Social Nudges for Vaccination: How Communicating Herd Behaviour Influences Vaccination Intentions

"...social-benefit appeals increase vaccination intentions..."

This study explored intervention strategies to tackle vaccine hesitancy that harness social processes to motivate vaccination. More specifically, it tests the following three social nudges: (i) the communication of herd immunity - i.e., the more people in a community who are vaccinated against a disease, the less probable it is that the disease will spread; (ii) communication of herd immunity threshold, i.e., the proportion of the population that must be immunised to stop the infection from spreading and protect everyone; and (iii) descriptive social norm communication - i.e., the level of vaccination coverage in one's country (what most people are doing).

The researchers ran an online experiment with a 2 (herd-immunity explanation: present vs. absent) × 2 (herd-immunity threshold: present vs. absent) × 3 (descriptive norm: high vs. low vs. absent) between-subjects fractional design with seven groups. Group 7, the control, did not receive any experimental intervention to serve as a benchmark for the effect of herd-immunity communication.

Study participants were 543 people (aged 18-64) residing in the United Kingdom (UK). Data were collected at the time of the COVID-19 pandemic, between October 5 and November 24 2020. The second half of the data collection period encompassed the second national lockdown but ended before COVID-19 vaccinations were first rolled out in the UK. Participants first received an explanation of herd immunity emphasising social benefits (protecting others) in both textual and animated-infographic form. Next, they were were asked to imagine themselves in a scenario in which they had to decide whether to get vaccinated against a fictitious disease. The scenario informed participants about the disease and the vaccine, the herd-immunity threshold (90%), and the level of the vaccination coverage in their country (20% or 80%). Following scenario-recall questions, participants rated their intention to get vaccinated. Then, perceived riskiness of the infection and the disease were assessed. This was followed by a measure of vaccine hesitancy.

Key results:

• Communicating herd immunity via text and animated infographic significantly increased vaccination intentions compared to the control; the effect remained significant after controlling for sociodemographic variables.
• Intentions of the participants who were also informed about the herd immunity threshold were not significantly different from the intentions of the participants who were not informed about it.
• Communicating different descriptive norm messages (no-coverage vs. low-coverage [20%] vs. high-coverage [80%] message) alongside herd immunity was not effective in increasing vaccination intentions.
• The researchers detected no significant interaction between vaccine hesitancy and either of the three factors (herd-immunity explanation, herd-immunity threshold, or descriptive norm).

Reflecting on the findings, the researchers suggest that social norms and herd immunity threshold communication - on top of herd immunity communication - might have showed no effect precisely because all participants were familiar with the concept of herd immunity. When it comes to herd immunity, the observed effect size was smaller than in previous studies. This might be due to the pandemic context in which participants had been living. For example, in March 2020, herd immunity briefly came to be seen as the UK government's strategy to respond to COVID-19, sparking criticism and public backlash. The confusion stemmed from interviews in which government advisers appeared to suggest that one way to manage the epidemic would be to naturally reach herd immunity by aiming for 60% of the population to fall ill. Although the study materials mentioned the term "community immunity" only, explaining that it was generated through vaccination (not infection), some participants might have misinterpreted the materials due to confusing public messaging earlier that year.

Going forward: "To provide further empirical guidance for effective and scalable communication strategies that rely on social nudges, it might be useful to replicate this study design with real-world vaccine-preventable diseases; to conduct the studies in other countries and with samples that are representative of the population (also with respect to vaccine hesitancy); and to assess the long-term effects of providing people with information about herd behaviour."