Background: The Canadian Nosocomial Infection Surveillance Program (CNISP) is a sentinel network of teaching hospitals in Canada. After the emergence of the pandemic H1N1 influenza virus in April, 2009, CNISP enhanced surveillance for laboratory-confirmed influenza in hospitalized adults.
Objective: The objectives are to describe the outcomes and epidemiology of laboratory-confirmed influenza in adults in participating CNISP hospitals.
Methods: Cases were hospitalized patients more than 16 years old with laboratory confirmation of influenza by any test method. Data on cases were collected prospectively by hospital infection prevention and control professionals. Variables of interest included intensive care unit (ICU) admissions, intubation or mechanical ventilation due to influenza, and death attributed to influenza assessed 30 days following initial diagnosis.
Results: Between June 1 and December 31, 2009, 23 hospitals submitted data on 503 cases of laboratory-confirmed influenza. Pandemic H1N1 influenza accounted for 346 (69%) cases; 154 (31%) cases were non-subtyped Influenza A; and 3 (0.6%) cases were Influenza B.
Two waves of influenza were observed. The first peaked mid-June and the second peaked the first week of November (Figure 1). Patients in the first wave were more likely to have at least one underlying medical condition (OR=2.6, p = 0.03); were more likely to be admitted to the ICU due to influenza-associated complications (OR=1. 9, p=0.02); and were more likely to require intubation (OR=1.8, p=0.04). Women in the first wave were more likely to be pregnant (OR=4.8, p<0.001).There was no difference in mortality rates between the two waves (p=0.4).
A total of 24 (5%) patients died: influenza was the primary cause of death in 11 (46%) patients, contributed to death in 11 (46%) patients and was unrelated to death in 1 (4%) patient. The relationship between influenza and death was not able to be determined for one case. Mortality attributable to influenza was 4.4%. Females were more likely to die of H1N1 than males (OR=3.5 p=0.02). None of the 24 pregnant women died. There was no significant difference in age (p=0.54), aboriginal status (p=0.94) or presence of co-morbidities (p=0.1) between those that died and those that survived.
Conclusions: Canada experienced two waves of the pandemic H1N1 (2009) influenza virus outside of the typical North American influenza season. Mortality attributed to influenza as a primary or contributing cause of death was 4.4%. CNISP enhanced surveillance allowed for timely description of outcomes and epidemiology of a novel strain of influenza.