Herd immunity communication counters detrimental effects of selective vaccination mandates

Low vaccine uptake results in regular outbreaks of severe diseases, such as measles. Selective mandates, e.g. making measles vaccination mandatory (as currently implemented in Germany), could represent a viable solution to the problem. However, prior research has shown that making only some vaccinations mandatory while leaving the rest to voluntary decisions can result in psychological reactance (anger) and decreased uptake for voluntary vaccines. Since communicating the concept of herd immunity has been shown to increase the willingness to vaccinate, we assessed whether it can buffer such reactance effects. A total of 576 participants completed a preregistered 2 (policy: selective mandate vs. voluntary decision) × 2 (communication: herd immunity explained yes vs. no) factorial online experiment. In a first scenario, the concept of herd immunity was either introduced or not and vaccination was either mandatory or voluntary, depending on condition. The dependent variable was the intention to vaccinate in the second scenario where vaccination was always voluntary. Additionally, we explored the mediating role of anger between policies and intentions. Herd immunity communication generally increased vaccination intentions; selective mandates had no overall effect on intentions, and there was no interaction of the factors. However, selective mandates led to increased anger when herd immunity was not explained. This increased anger led to lower subsequent vaccination intentions. Consequently, explaining herd immunity can counter potential detrimental effects of selective mandates by preventing anger (reactance). Key messages When introducing selective mandates, the social benefits of herd immunity should be communicated. Otherwise, the uptake for still voluntary vaccines may decline, putting public health at risk.