Background: In 2017, a new adjuvanted recombinant zoster vaccine (RZV) was recommended for adults ≥50 years as two-dose series 2–6 months apart. We evaluated two-dose RZV completion and factors associated with completion. Methods: The study included Kaiser Permanente Southern California members ≥50 years who received an RZV dose during April-November 2018 and had continuous membership 12 months before to 9 months after the first RZV dose. Completion was defined as receipt of the second dose ≥4 weeks to 9 months after the first dose (allowing 3-month grace period). Characteristics including age, sex, race/ethnicity, Medicaid status, neighborhood level income and education, comorbidities, history of zoster, health care utilization before and after the first dose, receipt of influenza vaccine, vaccination month (supply shortage proxy), concomitant vaccine, medical center, and medically attended reactions, pain, or gout after the first dose were compared between completers and non-completers. Adjusted odds ratios and 95% confidence intervals for factors associated with completion were estimated by multivariable logistic regression. Results: Among 31,120 first dose recipients, 67.2% completed the series within 9 months. In adjusted analyses, higher completion was associated with White compared with Black or Hispanic race/ethnicity, higher neighborhood income and education, no chronic pulmonary disease, diabetes, or dementia, more outpatient visits and fewer emergency department visits before or after the first dose, no hospitalizations after the first dose, receipt of influenza vaccine, receipt of the first dose in June-November rather than April-May 2018, and no concomitant vaccine with the first dose. Systemic reactions or pain after the first dose was not associated with completion. Conclusion: Completion of RZV series appears suboptimal in the early phase of implementation. Despite similar accessibility in a health care system, completion varied by race/ethnicity, socioeconomic status, health status, and care seeking behavior, suggesting areas to target for improvement.