of the disease.

Results: The study enrolled 121 of the 125 patients hospitalized due to 2009 pandemic influenza A (H1N1) virus infection. The median age was 31 years (range, 18-83); 5% of the patients were aged more than 65 years. Pregnant and postpartum women comprised 26% of all hospitalized cases. Nearly half (49.5%) of those who underwent chest radiography had findings consistent with pneumonia, which was bilateral in one-third of cases. The risk to have pandemic influenza complicated by pneumonia increased significantly with one-day delay from symptom onset to antiviral treatment (OR, 2.241; 95% CI, 1.354-3.710). More than half (57%) of the patients received antiviral treatment. In 45% of the treated patients, antiviral drugs were administered within 48 hours from symptom onset. Intensive care was required in 7.4% of the cases. The overall mortality was 5% (6/121). The median age of the patients who died was 43.5 years (range, 23-62); 4 patients had been previously healthy, 1 patient suffered from chronic lympholeukemia, and 1 patient was a pregnant woman.

Conclusion: The 2009 pandemic influenza A (H1N1) caused considerable morbidity in a significant proportion of hospitalized adults. The main risk factor associated with the complicated course of illness was delayed antiviral treatment.