Informational briefing followed by LLM-guided motivational interview increases MMR vaccination intent among hesitant parents

*Background*: MMR vaccination rates are declining in the U.S. (1), contributing to recent measles outbreaks. If the decline continues, we risk the return of endemic measles, along with other vaccine- preventable diseases. Evidence-based, scalable approaches to support hesitant parents are needed.*Methods*: We ran a preregistered two-arm online randomized controlled trial (Prolific, August- September 2025) among MMR-hesitant U.S. parents. Participants with baseline intention ≤ 6 (1–7 scale) were randomized 1:1 to: (i) Experimental, a timer-gated four-panel MMR carousel followed by an LLM-guided motivational interview, or (ii) Active Control, a format-matched car seat safety carousel and chat. The primary outcome was post-intervention MMR intention adjusted for baseline using ANCOVA (HC3 robust SEs). Sensitivity included a rank-inverse-normal (RIN) transform and exclusion of two borderline low-quality cases. An exploratory analysis of the durability of the effect used delayed post-intent from a follow-up survey on a separate sample of participants.*Results*: Among N = 180 randomized participants, the adjusted arm effect (Treatment vs Control) was β ≈ 1.03 points (95% CI 0.72–1.34) on the 1–7 scale. In the follow-up sample (N = 66), the adjusted effect was positive and of similar magnitude (β ≈ 1.09; 95% CI 0.52–1.66). Sensitivity analyses yielded consistent inferences.*Conclusions*: A brief appointment-framed MMR content review plus LLM conversation produced a clear increase in vaccination intention relative to a structure-matched active control, with encouraging signs of short-term durability. These findings motivate pragmatic trials testing real-world vaccination uptake.