Different commentators are often invited in the media in order to discuss medical and health-related advances, such as the deployment of new vaccines or prevention tests. How do the expertise and opinions of such intermediaries affect public trust towards them? Do these factors also influence the public beliefs and decisions regarding those medical advances? We presented to 1984 French participants new (fictitious) medical tools that have been recently made available and commented on by individuals of different degrees of expertise and having distinct opinions. The results indicate that both factors significantly influenced participants' trust in the commentator's message. The commentator's opinion also affected (although to a smaller extent) the public attitude towards the tool and their willingness to use it. Crucially, participants recognized that commentators' assertiveness in expressing their opinions might unduly bias their beliefs. The study highlights the importance of considering both para-verbal and contextual cues in health communication, advocating for strategies to mitigate (or better use) their influence on public trust, beliefs, and decision-making.

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.