Major Finding: Interval to treatment, First Nations ethnicity, and medical comorbidities independently predicted ICU admission for 2009 pandemic H1N1 influenza in Manitoba, Canada.
Data Source: Cumulative case-control study of 795 Manitoba residents with confirmed H1N1 infection.
Disclosures: None declared.
The time interval between the onset of symptoms and antiviral treatment was the strongest predictor of intensive care admission for severe infection with 2009 pandemic influenza A(H1N1) and increased the risk for ICU admission up to eightfold in a cumulative case-control study of 795 patients in Manitoba, Canada.
Other predictors of ICU admission, compared with more mild disease cared for in the community, included First Nations ethnicity and the presence of a medical comorbidity, said Dr. Ryan Zarychanski of the University of Manitoba, and his associates.
“These data may have implications for proactive public health and primary care using outreach models at the community level, whether for public health education, prioritization of vaccination efforts, or strategies for antiviral treatment,” the investigators said.
The 795 study patients came from a total 894 confirmed H1N1 cases among Manitoba residents for whom the location of care could be determined: 569 remained in the community, 181 were admitted to the hospital but not the ICU, and 45 were admitted to the ICU. The mean age of the 795 infected individuals analyzed was 25.3 years, and 52% were female. Of 588 for whom ethnicity was known, 37% described their ethnicity as “First Nations,” one of three officially recognized groups of Canadian aboriginal peoples.
First Nations residents accounted for 28% of the 410 community cases for whom ethnicity was known, compared with 54% of those admitted to the hospital and 60% of 42 admitted to the ICU. Females represented 52% of the total 569 community cases and 50% of the 181 hospital admissions, but 69% of the 45 ICU cases. The presence of an underlying medical condition—including heart or lung disease, diabetes, malignancy, and substance abuse as well as pregnancy—also increased with severity of disease, from 35% of the community cases to 57% of the hospital admissions to 76% of those admitted to the ICU, Dr. Zarychanski and his associates reported.
Two-thirds of the 34 adults with comorbid conditions admitted to the ICU were clinically obese (body mass index greater than 30), but height and weight were not recorded consistently enough for patients in the two control groups to allow analysis of obesity as a risk factor for severe outcomes, they noted.
Antiviral therapy was known to have been prescribed for 34% of patients in the community group, 54% admitted to the hospital, and 95% of those who ended up in the ICU. Severity of illness correlated with increasing interval from symptom onset to the start of antiviral therapy, with the median interval 2 days for the community patients, 4 days for those hospitalized but not in ICU, and 6 days for those admitted to the ICU (CMAJ 2010, [doi:10.1503/cmaj/.091884
In a multivariate analysis that accounted for age, sex, First Nations ethnicity, medical comorbidity, interval from symptom onset to antiviral initiation, urban vs. rural status, and income quintile, a treatment interval of greater than 2 days vs. 2 days or fewer increased the risk for ICU admission, compared with being treated in the community by more than eightfold, with an odds ratio of 8.24. First Nations ethnicity increased the ICU risk by an odds ratio of 6.52 and medical comorbidity by an odds ratio of 3.19.
Similarly, in an analysis comparing those admitted to the ICU with those admitted to the hospital but not the ICU, First Nations ethnicity increased the risk by more than threefold, with an odds ratio of 3.23. In this comparison, the odds ratio for treatment interval of more than 2 days vs. 2 or fewer was 2.44 but did not reach statistical significance, the investigators noted.
The finding of increased illness severity among First Nations people is consistent with historical records from the 1918 influenza pandemic and more recent studies. Some evidence suggests a genetic predisposition, Dr. Zarychanski and his associates commented.
The Public Health Agency of Canada provided salary support to help facilitate data collection and statistical consultation.