In this article, we conducted a comprehensive survey of the publications from "Médicos por la Verdad" (Doctors for the Truth), one of the main anti-vaccine groups in Latin America, on the social media platform Twitter during the COVID-19 pandemic. We categorized their types of reasoning and the content of their messages, and we showed that the existing proposals for the analysis of anti-scientific discourses cannot be applied to this particular case. Consequently, we propose a new categorization and its focused application using tools available for science communicators on one hand, and the non-specialized public on the other.