Nudging Toward Vaccination: A Systematic Review

"In light of the COVID-19 pandemic and public discourse around risks and chances of novel vaccines, exploring new pathways for interventions, including nudges, that foster vaccine trust and confidence is required."

Several health promotion approaches have been proposed and tested to address vaccine hesitancy (VH), including "nudging", defined as altering the environmental context in which a decision is made or a certain behaviour is enacted. Nudging has been used, for example, to improve malaria and HIV testing in Nigeria and Ecuador, respectively. However, little information on nudging as a VH strategy is available from low- and middle-income countries (LMICs), many of which are currently facing declining public confidence in vaccines. With a global scope, this systematic review sought to determine if nudging can mitigate VH and improve vaccine uptake.

The search, performed from May 15 2020 to May 31 2021, resulted in a list of 48 articles, the key characteristics of which are detailed in table 2 of the paper. Reflecting the dearth or research in this field from LMICs, 46 of 48 of the studies were conducted in high- or upper-middle-income countries. A majority of studies (28 of 48) are randomised controlled trials (RCTs) testing interventions. The researchers evaluated the studies using the Mixed Methods Appraisal Tool (MMAT) and the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) framework. Identified interventions are presented according to the MINDSPACE framework for behaviour change.

The seven types of nudging interventions employed across the selected studies include, in descending order of strength of evidence:

1. Make available information salient: 21 of 48 studies used different ways of salient messaging (e.g., novel, accessible, and simple information) to capture attention and directly relate it to the focus population's personal experiences to promote vaccination behaviours. For example, in Bangladesh, an RCT demonstrated that the use of an application called mTika to automatically generate reminder messages for mothers in hard-to-reach rural and urban areas was effective in improving vaccination coverage.
2. Offer incentives: 5 studies used incentives as a mechanism to motivate behaviour change. For example, in rural India, an RCT evaluated vaccination campaigns with and without incentives, finding that parents who received non-financial incentives were more likely to complete vaccination compared with controls, as long as vaccines were reliably available.
3. Change defaults: Setting defaults has been used with regard to vaccination-related patient–provider communication. For example, a cross-sectional study in the United States (US) used participatory (built on the principle of shared decision making) and presumptive (presupposing those parents would decide to vaccinate their child the same day) approaches to increase vaccination acceptance. The presumptive format was associated with an increase in parental vaccine acceptance but also with reduced satisfaction in the clinical experience, while the participatory format showed the opposite pattern.
4. Change the messenger: Two studies focused their nudging intervention on changing the messenger assigned with communicating vaccine-related health information. In the US, a mixed-methods study used a community approach ("immunity community") to mobilise parent advocates to establish dialogues about vaccines through approaches such as one-on-one communication with peers, social media advocacy, events, and distribution of immunisation educational materials. The results showed statistically significant improvements in parental knowledge and attitudes towards vaccines, but parental vaccine-related behaviours did not change significantly.
5. Change the way outcomes are framed: Some interventions explore use of a gain-framed message (describing the benefits of getting the vaccine) versus a loss-framed one (describing the risks of not getting the vaccine). To cite an example of another kind of framing: In the United Kingdom, an RCT demonstrated that the use of personal benefit messages was more effective among individuals who are strongly hesitant about COVID-19 vaccines compared with those who received information on collective benefits.
6. Invoke social norms: Three studies highlighted the use of social norms (e.g., normalising vaccination as a socially acceptable behaviour or featuring a celebrity to establish a norm) in existing social networks (e.g., network members, friends, family, and healthcare providers) to shape knowledge and intention to vaccinate. For example, a pilot RCT among Cambodian-Americans found that the use of a storytelling narrative video could increase engagement and persuasion. The video showed Khmer mothers and Khmer physicians discussing issues around getting vaccinated, eventually normalising vaccines and depicting vaccination as a desirable behaviour that benefits the entire community. Results showed higher intention to vaccinate in the intervention group versus the control group, but vaccine uptake was not evaluated.
7. Encourage emotional affects: Six studies employed dramatic narratives and graphics in the context of vaccine messaging. For example, an RCT in the US found that a vaccination-related radionovela (a dramatic story broadcast on the radio) of a young girl's journey of hearing about and ultimately receiving the human papillomavirus (HPV) vaccine improved knowledge and attitudes about vaccines among US Hispanic parents or guardians of young children.

A variety of outcome measures were used to assess the effect of nudging interventions (see table 4). In 10 of 16 studies assessing how participants perceived a given nudging intervention, participants reported that the approach was acceptable, feasible, and satisfactory. In 6 of the 10 studies that assessed vaccination knowledge and awareness, the nudge of providing incentives, saliency of information, framing messages, encouraging emotional affect, and changing the messenger were reported to have a positive effect. Among the 19 studies that looked at vaccination uptake as the primary outcome of interest, 12 studies found statistically significant increases in vaccination coverage compared with control groups. Roughly half (12 of 21 studies) showed a positive effect on vaccination intention among study participants.

In short, this study has identified a number of nudging approaches, "several of which have shown promising results in changing vaccination attitudes or behaviours", including some co-developed by scientists, community members, and healthcare workers (HCWs). The review finds that the most compelling evidence exists for nudges that offer incentives to parents and HCWs, that make information more salient, that change defaults, or that engage trusted messengers. However, the effectiveness of nudging interventions and the direction of the effect varies substantially by context. Evidence for some approaches is mixed, highlighting a need for further research, including how successful interventions can be adapted across settings. Notably, "nudging interventions are often considered to be less costly than many other large-scale health intervention programmes,...highlighting their potential for LMICs or other resource-limited settings."

Some of the identified nudging interventions are driven by emotional appeals, with evidence showing they could increase vaccination intentions and uptake. However, the researchers urge that this approach be used only with "parents who are 'fence sitters' with concrete doubts or questions regarding vaccines....Some studies suggest that using nudging interventions with those groups already experiencing strong and conflicting feelings towards vaccination can backfire....[More generally,] the antivaccination community must be considered when promoting a new intervention, especially on platforms and social media with a history of a highly emotionalised vaccine-related discourse (eg, Facebook)....Similarly, emotionalised stories and shocking images designed to inflict fear can increase vaccine safety concerns among parents."

In conclusion: "Successful public health programmes designed to increase vaccination uptake require context-specific adaptations to address target groups' specific concerns and the understandings of vaccine-hesitant individuals. Further research, specifically in relation to nudging interventions in LMICs, is required."