Influenza Vaccine Effectiveness among Cancer Patients: A Population-Based Study Using Health Administrative and Laboratory Testing Data from Ontario, Canada

Background: Seasonal influenza vaccination is recommended for cancer patients despite concerns that disease- or treatment-associated immunosuppression may decrease vaccine effectiveness (VE). The objective of this study was to evaluate VE against laboratory-confirmed influenza among cancer patients. Methods: We conducted an observational test-negative design study of previously diagnosed cancer patients aged ≥18 years who were tested for influenza during the 2010-11 to 2015-16 influenza seasons in Ontario, Canada. We linked individual-level cancer registry, respiratory virus testing, and health administrative data. Vaccination status was determined from physician and pharmacist billing claims. We used multivariable logistic regression to estimate VE, adjusting for age, sex, rurality, neighborhood income, cancer characteristics, chemotherapy exposure, comorbidities, previous healthcare use, influenza season, and calendar time of testing. Results: We identified 24,668 cancer patients who underwent influenza testing, with 3991 (16%) testing positive and 10,929 (44%) vaccinated. Mean age was 70 years, 52% were male, mean time since cancer diagnosis was 6 years, 79% had a solid tumor malignancy, and 24% were receiving active chemotherapy. The overall adjusted VE (aVE) against laboratory-confirmed influenza was 21% (95%CI, 15%, 27%). The aVE among patients with a solid tumour malignancy was 25% (95%CI, 19%, 31%) as compared with –2% (95%CI, –20%, 14%) amongst patients with a hematologic malignancy (p-value for interaction <0.01). The aVE among patients receiving active chemotherapy was 9% (95%CI, –7%, 23%), compared with 23% (95%CI, 17%, 29%) among patients not on active chemotherapy (p-value for interaction=0.13). Conclusions: Our results support recommendations for influenza vaccination among cancer patients and survivors. Influenza vaccination appears to be less effective among those undergoing active chemotherapy and we observed uncertain effectiveness among hematologic cancer patients. Strategies to improve VE and influenza vaccine uptake among cancer patients and their families are warranted. Legal entity responsible for the study: Phillip Blanchette and Jeff Kwong. Funding: The study was funded by The Canadian Institute for Health Research and Ministry of Health and Long-Term Care in Ontario which provide funding support for Public Health Ontario (PHO) and The Institute of Clinical Evaluative Science (ICES). Disclosure: K.I. Pritchard: Consultancy: Pfizer, Roche, Amgen, Novartis, Eisai, Genomic Health. A. McGeer: Research funding: Sanofi Pasteur, GlaxoSmithKline. All other authors have declared no conflicts of interest.