A Systematic Literature Review to Clarify the Concept of Vaccine Hesitancy

"...the field should first reach consensus on the definition and then measure VH [vaccine hesitancy] accordingly. This approach allows for a much-needed comparison between studies to improve our understanding of VH determinants, correlates and consequences on an individual and societal level."

The literature on vaccine hesitancy (VH) is characterised by a lack of conceptual clarity, where VH is variously conceptualised as a psychological state, as a type of behaviour, or even as a catch-all category, aggregating many different concepts. Confusion among researchers is then illustrated by inconsistencies in the applied definitions; arguably, this muddiness is impeding progress in the research field. The purpose of this systematic review was to provide an overview of how VH is operationalised in the literature in terms of conceptualisations, subpopulations, and measurements. On the basis of an interpretation of the findings, the researchers propose a renewed definition of VH.

Relevant publications were searched using the PubMed, Embase, and PsycINFO databases. The search strategy yielded 7,427 publications, 420 of which met the inclusion criteria. Seven additional studies were found through citation searching, 2 of which met the inclusion criteria, adding up to 422 studies. Some studies met the criteria of more than one category, with 36 studies categorised under VH conceptualisations, 63 under VH subpopulations, and 373 under VH measurements.

The characteristics of included studies are described in more detail in Supplementary Table 1 in the paper but, in brief, they cover a wide geographical distribution (including 43% originating in low- and middle-income countries - LMICs). The majority (60%) were published in 2021 and 2022. VH was discussed in relation to vaccination in general or specifically with regard to childhood vaccines in 19 and 13 of the studies, respectively. The remaining 4 studies discussed VH in relation to COVID-19 and influenza.

From the 36 studies on VH conceptualisation, the thematic analysis revealed 3 predominant conceptualisations:

1. Cognitions or affect - e.g., having or expressing concerns, doubts, or questions and being reluctant or unsure. Many authors describe VH as pertaining to beliefs, attitudes, or both.
2. Behaviour - e.g., a delay in acceptance or refusal of vaccines. Some authors describe VH as a range or continuum between the extreme ends of accepting all vaccines and refusing all vaccines.
3. Decision making - e.g., individuals who are undecided, indecisive, or under consideration, and not yet having made a final vaccine decision.

These 3 conceptualisations overlap in the majority of the studies and study excerpts. Illustrative excerpts of each conceptualisation are presented in Table 1 of the paper.

VH is often described in relation to other concepts. The researchers extracted 142 excerpts from 31 studies describing closely related concepts - most commonly, confidence or trust, complacency, and convenience (the "3Cs", described in 69 of 142 (49%) of the excerpts). Most often, the 3 Cs are described as having a causal relationship with VH and as representing determinants.

Of the 422 included studies, 63 identified various VH subpopulations. The analysis identified themes aligned with the above 3 VH conceptualisation categories. An overview is provided Supplementary Table 3.

Of the 422 studies, 373 report a measurement of VH in individuals. An overview is provided in Supplementary Table 4, grouping the studies according to the instruments used. The most common, albeit highly heterogenous, method used in 210 (56%) studies is a brief VH assessment comprising 1-3 questions. The majority of questions used in this method cover operationalisations of VH that did not emerge from the conceptual analysis (the above 3 categories); for example, 124 studies assess VH by asking about vaccination intention. The second most common method, applied by 132 (35%) studies, is the use of a validated instrument - e.g., the parent attitudes about childhood vaccines (PACV) survey, used in 70 studies. "Particularly during 2021, there has been a plethora of studies reporting VH measurements that, due to divergent definitions and methods, have been of questionable value."

Reflecting on the findings, the researchers argue that conceptualising VH as vaccination behaviour is untenable, as mere behaviour is insufficiently discriminating between hesitant and non-hesitant individuals. For instance, people may accept vaccines with or without hesitation or reject vaccines with or without hesitation. Although the researchers agree that certain types of vaccination behaviour may be manifestations of VH, they agree with others who have, for example, commented on the blurred distinction between VH and refusal of vaccines.

This argument logically results in the proposition that VH should be considered a psychological construct, yet the researchers argue that cognitions or affect are insufficiently distinctive to define VH. Instead, they hold that it is the element of indecision that provides a unique and distinctive feature for VH. (In addition, they found that this conceptualisation is predominantly discussed in studies in the public health field. Such studies take a more pragmatic approach, examining the presence of VH at a stage where people have been offered a vaccine or to anticipate public sentiments around willingness to accept a vaccine when it is offered.)

Thus, the researchers argue that VH is a psychological state of being undecided, indecisive, or not yet making a decision regarding vaccination. They acknowledge that experiencing concerns, doubts, or reluctance regarding vaccination may play a vital role in shaping VH. However, their suggestion is that these factors are relevant constructs integral to shaping VH, rather than synonymous to VH. Operationalising VH by measuring or distinguishing subpopulations should ideally be directed at this state of indecision.

"The conclusion of this review is that VH is, for now, an impracticable concept, due to the confusing use of multiple, varied operationalizations. To aid further research, the VH concept must be clearly conceptualized and adapted from its broad and inclusive form to a pragmatic and refined alternative....Our way forward is to simplify and clarify the operationalization of VH by returning to its root core of indecisiveness."