Degrees of disagreement and reliability of information sources in pro- and anti-vaccination comments on Facebook

During the COVID-19 vaccination campaign, the Spanish Ministry of Health shared informative posts on platforms like Facebook, sparking heated debates. This paper utilizes a custom corpus of Facebook comments with evidential elements to explore the disagreement and confrontation in online comments from pro- and anti-vaccine advocates. The study also analyses the types of evidence employed by posters to support their positions, revealing potential hierarchies of information sources in terms of reliability and validity. Findings indicate that anti-vaccine advocates (i) engage in stronger disagreement than vaccine supporters; (ii) use disqualification and hostile speech acts slightly more; and (iii) employ more impolite strategies. Moreover, the study shows differences between these two user groups with regard to the sources of the information they chose to use: anti-vaccine posters employ a higher percentage of more objective types of evidence, while pro-vaccine posters resort to evidence based on more subjective, and personal sources.