Measuring Trust in Vaccination: A Systematic Review

Vaccination decisions occur within the context of trust held in the various actors who interpret and make decisions based on the available evidence. This systematic review analyses vaccine research literature that explicitly refers to the concept of trust within their stated aims or research questions. Its objectives are to: investigate how studies conceptualise and measure the concept of trust as a prominent factor in vaccine intention or uptake; discuss how the research compares to the wider literature on trust in the context of health decisions; and explore the different dimensions of trust and their relationships as they influence vaccine uptake.

Trust is defined here as "a relationship that exists between individuals, as well as between individuals and a system, in which one party accepts a vulnerable position, assuming the best interests and competence of the other, in exchange for a reduction in decision complexity." Vaccine acceptance involves multiple levels of trust: trust in the product (the vaccine), the provider (the specific healthcare professionals or administrative staff that are involved in providing and administering vaccination), and trust in the policymaker (the health system, government, and public health researchers involved in approving and recommending the vaccine). In addition to influences on trust in the context of immunisation, there are a number of external factors that influence trust, which the article examines in turn: generalised trust, historical influences on trust, and external influences. Also explored here are mechanisms by which trust-based cooperation is built or eroded.

Ten different medical and social science literature databases were searched for peer-reviewed articles on trust in vaccines or vaccination programmes. A set of keywords was created to reflect the core concepts: vaccination and public perceptions, decision-making, and vaccination behaviour. The search was run across all databases during the period November 12-19 2012, then on December 15 2014, and, finally, on November 17 2017.

Of the 35 included studies, 11 studies focused on childhood vaccinations, 14 studies focused on adult vaccinations, and 3 focused on the adolescent human papillomavirus (HPV) vaccination. Investigated trust factors predominantly included the information from and/or the trust placed in the health system, healthcare professionals (HCPs), the government, science or trusted others (e.g., friends, family, alternative HCPs, non-official internet sources, celebrities). Three studies investigated the concept of generalised trust.

Findings indicated that trust is often referred to implicitly (19/35), rather than explicitly examined in the context of a formal definition or discussion of the existing literature on trust in a health context. Within the quantitative research analysed, trust was commonly measured with a single-item measure (9/25); only 2 studies used validated multi-item measures of trust. While trust is shown to have a positive effect on vaccination intention and uptake in most of the studies reviewed, few explored trust factors or concepts beyond those of trust in the health system (21 studies), the government (10 studies), or HCPs (9 studies). Only 2 studies specifically measured trust in the vaccine (e.g., "Overall, how much do you trust the flu vaccine?"). Furthermore, factors outside of the vaccination programme were rarely measured.

The researchers found that "The theme of historic neglect or abuse from a government or health system was often seen as an underlying reason for distrust in vaccines among marginalised groups....By shifting the burden of distrust onto the minority individual or community, and away from the trustworthiness of institutions, the genuine drivers of trust and distrust may actually be obscured." Furthermore, only one study that met inclusion criteria was based in a middle-income country, and none of the studies focused on low-income countries. "With trust playing such a key role in influencing vaccine acceptance, more research is needed in middle and low-income settings to truly understand whether findings in high-income countries have relevance in low and middle-income countries."

Envisioning future research directions, the researchers indicate that the prevalence of single-item measures, where the definition of trust was left as implicit, suggests that a thorough understanding of trust as it relates to vaccine acceptance needs more exploration. "Furthermore, a lack of experimental or longitudinal studies that investigate how trust can be eroded or built over time demonstrates that there is great potential for new contributions to our understanding of the temporal dynamics and levers of trust in relation to vaccination."

To sum up: "The findings of this review indicate a disconnect between the current vaccine hesitancy research and the wider health-related trust literature, a dearth in research on trust in low and middle-income settings, and a need for studies on how trust levels change over time and investigations on how resilience to trust-eroding information can be built into a trustworthy health system."