Abstract
BACKGROUND: The cell-propagated inactivated quadrivalent influenza vaccine (ccIIV4) may offer improved protection in seasons where egg-derived influenza viruses undergo mutations that affect antigenicity. This study estimated the relative vaccine effectiveness (rVE) of ccIIV4 versus egg-derived inactivated quadrivalent influenza vaccine (eIIV4) in preventing influenza-related medical encounters in the 2018-2019 US season. METHODS: A dataset linking primary care electronic medical records with medical claims data was used to conduct a retrospective cohort study among individuals ≥ 4 years old vaccinated with ccIIV4 or eIIV4 during the 2018-2019 season. Adjusted odds ratios (ORs) were derived from a doubly robust inverse probability of treatment-weighted approach adjusting for age, sex, race, ethnicity, geographic region, vaccination week, and health status. rVE was estimated by (1 - OR) × 100 and presented with 95% confidence intervals (CI). RESULTS: Following the application of inclusion/exclusion criteria, the study cohort included 2 125 430 ccIIV4 and 8 000 903 eIIV4 recipients. Adjusted analyses demonstrated a greater reduction in influenza-related medical encounters with ccIIV4 versus eIIV4, with the following rVE: overall, 7.6% (95% CI, 6.5-8.6); age 4-17 years, 3.9% (95% CI, .9-7.0); 18-64 years, 6.5% (95% CI, 5.2-7.9); 18-49 years, 7.5% (95% CI, 5.7-9.3); 50-64 years, 5.6% (95% CI, 3.6-7.6); and ≥65 years, -2.2% (95% CI, -5.4 to .9). CONCLUSIONS: Adjusted analyses demonstrated statistically significantly greater reduction in influenza-related medical encounters in individuals vaccinated with ccIIV4 versus eIIV4 in the 2018-2019 US influenza season. These results support ccIIV4 as a potentially more effective public health measure against influenza than an egg-based equivalent.