To assess COVID-19 and influenza vaccination rates across Indiana's 92 counties and identify county-level factors associated with vaccination. We analyzed county-level data on adult COVID-19 vaccination from the Indiana vaccine registry and 2021 adult influenza vaccination from the Centers for Disease Control and Prevention. We used multiple linear regression (MLR) to determine county-level predictors of vaccinations. COVID-19 vaccination ranged from 31.2% to 87.6% (mean = 58.0%); influenza vaccination ranged from 33.7% to 53.1% (mean = 42.9%). In MLR, COVID-19 vaccination was significantly associated with primary care providers per capita (b = 0.04; 95% confidence interval [CI] = 0.02, 0.05), median household income (b = 0.23; 95% CI = 0.12, 0.34), percentage Medicare enrollees with a mammogram (b = 0.29; 95% CI = 0.08, 0.51), percentage uninsured (b = -1.22; 95% CI = -1.57, -0.87), percentage African American (b = 0.31; 95% CI = 0.19, 0.42), percentage female (b = -0.97; 95% CI = -1.79, ‒0.15), and percentage who smoke (b = -0.75; 95% CI = -1.26, -0.23). Influenza vaccination was significantly associated with percentage uninsured (b = 0.71; 95% CI = 0.22, 1.21), percentage African American (b = -0.07; 95% CI = -0.13, -0.01), percentage Hispanic (b = -0.28; 95% CI = -0.40, -0.17), percentage who smoke (b = -0.85; 95% CI = -1.06, -0.64), and percentage who completed high school (b = 0.54; 95% CI = 0.21, 0.87). The MLR models explained 86.7% (COVID-19) and 70.2% (influenza) of the variance. Factors associated with COVID-19 and influenza vaccinations varied. Variables reflecting access to care (e.g., insurance) and higher risk of severe disease (e.g., smoking) are notable. Programs to improve access and target high-risk populations may improve vaccination rates. ( 2024;114(4):415-423. https://doi.org/10.2105/AJPH.2023.307553).