question: ObjectiveTo evaluate the effect of presenting positively attribute-framed side effect information on COVID-19 booster vaccine intention relative to standard negatively-framed wording and a no-intervention control. Design setting and participantsA representative sample of Australian adults (N=1,204) were randomised to one of six conditions within a factorial design: Framing (Positive; Negative; Control) * Vaccine (Familiar (Pfizer); Unfamiliar (Moderna)). InterventionNegative Framing involved presenting the likelihood of experiencing side effects (e.g., heart inflammation is very rare, 1 in every 80,000 will be affected), whereas Positive Framing involved presenting the same information but as the likelihood of not experiencing side effects (e.g., 79,999 in every 80,000 will not be affected). Primary OutcomeBooster vaccine intention measured pre- and post-intervention. ResultsPositive Framing (M=75.7, SE=0.9, 95% CI[73.9, 77.4]) increased vaccine intention relative to Negative Framing (M=70.7, SE=0.9, 95% CI[68.9, 72.4]) overall (F(1, 1192)=4.68, p=.031,{eta} p2=.004). Framing interacted with Vaccine and Baseline Intention (F(2, 1192)=6.18, p=.002,{eta} p2=.01). Positive Framing was superior, or at least equal, to Negative Framing and Control at increasing Booster Intention, irrespective of the participants pre-intervention level of intent. Side effect worry and perceived severity mediated the effect of Positive vs. Negative Framing across vaccines. ConclusionPositive framing of side effect information appears superior for increasing vaccine intent relative to the standard negative wording currently used. Pre-registrationSee: aspredicted.org/LDX_2ZL