Social Norms and Free-Riding in Influenza Vaccine Decisions: An Online Experiment

Presenter: Krystal Lau

Twitter: '@krystallau9209

Co-authors: Marisa Miraldo, Matteo Galizzi, Katharina Hauck

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‘Nudge’-based social norm messages that signal uptake strength of a certain behaviour within a population can be used in public health interventions to shape individuals’ decisions to adopt that behaviour. Messages conveying high influenza vaccination coverage levels signal a strong social norm, encouraging vaccination, but also a low risk of infection, discouraging vaccination and promoting free-riding. The complex interplay between these two signals can result in ambiguous vaccination decision-making behaviour, in particular as coverage levels vary. We aimed to causally measure different vaccination coverage levels’ impact on influenza vaccination intention. In an online experiment, we randomly assigned UK residents aged 18+ years to a control group (with no message) or one of seven treatment groups with different messages of vaccination coverage levels, measured by the proportion of people vaccinated (10%, 25%, 50%, 65%, 75%, 85%, or 95%) in the respondents’ environment. Impact on respondents’ vaccination intention was measured with self-reported intention and three elicited behaviour measures: (a) opening an online map locating nearby private flu jab providers; (b) time looking at this map; and (c) downloading a calendar reminder to vaccinate. Linear regressions, probit, logistic and double hurdle models were used, controlling for population behaviour perceptions, risk attitudes, behavioural and socio-economic characteristics collected through individual questionnaires. We randomly allocated 1,365 participants into one of the treatment (N = 1,163) or control (N = 202) groups. Treatment groups had significantly higher vaccination intention than the control. Groups treated with higher coverage levels had greater vaccination intention than groups treated with levels below the 75% to 85% threshold. Groups treated with levels above this threshold had lower vaccination intention than those treated with levels below. Average vaccination intention was higher at lower coverage levels (potential social norm compliance) but lower at higher coverage levels (possible free-riding behaviour). Policymakers should consider personalizing interventions that use social norm messages to ‘nudge’ vaccination.