The 2009 H1N1 influenza caused severe illness requiring hospitalization during the outbreak’s peak in the Northern Hemisphere last spring and was a burden on ICUs in the Southern Hemisphere during their winter season, according to two studies published this week in the New England Journal of Medicine.

Seema Jain, MD, and colleagues from the CDC, and researchers from the Australian and New Zealand Intensive Care study noted many of the patients in both their studies had underlying medical conditions.

In the CDC report, which examined medical charts of 272 patients who were hospitalized for at least 24 hours with the novel influenza from April 2009 to mid-June 200, researchers wrote that nearly three-quarters of the patients had one or more underlying medical conditions. Twenty-five percent of these patients were admitted to an intensive care unit and 7% died. Forty-five percent of the patients were younger than 18.

Researchers in the Southern Hemisphere reported 856 patients were admitted to the ICU with influenza A during the study period of June 1 through Aug. 31; of these patients 722 had confirmed novel influenza A (H1N1).

The Australian researchers estimate the proportion of patients with asthma or other chronic pulmonary disease in the general population to be around 13% but they said 32.7% of the hospitalized patients had asthma or another chronic pulmonary disease. Indigenous groups were also over-represented among the ICU patients, the researchers noted.

The researchers in the Southern Hemisphere noted that almost half the pandemic patients had acute respiratory distress syndrome or viral pneumonitis, which was a sharp increase from previous years.