Evidence from a statewide vaccination RCT shows the limits of nudges

In a randomized controlled trial (RCT) five to eight weeks after all adults in the study population (n = 142,428) were eligible for vaccination, we find that the best-performing nudge in previous studies2,6 and seven additional messages-stressing vaccines' safety, efficacy, minimization of bad outcomes, accessibility (free, no identification required), protection of recipients' families or widespread adoption-had no detectable effect among people who had not been vaccinated according to state records. Public health authorities should consider simple messages to encourage vaccination at key inflection points (for example, rollout of paediatric COVID-19 vaccines and full Food and Drug Administration approval for adults), but may see diminishing returns if using them to encourage the more hesitant. [...]it has garnered considerable media attention9, with pieces advocating it in The Washington Post, Fortune, The Guardian, U.S. News & World Report and this journal10. Furthermore, we see no evidence of differences in vaccination rates (however measured) between the control and an aggregated 'any message' condition (estimated difference in proportions vaccinated -0.001, 95% confidence interval (CI) -0.004 to 0.001, CMH test, P = 0.27), nor between the arms taken all together (CMH test for 9x2x13 table, P = 0.12).