The safety of seasonal influenza vaccination among patients prescribed immune checkpoint inhibitors: A self-controlled case series study using administrative data

6606 Background: Immune checkpoint inhibitor (ICI) therapy for cancer patients carries a risk of severe immune-related adverse events (IRAEs). Since vaccines are immunomodulatory, administering seasonal influenza vaccinations to individuals on ICI therapy may exacerbate this risk. However, these vaccines provide substantial benefit to this at-risk population. As severe IRAEs are rare, previous vaccine safety studies in this population have been underpowered and have shown conflicting results. Methods: We used health administration data on adult Ontarians who initiated ICI therapy and received an influenza vaccine between January 1, 2012, and December 31, 2019. We conducted a self-controlled case series study, with a pre-vaccine control period (12-weeks post-ICI initiation to 14 days before vaccine), risk period (42 days post-vaccine), and a post-vaccine control period (until observation end). Each individual contributed a maximum of 2 years of person-time. Emergency department (ED) visit(s) and/or hospitalization for any cause was used as a surrogate measure of severe IRAE frequency. We fitted a fixed effect Poisson regression model incorporating seasonality and time to estimate incidence. Results: We identified 1133 patients who received an influenza vaccine on ICI therapy. The majority were aged ≥66 years (72.7%), male (62.8%), and had lung cancer (53.9%). A quarter (25.9%) experienced an ED visit and/or hospitalization during the observation period. The rate of ED visits and/or hospitalization per person-days in the risk and control periods were similar, with an incidence rate ratio of 1.04 (95% CI: 0.75, 1.45). Subgroup and sensitivity analyses revealed similar findings. Conclusions: Receipt of a seasonal influenza vaccine was not associated with an increased incidence of ED visit or hospitalization among adults on ICI therapy. The results from this analysis suggest no significant safety concern with administering influenza vaccines to this patient population.