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Background Data are limited on influenza vaccine effectiveness (VE) in the prevention of influenza-related hospitalizations in older adults and those with underlying high-risk comorbidities. Methods We conducted a prospective, test-negative, case-control study at 2 US hospitals from October 2018–March 2020 among adults aged ≥50 years hospitalized with acute respiratory illnesses (ARIs) and adults ≥18 years admitted with congestive heart failure (CHF) or chronic obstructive pulmonary disease (COPD) exacerbations. Adults were eligible if they resided in 1 of 8 counties in metropolitan Atlanta, Georgia. Nasopharyngeal and oropharyngeal swabs were tested using BioFire FilmArray (bioMérieux, Inc.) respiratory panel, and standard-of-care molecular results were included when available. Influenza vaccination history was determined from the Georgia vaccine registry and medical records. We used multivariable logistic regression to control for potential confounders and to determine 95% confidence intervals (CIs). Results Among 3090 eligible adults, 1562 (50.6%) were enrolled. Of the 1515 with influenza vaccination history available, 701 (46.2%) had received vaccination during that season. Influenza was identified in 37 (5.3%) vaccinated versus 78 (9.6%) unvaccinated participants. After adjustment for age, race/ethnicity, immunosuppression, month, and season, pooled VE for any influenza-related hospitalization in the eligible study population was 63.1% (95% CI, 43.8–75.8%). Adjusted VE against influenza-related hospitalization for ARI in adults ≥50 years was 55.9% (29.9–72.3%) and adjusted VE against influenza-related CHF/COPD exacerbation in adults ≥18 years was 80.3% (36.3–93.9%). Conclusions Influenza vaccination was effective in preventing influenza-related hospitalizations in adults aged ≥50 years and those with CHF/COPD exacerbations during the 2018–2020 seasons. [ABSTRACT FROM AUTHOR]