What Is the State-of-the-Art in Clinical Trials on Vaccine Hesitancy 2015–2020?

Background: Vaccine hesitancy is related to a delay in acceptance or refusal of vaccination. Aim: to perform a systematic review of clinical trials on vaccine hesitancy (2015–2020). Methods: a systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses criteria (PRISMA). Five databases were screened—PubMed, Cochrane Library, DOAJ, SciELO and b-on—which comprise multiple resources. Keywords: “Vaccine hesitancy” and (“randomized controlled trial” or “clinical trial”). Inclusion criteria: trials about “vaccine hesitancy” enrolling patients and/or health professionals (2015–2020). Exclusion criteria: studies about other topics, repeated and qualitative studies, reviews and papers written in languages other than English, Portuguese, French or Spanish. Results: a total of 35 trials out of 90 were selected (19 PubMed, 14 Cochrane Library, 0 DOAJ, 0 SciELO and 2 b-on). Selected trials were classified into five topics: children/pediatric (n = 5); online or electronic information (n = 5); vaccination against a specific disease (n = 15) (e.g., influenza or COVID-2019); miscellaneous (n = 4); and educational strategies (n = 6). Conclusion: the provision of online or electronic information (e.g., through virtual reality, social websites of experts, or apps), communication-based interventions and training of health professionals, residents or subjects seemed to improve vaccine hesitancy.