Parental Childhood Vaccine Hesitancy and Predicting Uptake of Vaccinations: A Systematic Review

"Potential vaccination barriers must be identified to inform initiatives aimed at increasing vaccine awareness, acceptance, and uptake."

Concern from parents, decision-makers, and the media regarding the safety of recommended immunisations has increased in recent years. Vaccine hesitancy (VH) recognises a continuum between vaccine acceptance and vaccine refusal. The aims of this systematic review are to: (i) identify relevant quantitative research on parental childhood VH, with vaccine uptake and vaccination intention being relevant outcomes, and (ii) map the gaps in knowledge on VH to develop suggestions for further research and to guide interventions in this field.

This systematic review uses Protection Motivation Theory (PMT) and the World Health Organization (WHO)'s Strategic Advisory Group of Experts on Immunization (SAGE) Working Group (WG) model as theoretical frameworks for understanding VH and its drivers. PMT suggests that people will be likely to protect themselves (e.g., by obtaining a vaccine) if they have firm beliefs about the threat posed by the disease itself (severity and vulnerability). PMT predicts behavioural intentions based on attitudes and perceptions. The SAGE WG's 3C model of VH centres around 3 different types of VH determinants: confidence, complacency, and convenience. Grounded in the 3C model and other validated VH and acceptance models, the 5C model retains the terms "confidence" and "complacency" but replaces "convenience" with "constraints". The final Cs on the 5C model are calcuation and collective responsiblity.

The search process and resulting analysis followed the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-analyses) approach, with the initial search conducted from December 31 2020 to January 21 2021. The publication dates of interest were limited to the period between January 1 1998 and December 31 2020. The starting year was chosen as it was the year of publication of Andrew Wakefield's since-retracted article that linked the measles, mumps, and rubella (MMR) vaccine with the occurrence of autism and behavioural abnormalities in children. The controversy fuelled the anti-vaccination movement.

Using PRISMA guidelines, 34 papers were included that focused on childhood immunisations, employed multivariate analysis, and were published in English in a peer-reviewed publication. Relevant research about VH was found across all WHO regions, and there was an increase in research on parental childhood VH across all these regions over the period 1998-2020.

The 34 studies recognised the complex nature of VH, evidenced by the range of factors identified as determinants of vaccination behaviour (see Table 5 in the paper). These factors clustered around the core concepts of commonly used social cognitive models such as the Health Belief Model (HBM), Theory of Reasoned Action, Theory of Planned Behaviour (TPB), Social Cognitive Theory, Health Action Process Approach, and the PMT. With respect to the objectives of this review, these findings validate the determinants of VH outlined in the SAGE WG model.

The researchers observe that only a few of the studies expanded the field of VH research using novel approaches drawn from the core concepts of social cognitive models. For example, a cross-sectional study in the Netherlands was conducted to determine parental attitudes towards future childhood immunisations. Guided by the HBM, the authors developed a questionnaire for distribution to 800 highly educated parents of children attending daycare centres. This study highlighted the need for continuous health education to ensure the success of vaccination programmes. However, the low predictive capability of HBM variables was evident in this research, given the lack of depth of the questions used to assess and predict behaviour change among the respondents.

Significant challenges to vaccine uptake or intention were identified in the 34 studies. In brief:

• Contextual influences included: socioeconomic factors; communication and media environment; and trust in pharmaceutical companies.
• Individual and social group influences, including:
  • Past experiences: Negative past experiences with vaccination services, such as side effects, poor continuity of care, or lack of compassionate or comprehensive care, were significant predictors of VH among parents in 12 of the studies reviewed.
  • Beliefs and attitudes: All 34 studies characterised beliefs about vaccine safety and efficacy, as well as general attitudes and trust, as significantly associated with the vaccination status of children. For instance, having a positive attitude towards vaccination and a belief in the scientific efficacy of vaccines was identified as a promoter of vaccination.
  • Knowledge and awareness: Knowledge about the severity of a disease and awareness of disease susceptibility were important determinants of the vaccination status of children in several countries studied in the review's papers.
  • Risk-benefit perception (perceived threat vs perceived rewards): Several studies (22/34) highlighted the influence that perceived risks and benefits have on vaccination behaviour. These determinants are in line with the Threat Appraisal Pathway of the PMT: Perceived Threat (Severity, Vulnerability), and Perceived Rewards (Intrinsic Rewards, Extrinsic Rewards).
  • Vaccination as a social norm: Vaccine uptake was influenced by the presence of peers or relatives that are in support of vaccination.
• Vaccine- and vaccination-specific issues included: accessibility, introduction to a new vaccine, and role of healthcare professionals (all 34 studies acknowledged that advice or recommendation from health professionals could be an important determinant of vaccine acceptance).

Thus, several determinants of VH were identified by the studies included in this review. All the explanations for not being vaccinated as stated by the 5C model were recognised as major barriers to vaccine acceptance. Constraints and calculation, however, were less significant drivers. "Campaigns aimed at raising parental vaccination intention would most likely be effective if they emphasise building confidence and collective responsibility while reducing complacency."

Based on the constructs of relevant theoretical models, the researchers emphasise the need for parents and stakeholders to be actively engaged in the vaccination decision-making process from an early stage. "It is clear, however, that additional information sources are needed to ensure these models adequately account for the influence of broader contextual factors, particularly in regions with limited peer-reviewed literature."

On that note, no single algorithm was applicable to all studies, as each factor was independent and varied across time, place, and vaccines, reflecting the complex interplay of other variables and the context-specific nature of VH. Understudied themes were also discovered during the review. Different research methods were applied, and most of the studies were cross-sectional, thus making it difficult to draw conclusions about the influence of single or multiple determinants of vaccine acceptance at the individual or collective level. Per the researchers, future research should consider qualitative studies to help fill these gaps and contribute to existing knowledge and understanding of the many factors that influence parental decision-making.

The researchers suggest that theoretical approaches to quantifying VH "will continue to strengthen the body of knowledge needed to develop successful evidence-based interventions. The efficacy of vaccine advocacy campaigns could be increased and the burden of VPDs [vaccine-preventable diseases] could be lowered by adopting clinical, patient-centred techniques to measure and overcome VH. A combination of local, regional, and universally driven initiatives will be critical in the early detection of parental concerns."

In conclusion: "When the findings of this systematic review are integrated with conceptual frameworks such as the 5C model, important revelations about modifiable behaviours can emerge."