The aim is to examine how vaccine misinformation shapes perceived social norms and assess their mediating role in vaccination intentions during pandemics, an underexplored mechanism in misinformation’s influence on vaccine decisions. In a pre-registered online experiment, UK residents (n = 332) were randomly assigned to either a misinformation or control condition in a hypothetical pandemic scenario. I measured changes in vaccination intentions, first-order normative beliefs (perceptions of others’ vaccination intentions) and second-order normative beliefs (perceptions of others’ beliefs about vaccine safety) before and after exposure. Causal mediation analysis using inverse odds ratio weighting assessed the indirect effects of misinformation through changes in normative beliefs. The pre–post comparison revealed that vaccine intentions declined 2.5% more in the misinformation condition compared to the control group (p = 0.024, d = 0.24). In the misinformation group, average vaccine intentions dropped from 62.4% to 59.3%, while the control group showed minimal change from 60.8% to 60.2%. Changes in first-order normative beliefs mediated 39.52% of misinformation’s total effect on vaccination intentions. The findings reveal that vaccine misinformation operates through dual pathways: directly affecting individual beliefs while simultaneously distorting perceptions of social consensus about vaccination.