Vaccination Demand and Acceptance: A Literature Review of Key Behavioural Insights

"Behavioural factors such as choice overload, perception biases, uncertainty, unstable preferences over time and the impacts of peers have been recognized over time and they are important determinants of health choices, including decisions concerning vaccines."

High confidence in vaccination programmes is crucial for maintaining high coverage rates, especially at levels that exceed those required for herd immunity. Across the European Union (EU), however, vaccine delays and refusals are contributing to declining immunisation rates in several countries, leading to increases in disease outbreaks. A variety of factors contribute to modern vaccine hesitancy (VH), such as the layperson's heuristic thinking (cognitive shortcuts or rules of thumb) when it comes to balancing risks and benefits. This literature review examines key behavioural insights that can help explain and support vaccination demand and acceptance.

Using a time horizon from the year 2000 until 2020, The review included 209 papers that are quantitative, including literature review papers, mixed-method papers, and a few qualitative papers, as well as theory papers that are in the domain of behavioural economics at the intersection of vaccination. The researchers employed the following keywords: childhood vaccination behavioural economics experiment; vaccination hyperbolic discounting; vaccine hesitancy Europe; vaccination "peer effects"; and vaccination "social norms Europe. (Thus, the review focused on but was not limited to the EU.)

The report discusses the results along the lines of four broad behavioural factors, plus a group with behavioural spillover:

1. Involvement: Being involved in a vaccination decision suggests a state of mindfulness, rational, and educated decision making and a motivation to act upon one's intentions. Individuals are often involved in decision-making for others. For example, parents are large subset of individuals who make proxy decisions for their children. They need to be aware of the correct information available. The topic is quite complex and may cover information overload, choice overload, information avoidance, risk and benefit perceptions, psychic costs, salience and proxy vaccination.
2. Vaccination decisions under uncertainty: Vaccination decision-makers need to evaluate perceived risks of vaccination and non-vaccination. Those perceptions might be different based on whether someone makes decisions for oneself or for others. Vaccination is a decision made under uncertainty. One either faces the uncertain prospects of getting sick due to non-vaccination or the uncertain prospects of side-effects from the vaccine with a minutely small propensity. Incorrect evaluations of those prospects or ambiguity, i.e. the state of not knowing the probabilities with which events occur, may harm overall coverage rates. Regret aversion and overweighting of small probability events in the spirit of prospect theory deserves more systematic attention for side effects with small propensities.
3. Vaccination decision-making over time: Some vaccines, like influenza, are recommended to be taken every year due to mutations of the virus strains. Other vaccines only need to be administered during childhood. In either event, the individual might face a trade-off decision between incurring costs now for prevention and receiving benefits in the future of reducing the propensity to become sick while never being able to know the counterfactual state with certainty. Decision-makers might also be present-biased individuals who incorrectly overweight their utility today and underweight their utility in the future. Reducing procrastination can therefore, assist in increasing vaccination coverage for any vaccine. "Research in this domain has the potential to increase coverage substantially."
4. Peer effects: Peers, such as other parents, friends, colleagues, or other close individuals within a network, may influence vaccination coverage. Peers generally can influence behaviour either through beliefs about the actions of others, i.e. via nonconformity, or by following one's peers through conformity. We know from economic theory that externalities can be generated through peers - e.g. by receiving additional information, or through social norms. Being a role model for others might have an impact on overall vaccination coverage. While there seems to be some divergence in the literature on whether peers are improving coverage or reducing coverage, it is not clear by how much free riding might offset conformity. Further investigations into the mechanisms, such as learning from peers or information spillover, are crucial to understand which policies to recommend.
5. Healthcare professionals (HCPs): As a special group with potential of behavioural spillover, interventions engaging HCPs as a behavioural supply side factor of vaccination have potential. However, only few studies provide credible causal interventions that can address behavioural factors to encourage trust, to improve information, and to educate HCPs in order to ultimately increase vaccination coverage of patients through the individuals who recommend and provide vaccinations. In particular, leveraging networks and contracts may allow a reduction in behavioural biases, which can improve overall vaccination coverage.

Each of the above has a dedicated chapter in the report, and the literature findings for each is split into suggestive evidence vs causal evidence. Sample findings:

• Key attributes of vaccines are safety (risk of side effects) and accessibility. Perceived vaccine safety is correlated with positive parental response to childhood vaccination. Also, individuals accept higher probabilities of potential side effects as the effectiveness of the vaccine increases.
• Being exposed, even for a limited amount of time, to vaccine-critical information increases vaccine risk perceptions. Being the first voice heard in the vaccination debate and counteracting false claims and misinformation are vital actions to combat VH.
• The greater the trust in government, healthcare systems and professionals, the greater the vaccination acceptance. HCPs' quality, in particular, is correlated with vaccination uptake. Making sure that HCPs can build good relationships with their patients and know how to deliver complex messages around vaccination is essential to combat VH.
• There is a large potential for forgetfulness, memory issues, and present-biasedness to be drivers of low vaccination coverage rates. Opt-out vaccination appointments and multiple SMS (short messaging service, or text message) reminders are an easy yet effective tool to overcome vaccination barriers.
• Prospect theory can explain a portion of VH: People overweigh small probabilities (vaccination side effects) and underweight relatively large ones (contracting a vaccine-preventable disease). Also, individuals prefer known risks over unknown risks: The future risk of contracting a known disease might be even preferred to an immediate risk of experiencing the unlikely adverse effects of new vaccine.
• Vaccinated individuals may engage in risk-related trade-offs. For example, they could disregard other preventive measures once they receive the vaccine.
• Homophily (selecting the people to interact with based on similar beliefs) and contagion (how the behaviour of others travels across social networks) are central in understanding vaccination behaviour. In particular, HCPs can have a large impact on vaccination coverage: They are key to positive attitudes.

The report also notes that while existing literature is useful in understanding attitudes towards COVID-19 vaccines, the unique circumstances of the pandemic limit the generalisability of these findings. First, although the disease is generally perceived as serious, a substantial fraction of the population chooses to remain unvaccinated. Second, such perception is a poor predictor of vaccination acceptance at the individual level. Furthermore, COVID-19 VH can be associated with the other side of the novelty coin: the novelty of the vaccine itself, making patients uncertain about its efficacy, and, crucially, its safety. Evidence suggests that building up trust in vaccinations in general (their safety and efficacy) will have a sizable impact on COVID-19 vaccination uptake. This strategy has an appeal, given the evidence of overload and active avoidance of messages related to COVID-19. The major practical difficulty is that it is very hard to foster trust in vaccinations in general.

The report concludes by emphasising the need for a new research agenda to build trust in the medical system, government, and pharmaceutical companies, and it highlights the importance of HCPs in addressing the challenges of vaccination programmes, as they are the most trusted source of health information. The report advocates for a multidisciplinary approach that includes insights from the behavioural sciences to shed light on the mechanisms at work when people make decisions about vaccination. "A substantial amount of work would still need to be done to improve our understanding of vaccination behaviour and to recommend policies that allow improving overall societal welfare in a world where coverage from each individual is important in itself and generates positive externalities."