Public acceptance of vaccination and Genetically Modified (GM) food is low and opposition is stiff. During two science festivals in France, we discussed in small groups the scientific evidence and current consensus on the benefits of vaccination and GM food safety. Our interventions reinforced people's positive opinions on vaccination and produced a drastic positive shift of GM food opinions. Despite the controversial nature of the topics discussed, there were very few cases of backfire effects among the 175 participants who volunteered. These results should encourage scientists to engage more often with the public during science festivals, even on heated topics.