Nudges for COVID-19 Voluntary Vaccination: How to Explain Peer Information?

"Vaccination promotion is a crucial strategy to end the COVID-19 pandemic; however, individual autonomy should also be respected."

Nudging, which is an application of behavioural economics, has been used to promote prosocial behaviours, including vaccination. Even if nudge-based interventions can promote vaccinations at present, they may decrease the levels of other infection control measures and future vaccination: Research indicates that some nudges potentially force people to make choices they may not wish to make and thus reduce cooperative behaviours. Conducted in Japan, this study explores nudges that can reinforce people's intention to receive the COVID-19 vaccine without impeding their autonomous decision-making.

As a literature review opening the paper explains, other-regarding nudges, which this study focuses on, are one type of information nudge. Behavioural economics calls these nudges "social comparison nudges" in that they make people aware of negative emotions from not behaving in the same way as others. In the context of nudging around vaccination, social comparision messages focus on one's own and others' decisions, rather than on the vaccine's performance itself (the vaccine's efficacy/effectiveness in preventing infection, disease onset, or severe disease) or on the benefits to individuals or society as the consequences. For instance, field experiments have examined the effectiveness of emphasising "vaccine ownership", with Dai et al. (2021) finding that the message "a COVID-19 vaccine has just been made available to you..." increased COVID-19 vaccine uptake. By receiving these messages, people may feel reassured that their vaccine is ready and think that if they do not take the vaccine, it may go to someone else. As an example of this approach, Japanese local governments sent coupons for free COVID-19 vaccines to people before accepting vaccination reservations, which may have the similar effect of making people feel that their vaccine was secured.

In providing some context for the experiment, the researchers note that there may still be a certain proportion of Japanese people who have strong hesitancy against vaccines due to a history of controversy with the human papillomavirus (HPV) vaccine in that country. After public subsidies for the HPV vaccine started in 2010, in June 2013, the Ministry of Health, Labour, and Welfare announced refraining from actively recommending the vaccine following repeated media coverage of various symptoms related to vaccination and subsequent lawsuits. Consequently, the HPV vaccination rate dramatically decreased and almost halted. In general, Japanese people have a lower trust level for vaccines on an international scale.

From March 16-18 2021, the researchers conducted an online experiment with 1,595 people living in Japan, which had not yet at that time begun offering COVID-19 vaccinations to ordinary people. Study participants were defined as "older", being at higher risk of severe disease and able to be vaccinated earlier, as aged 65-74 years, or "younger" (aged 25-34 years). The researchers randomly assigned them either a control group or one of three treatment groups that received messages differently describing peer information: control, comparison (message conveyed the proportion of people willing to receive the COVID-19 vaccine), influence-gain (message was: "Your vaccination uptake can encourage the vaccination uptake of the people around you."), and influence-loss (message was: "If you do not receive the vaccine, the people around you may also not do so."). In line with Tversky and Kahneman's (1979, 1981) prospect theory, loss framing tends to place a heavier emotional burden on the individual and to inhibit their autonomous decision-making in the context of vaccination. The researchers compared each message's effects on vaccination intention, autonomous decision-making, and emotional response.

The proportion of older respondents in the control group who were willing to receive the free vaccine was 84.4%, which was higher than for young adults, at 67.0%. Figure 3 in the paper shows the effects of nudge-based messages on vaccination intentions. In short, the findings indicate that the influence-gain nudge was effective in increasing the number of older adults who newly decided to receive the vaccine. The comparison and influence-loss nudges further reinforced the intention of older adults who had already planned to receive it. However, the influence-loss nudge increased participants' negative emotion (e.g., they were "distressed" and the messages "intensified the interpersonal and intrapersonal pressure" to get vaccinated, etc.). These messages had no promoting effect for young adults with lower vaccination intentions at baseline.

In reflecting on the findings, the researchers suggest that governments should use different messages depending on purposes. Namely:

• The gain-framed nudge may be effective for increasing the number of older adults who newly decide to receive the vaccine. One option is to include this message on public posters and websites.
• The comparison nudge may be effective for reinforcing the intentions of older adults who have already intended to receive the vaccine and ensuring their vaccination is carried out.
• Regarding social well-being, the governments should use the comparison nudge, instead of the loss-framed nudge, which has the similar promoting effect but increases the emotional burden of those who receive the message. However, since the comparison nudge may further weaken the vaccination intentions of older adults with originally lower intentions, this message should be displayed only to those who wish to get vaccinated. One option is to include this message on the reservation screen for vaccinations or in reminder emails.

The researchers note that the Japanese have long been considered to be a collectivist culture. The effects of other-regarding nudges may depend on this and other characteristics. However, since COVID-19 vaccination is being promoted worldwide, the researchers suggest that the findings could contribute to countries and cultures that share common characteristics with Japan and may have implications for vaccination programmes in future pandemics.