Effectiveness of the Cell-Derived Inactivated Quadrivalent Influenza Vaccine in Individuals at High Risk of Influenza Complications in the 2018-2019 United States Influenza Season

Background. Higher rates of influenza-related morbidity and mortality occur in individuals with underlying medical conditions. To improve vaccine effectiveness, cell-based technology for influenza vaccine manufacturing has been developed. Cell-derived inactivated quadrivalent influenza vaccines (cIIV4) may improve protection in seasons in which egg-propagated influenza viruses undergo mutations that affect antigenicity. This study aimed to estimate the relative vaccine effectiveness (rVE) of cIIV4 versus egg-derived inactivated quadrivalent influenza vaccines (eIIV4) in preventing influenza-related medical encounters in individuals with underlying medical conditions putting them at high risk of influenza complications during the 2018-2019 US influenza season. Methods. An integrated dataset, linking primary care electronic medical records with claims data, was used to conduct a retrospective cohort study among individuals aged >= 4 years, with >= 1 health condition, vaccinated with cIIV4 or eIIV4 during the 20182019 season. Adjusted odds ratios (ORs) were derived using a doubly robust inverse probability of treatment-weighting (IPTW) model, adjusting for age, sex, race, ethnicity, geographic region, vaccination week, and health status. Relative vaccine effectiveness was estimated by (1 - OR) x 100 and presented with 95% confidence intervals (CIs). Results. The study cohort included 471 301 cIIV4 and 1 641 915 eIIV4 recipients. Compared with eIIV4, cIIV4 prevented significantly more influenza-related medical encounters among individuals with >= 1 health condition (rVE, 13.4% [95% CI, 11.4%-15.4%]), chronic pulmonary disease (rVE, 18.7% [95% CI, 16.0%-21.3%]), and rheumatic disease (rVE, 11.8% [95% CI, 3.6%-19.3%]). Conclusions. Our findings support the use of cIIV4 in individuals >= 4 years of age at high risk of influenza complications and provide further evidence supporting improved effectiveness of cIIV4 compared with eIIV4.