Relative Effectiveness of the Cell-Based Quadrivalent Influenza Vaccine in Preventing Cardiorespiratory Hospitalizations in Adults Aged 18–64 Years During the 2019–2020 US Influenza Season

BACKGROUND: The mammalian cell-based quadrivalent inactivated influenza vaccine (IIV4c) has advantages over egg-based quadrivalent inactivated influenza vaccine (IIV4e), as production using cell-derived candidate viruses eliminates the opportunity for egg adaptation. This study estimated the relative vaccine effectiveness (rVE) of IIV4c versus IIV4e in preventing cardiorespiratory hospitalizations during the 2019–2020 US influenza season. METHODS: We conducted a retrospective cohort study using electronic medical records linked to claims data of US individuals aged 18–64 years. We assessed rVE against cardiorespiratory hospitalizations and against subcategories of this outcome, including influenza, pneumonia, myocardial infarction and ischemic stroke, and respiratory hospitalizations. We used a doubly robust inverse probability of treatment weighting and logistic regression model to obtain odds ratios (ORs; odds of outcome among IIV4c recipients/odds of outcome among IIV4e recipients) adjusted for age, sex, race, ethnicity, geographic region, vaccination week, health status, frailty, and healthcare resource utilization. rVE was calculated as 100(1 − OR(adjusted)). RESULTS: In total, 1 491 097 individuals (25.2%) received IIV4c, and 4 414 758 (74.8%) received IIV4e. IIV4c was associated with lower odds of cardiorespiratory (rVE, 2.5% [95% confidence interval, 0.9%–4.1%]), respiratory (3.7% [1.5%–5.8%]), and influenza (9.3% [0.4%–17.3%]) hospitalizations among adults 18–64 years of age. No difference was observed for the other outcomes. CONCLUSIONS: This real-world study conducted for the 2019–2020 season demonstrated that vaccination with IIV4c was associated with fewer cardiorespiratory, respiratory, and influenza hospitalizations compared with IIV4e.