Study: Hospitalizations for H1N1 differ from hospitalizations associated with typical flu

Pregnant women represented a greater percentage of those hospitalized than in a typical flu year

When compared to a typical flu, the H1N1 virus resulted in a higher proportion of patients requiring specialized hospital services - and affected younger people more - according to a new study released today by the Canadian Institute for Health Information (CIHI).

The study, H1N1 in Canada - A Context for Understanding Patients and Their Use of Hospital Services, is the first of its kind to examine at a pan-Canadian level how hospitalizations for H1N1 differed from hospitalizations associated with a typical flu. It compares Public Health Agency of Canada FluWatch analysis from April to December 2009 with CIHI hospital statistics for the baseline year 2007-2008. The comparison group included patients with influenza and/or pneumonia, the most common complication of influenza.

"The H1N1 pandemic was met with an unprecedented response from public health officials. As the spread of the virus subsides, it is important to pause and look at how H1N1 compares to a typical flu season," says Jean-Marie Berthelot, Vice President of Programs at CIHI. "Our study shows that proportionately more H1N1 patients needed specialized and intensive hospital services than what we've come to expect in a typical flu season."

Specifically, the study found that from April 12, 2009, to January 2, 2010, the proportion of hospitalized H1N1 patients requiring intensive care was 50% higher than for those admitted to hospital with influenza or pneumonia in the baseline year - with almost one in six H1N1 hospitalized patients admitted to an intensive care unit (ICU). In addition, an estimated 1 in 10 patients admitted to hospital with H1N1 required ventilation to assist with breathing; proportionally, this was more than double the influenza/pneumonia group.

Patients with H1N1 tended to be younger

The study found the H1N1 virus was especially hard-hitting in younger age groups, compared to the influenza/pneumonia patients in a typical flu year. The median age for patients hospitalized with H1N1 was 28, while the median age for influenza/pneumonia was 71.

The median age of people with H1N1 who died was about 30 years younger than that of people who died with influenza/pneumonia: the early 50s compared to the early 80s.

Within the ICU, H1N1 patients were younger than baseline influenza/pneumonia patients; the median age for H1N1 patients was the mid-40s, compared to baseline patients with influenza/pneumonia, who had a median age of 68.

Impact on pregnant women requires more study

Early research studies on H1N1 raised concerns that pregnant women were more significantly affected by H1N1 than by the regular flu. Our study found that the proportion of pregnant women hospitalized with H1N1 was higher than what would be expected in a typical flu year.

From April to December 2009, more than one in five (21%) women of child-bearing age (15 to 44) who were hospitalized with H1N1 were pregnant, compared to 13% of women in this age group who were hospitalized for influenza/pneumonia in 2007-2008. There were four deaths among pregnant women with H1N1 during this time period, compared to no deaths among pregnant women with influenza/pneumonia in all of 2007-2008. ICU use by hospitalized pregnant women was proportionately similar among both H1N1 and baseline influenza/pneumonia patients. In both groups, 12% of ICU patients were pregnant.

"More study is needed to understand why pregnant women account for a higher proportion of hospitalized patients for H1N1 compared to regular influenza/pneumonia," explains Kathleen Morris, head of Emerging Issues at CIHI. "This preliminary comparison will hopefully be a springboard for future research."