Krystal Lau (Imperial College)

‘Nudges’ based on social norm messages can be used in public health interventions to change behaviour. In the context of influenza vaccination, messages about high coverage levels may signal a bandwagoning effect that encourages vaccination, but also a low individual risk of infection, promoting a free riding effect that discourages vaccination. The interplay between these two factors can affect decision-making differently at varying population coverage levels, making the impact of nudges on intentions to vaccinate ambiguous to predict. We aim to causally measure the effects of different social norms via messages about population vaccination coverage rates on influenza vaccination intention. In an online experiment, we randomly assign a sample of n=1,365 UK residents aged 18+ years to a control group (with no manipulated information on coverage rate), or to one of seven treatment groups with different messages about the proportion (10%, 25%, 50%, 65%, 75%, 85%, or 95%) of people in their environment who received the flu vaccine. Vaccination intention is measured with self-reported intention and three elicited behavioural measures: (a) opening an online map locating nearby private flu vaccination providers; (b) time looking at this map; and (c) downloading a calendar reminder to vaccinate. Through linear, logistic, and double hurdle regression analysis, we find that individuals treated with greater coverage rates have higher stated vaccination intention than those treated with lower coverage rates. For map-related elicited vaccination intention, however, those with manipulated coverage rates up to 75%, exhibited greater intention at higher treatment intensities, demonstrating possible bandwagoning effects. In groups above this 75% threshold, however, the impact of the treatments when compared to the control – albeit still positive – is lower at greater treatment intensities, signalling the introduction of free riding behaviour and its interplay with bandwagoning above that threshold. Furthermore, individual perceptions on risk of infection and population coverage rates may have an amplifying moderator effect on the relationship between social norms treatments and vaccination intention. Our results imply that health policymakers ought to carefully design interventions based on social norm messages to nudge influenza vaccination in order to promote vaccination, rather than discourage it.