Background <p>Among health care workers, long-term care center (LTCC) staff are among the most hesitant to obtain COVID-19 vaccines and boosters. Our objective was to compare the effectiveness of enhanced usual care (EUC) to codesigned materials (Codesign) in increasing COVID-19 booster vaccination rates and willingness to promote the booster among long term care center (LTCC) staff.</p> Methods <p>We conducted a two-arm cluster-randomized trial in 40 LTCCs in Georgia (<i>n</i> = 21) and Washington (<i>n</i> = 19) from August 2021 to December 2023. EUC LTCCs distributed standard government COVID-19 booster promotion materials. LTCCs in the Codesign arm allowed staff to participate in virtual codesign teams. Over 10 weekly meetings, the teams developed tailored COVID-19 booster promotion materials. Materials were distributed to both arms in January 2023. Primary outcomes were LTCC-level COVID-19 booster vaccination and a Net Promoter Score (NPS) measuring booster promotion to coworkers. Secondary outcomes included staff-level vaccine hesitancy and confidence scores and booster promotion to friends and family.</p> Results <p>We did not detect a significant difference between the EUC and Codesign arms in COVID-19 booster rates or primary NPS measure. Similarly, secondary and exploratory outcomes were not significantly different between either arm.</p> Conclusions <p>The null findings of this trial are consistent with the challenges of other clinical trials of COVID-19 vaccine-promotion interventions in improving COVID-19 vaccine uptake among LTCC staff. Lessons for future research include identifying better ways to: (1) engage leaders and staff, (2) design more effective targeting messages, and (3) develop ways to target and customize dissemination.</p> Trial registration <p>This study is registered at <a href="http://www.clinicaltrials.gov.Clinicaltrials.gov">www.clinicaltrials.gov.Clinicaltrials.gov</a> #NCT05449418. Date of registration: 6/22/2022.</p>