The global rise of anti-vaccination movements has led to serious consequences for the public health such as the recent measles outbreak in the United States. The increased promotion of misleading information on vaccinations by social media influencers as well as by media outlets seems to have resulted into a more negative view on vaccinations. The popularity of these social media influencers and the good reputation and authority of the media outlets could have played a substantial role in these developments. The following experimental study will try to explore whether popularity or authority cues can have an impact on the evaluation of the credibility of these two types of sources via an online survey. Furthermore, it aims to examine whether said heuristic cues and resulting conformity effects or the source credibility in general could have an impact on the opinion of recipients on a potential autism-vaccination link. Results have shown that especially authority cues seem to have a positive impact on source credibility evaluations. Furthermore, a high general trust in the media positively influences assessments of source credibility of well-established news outlets. Popularity cues as well as authority cues seem to have a positive effect on the recipients´ opinion. However, authority cues and the resulting effects of informational conformity seem to lead to greater attitude changes. Particularly people with a high need for conformity exhibit considerable attitude changes when exposed to the well-established news outlet as a source. Also, a high perceived source credibility is positively correlated with a desired attitude change. Especially the high perceived credibility of the social media influencer resulted in significant attitude changes. The results underline the importance and positive impact that a perceived authority can have on source credibility assessments and on recipients’ opinions. This highlights the great necessity for self-proclaimed experts on social media platforms and particularly for well-established news outlets to increase their effort to thoroughly and accurately research health-related topics.