Effectiveness of H1N1 Influenza Vaccines in Manitoba, Canada

Excellent immune responses following 1 or 2 doses of the GSK monovalent AS03-adjuvanted pandemic H1N1 (pH1N1) vaccines (e.g., Arepanrix®) have been documented in several trials. Observational studies have found that these vaccines were effective in preventing laboratory confirmed influenza infections. However, it remains unclear whether vaccination during the pandemic was associated with reductions in more clinically meaningful outcomes, such as hospitalizations, severe illness, complications, and death. The investigators propose to evaluate the effectiveness of the GSK adjuvanted and non-adjuvanted pH1N1 vaccines used in Manitoba in preventing hospitalization and severe illness (defined as illness necessitating admission to intensive care or associated with major complications) due to influenza or pneumonia by means of a case-control study using data from Manitoba Health (MH) administrative databases and the database of the Cadham Provincial Laboratory. The primary outcome will be hospitalization with laboratory-confirmed influenza or pneumonia. A secondary outcome will be hospitalization with influenza or pneumonia. The investigators will also assess the effectiveness of the above vaccines for different age groups and among high-risk populations, e.g., immunocompromised individuals.