In a pandemic era, it is necessary to equip individuals with the ability to make informed decisions about health issues, especially in relation to viruses and vaccines. In order to achieve this goal, science educators need to explore students' decisions and reasoning about vaccination. The aim of the study reported in the paper, therefore, is to explore eighth graders' reasoning about vaccination throughout a 4-week implementation of small group and plenary discussion of false claims about vaccinations. The implementation consisted of a five-phase procedure including teacher presentation of false claims and related evidence texts about vaccination, small group discussions, a plenary discussion, and finally, an introduction to valid scientific content about vaccination. The explanations of the representatives from each group during the plenary discussion were video-taped and analyzed by the researchers independently to examine student decisions on each claim. Another data source of this study included student interviews in which the researchers videotaped and analyzed eight interviewees' responses. The findings revealed that including well-informed students in small group and plenary discussions may have a positive impact on other students' reasoning. This result indicated the benefit of encouraging students to provide evidence about vaccines during small group and plenary discussions in terms of their reasoning. The implications of this study suggest the necessity of emphasizing on scientific knowledge as well as argumentation for further investigations of students' reasoning on vaccination.