Should public communication of vaccination rates assume rationality, normativity or reasonableness? Insights from three preregistered experiments

The proportion of the population who are vaccinated against an infectious disease is significant – not only because vaccination keeps the virus from spreading, but also because learning about how many members of one’s community have decided to get vaccinated has been shown to affect individual vaccination intention. In three preregistered online experiments featuring country-level vaccination rates against a hypothetical disease, we tested two theoretical approaches which offer contrasting predictions on how public health messaging should leverage vaccination rates. If selfish rationality is assumed, a high uptake would tempt people to free-ride on herd immunity (so low uptake should be emphasized); conversely, if vaccination rates exert a descriptive normative influence, a high uptake would signal that vaccination is the best choice, and vice versa (so high uptake should be emphasized). In the pilot (N = 75) and Experiment 1 (N = 174), communicating a high (90%) vaccination rate (vs. 10% vs. no rate) increased vaccination intentions, with no detectable effect of a low vaccination rate. In Experiment 2 (N = 217), decisions to get vaccinated were frequently justified based on reasons involving self-protection, but also the protection of others and the collective, irrespective of the vaccination rate level (20% vs. 80%); participants, on the other hand, rarely endorsed any of the tested reasons for non-vaccination, including free-riding; furthermore, descriptive norms were perceived as more relevant for vaccination than non-vaccination decisions. Experiment 3 (N = 1,060) tested the effectiveness of different messages when the majority have been vaccinated (60%) but the coverage is still not optimal. Alongside a weak descriptive norm, the self-benefit message worked better than other- and collective-benefit messages. We argue that public health messaging should incorporate both theoretical approaches, closer to the notion of reasonableness (rather than pure rationality or normativity), which is context-sensitive and pragmatic.