Misinformation can decrease public confidence in vaccine safety and efficacy, and reduce vaccination intent. One strategy for countering these negative impacts comes from inoculation theory. Similar to biological vaccination, inoculation theory finds that exposure to a weakened form of misinformation can develop cognitive immunity, reducing the likelihood of being misled. Online games offer an interactive, technology-driven, and scalable solution using an active form of inoculation that engages and incentivizes players to build resilience against misinformation. The critical thinking game Cranky Uncle Vaccine synthesizes research into inoculation theory, critical thinking, humor in science communication, and serious games. The game content was iterated through a series of co-design workshops in Uganda, Kenya, and Rwanda. Workshop participants offered feedback on character design, gameplay experience, and the game’s content, helping to make the game more culturally relevant and avoid unintended consequences in East African countries. This presentation will present results from a pilot study conducted in Uganda, assessing the game’s performance in improving vaccine attitudes and the ability to discern facts from misinformation.