Emphasizing herd immunity in vaccine advocacy: A systematic review and three-level meta-analysis

Emphasizing herd immunity may increase or decrease vaccination motivation through prosocial or selfish motivations or have no effect at all. Despite extensive research, a quantitative synthesis of evidence is lacking. We conducted a systematic review and three- level meta-analysis regarding the impact of emphasizing herd immunity on vaccination motivation, synthesizing over a decade of experimental research. The synthesis utilized forward and backward citation searches, examining literature up to May 2024 across databases such as CINAHL, Google Scholar, PubMed, PsycINFO, Scopus, The Cochrane Library, and Web of Science. No language or formal exclusion criteria were applied. Studies were excluded if non-experimental or if they did not assess vaccination motivation. Risk of bias was evaluated using JBI checklists. The synthesis is preregistered on PROSPERO (CRD42024540536). The search identified 5,863 records; 3,935 were screened, and 281 full- text records assessed. Ultimately, 41 eligible studies (65 effect sizes) with 100,678 participants (50,826 intervention, 49,852 control) were included. The pooled effect size (Hedges g = 0.13, 95% CI [0.08, 0.18], p < 0.001) indicated a small positive effect of emphasizing herd immunity on vaccination motivation, with large between-study heterogeneity (I2 = 91.2%). Experiential communication methods (e.g., VR, simulations) showed stronger effects, with no evidence of publication bias and sufficient study quality. In conclusion, emphasizing herd immunityenhances vaccination motivation, particularly with experiential communication methods.