Methods: The Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN) conducted active surveillance for laboratory-confirmed influenza in adults (≥16 years) hospitalized across Canada during the 2011-2014 influenza seasons. Eligible patients who were admitted to hospital with any acute respiratory illness or symptom had a nasopharyngeal swab collected and tested for influenza virus using reverse transcriptase polymerase chain reaction (PCR). Demographic/clinical information, as well as in-hospital outcomes were collected. Frailty Index scores were also recorded at baseline and 30-days after discharge, when possible, in patients ≥65 years. Patients with influenza A and B were compared using descriptive statistics; discrete outcomes were compared using Chi-squared (χ2) tests; continuous outcomes were compared using student’s t-tests.
Results: Overall, there were 3484 influenza A cases and 1375 influenza B cases enrolled in the SOS Network from 2011-2014. Mean age was significantly different between influenza A and influenza B cases (mean age of influenza A: 65.8, mean age of influenza B: 71.2, p<0.01). A significantly larger proportion of influenza B patients were admitted from long-term care (A: 5.5%, B: 12.1%, p<0.01). There was no significant difference with respect to length of hospitalization (influenza A: 11.1 days, influenza B: 10.27 days, p=0.07) or mortality (A: 9.01%, B: 9.45%, p=0.63) between influenza A and B. Patients with influenza B were significantly more frail prior to the onset of illness (A: 0.21, B: 0.22, p<0.01).
Conclusion: Current attitudes consider influenza A to be the more significant virus in terms of morbidity and mortality in adults. However, influenza B is responsible for similar duration of hospitalization and similar mortality rates. In addition, influenza B predominantly affected the frail elderly and thus optimizing influenza B protection is important in this population.
M. K. Andrew, GSK: Grant Investigator , Research grant . Pfizer: Grant Investigator , Research grant . Sanofi Pasteur: Grant Investigator , Research grant .
T. F. Hatchette, GSK: Grant Investigator , Research grant . Pfizer: Grant Investigator , Research grant . Abbvie: Consultant , Speaker honorarium .
A. Ambrose, None
L. Ye, None
M. Elsherif, None
S. A. McNeil, GSK: Grant Investigator , Research grant . Pfizer: Grant Investigator , Research grant . Merck: Collaborator and Consultant , Contract clinical trials and Speaker honorarium . Novartis: Collaborator , Contract clinical trials . Sanofi Pasteur: Collaborator , Contract clinical trials .