Effectiveness of neuraminidase inhibitors in preventing hospitalization during the H1N1 influenza pandemic in British Columbia, Canada

A(H1N1)pdm09 pandemic for treatment of high-risk patients and/or anyone with moderate-to-severe illness. We assessed antiviral effectiveness (AVE) against hospitalization in that context. Methods: A population-based cohort study was conducted using linked administrative data. The cohort included all individuals living in BCduring the study period (1 September to 31 December 2009) with a diagnostic code con-sistent with influenza or pandemic H1N1. The main study period pertained to the second-wave A(H1N1)pdm09 circulation (1 October to 31 December 2009), with sensitivity analyses around the more specific pandemic peak (18 October to 7 November). Exposure was defined by same-day NI prescription. The main outcome was all-cause hospitalization within 14 days of the outpatient influenza diagnosis. Cox proportional hazards models assessed AVE with 1:1 propensity-score matching and covariate adjustment. Results: After matching, there were 304/58061 NI-exposed and 345/58061 unexposed patients hospitalized during the main study period. The very young [,6 months (35.0; 95 % CI 16.7–73.4)], the old [65–79 years (13.7; 95 % CI 10.1–18.6)] and the very old [≥80 years (38.7; 95 % CI 26.6–56.5)] had the highest hospitalization rate per 1000 patients overall. Fully adjusted AVE against all-cause hospitalization during the main study period was 16 % (95 % CI 2%–28%), similar to the pandemic peak (15%; 95 % CI24%–30%). Conclusions: The use of NIs was associated with modest protection against hospitalization during the 2009 pandemic, but appeared underutilized in affected age groups with the highest hospitalization risk