Objectives: To assess the effectiveness of plain language compared with standard language versions of COVID-19 recommendations specific to child health.Study Design and Setting: Pragmatic, allocation-concealed, blinded, superiority randomized controlled trial with nested qualitative component. Trial was conducted online, internationally. Parents or legal guardians (>18 years) of a child (!18 years) were eligible. Par-ticipants were randomized to receive a plain language recommendation (PLR) or standard (SLV) verison of a COVID-19 recommendation specific to child health. Primary outcome was understanding. Secondary outcomes included: preference, accessibility, usability, satisfaction, and intended behavior. Interviews explored perceptions and preferences for each format. Results: Between July and August 2022, 295 parents were randomized; 241 (81.7%) completed the study (intervention n = 121, con-trol n = 120). Mean understanding scores were significantly different between groups (PLR 3.96 [standard deviation (SD) 2.02], SLV 3.33 [SD 1.88], P = 0.014). Overall participants preferred the PLR version: mean rating 5.05/7.00 (95% CI 4.81, 5.29). Interviews (n = 12 parents) highlighted their preference for the PLR and provided insight on elements to enhance future knowledge mobilization of health recommendations.Conclusion: Compared to SLVs, parents preferred PLRs and better understood the recommendation. Guideline developers should strive to use plain language to increase understanding, uptake, and implementation of evidence by the public.& COPY; 2023 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).