Excluding numeric side-effect information produces lower vaccine intentions.

Encouraging vaccine uptake is important to reducing the impact of infectious disease. However, negative attitudes and vaccine hesitancy, due in part to worry about side effects, are obstacles to achieving high vaccination rates. Provided vaccine information sheets typically include a list of side effects without numeric information about their likelihoods, but providing such numbers may yield benefits. We investigated the effect of providing numeric information about side-effect likelihood (e.g., "1%") and verbal labels (e.g., "uncommon") on intentions to get a hypothetical vaccine, reasons for the vaccination decision, and risk overestimation. In a diverse, online, convenience sample (N = 595), providing numeric information increased vaccine intentions-70% of those who received numeric information were predicted to be moderately or extremely likely to vaccinate compared to only 54% of those who did not receive numeric information (p<.001), controlling for age, gender, race, education, and political ideology. Participants receiving numeric information also were less likely to overestimate side-effect likelihood. Verbal labels had additional benefits when included with numeric information, particularly among the vaccine hesitant. For these participants, verbal labels increased vaccine intentions when included with numeric information (but not in its absence). Among the vaccine-hesitant, 43% of those provided numeric information and verbal labels were predicted to be moderately or extremely likely to get vaccinated vs. only 24% of those given a list of side effects (p<.001). We conclude that the standard practice of not providing numeric information about side-effect likelihood leads to a less-informed public who is less likely to vaccinate.