Sometimes anti-vaxxers will point to the ostensibly high rate of serious adverse events exhibited by study participants in HPV vaccine trials. For example, in the study 'A 9-Valent HPV Vaccine against Infection and Intraepithelial Neoplasia in Women' published in The New England Journal of Medicine in 2015, in which the 9-Valent HPV vaccine was compared to the Quadrivalent version, the rate of serious adverse events was 3.3% and 2.6% respectively. This objection can be dismissed, as the study authors found that 0.0% of those serious adverse events were vaccine related.

Anti-vaxxers also sometimes object to studies like these on the basis that no placebo arm was included, and thus the true safety profile of the vaccine cannot be ascertained, but this can be flatly rejected because, as the study authors point out, '[s]ince HPV vaccination is widely recommended and has been shown to prevent HPV disease related to oncogenic HPV types, the use of a placebo was not considered to be acceptable for ethical reasons.'