Background: Quality data can be used to monitor immunization program performance and targets, reveal reasons for poor program performance, and provide evidence for decision-making at the frontlines. Challenges hampering immunization data quality include weak human resource capacity, weak monitoring supervision, and nonexistent or ineffectual feedback on data quality and use, especially at subnational levels. Interventions bordering on upskilling of available human resource capacity and supportive supervision can improve immunization system performance. We implemented a multicomponent intervention (training, supportive supervision, and peer mentoring) among health workers in Lagos to assess the reach (proportion of immunization staff who were reached), adoption (proportion of immunization staff who participated and completed the intervention), and effectiveness (data accuracy, completeness, timeliness, and quality index (QI) scores) of a multicomponent intervention on immunization data quality in Lagos State. Methods: This was a quasi-experimental study that employed a mixed-methods (quantitative and qualitative) approach to assess the reach, effectiveness, and adoption of the interventions. A pre- and posttest design was employed for the quantitative aspect of the study. The reach and adoption were assessed using a process indicator questionnaire, while the effectiveness of the intervention was assessed using the Data Quality Assessment (DQS) tool to obtain the accuracy ratio (AR) and detect any significant variation in the data recorded in the health facility (HF) registers, tally sheets, and District Health Information System version 2 for the pre- and post-intervention period. A total of 32 key informant interviews (KIIs) (eight KIIs in each local government area (LGA)) were conducted with respondents being key workers who were purposively selected. Data from both methods were triangulated and used to support the findings. Results: The study had a reach of 91% and a participation and completion (adoption) rate of 100%. At the HF level, the accuracy of data submitted (tally sheet vs immunization register) showed the most significant improvement from pre-intervention (11.8%) to post-intervention (100%) in one LGA, Second rural Local Government Area (R-IKD 2). Facilities in two LGAs, such as First urban Local Government Area U-IKJ 1 (64.7% to 94.1%) and Second urban Local Government Area U-LM 2 (88.2 to 100%) LGAs, showed the most significant improvement in timeliness and completeness of submitted reports, respectively. For QI scores, at the LGA level, the most significant improvement was recorded in the evidence of using the data domain (50% to 100%) for one LGA, U-IKJ 1. Conclusion: Challenges that hamper immunization data quality such as weak human resource capacity, weak monitoring, and supervision can be improved with a multicomponent intervention targeted at upskilling available human resource capacity and supportive supervision to strengthen immunization system performance.
