Parent-level barriers and facilitators to second or subsequent dose uptake in childhood vaccination: A mixed-methods systematic review protocol

Introduction

Vaccines play a critical role in reducing childhood morbidity and mortality and in safeguarding global health and economic security.¹ Since the rollout of the World Health Organisation’s (WHO) Expanded Programme on Immunisation, vaccines have saved over 154 million lives and generated more than 10 billion years of life lived in full health.² Achieving and sustaining these benefits depends on adherence to recommended vaccination schedules, including the completion of multi-dose primary series and booster doses series where appropriate.³

Despite substantial efforts, global childhood vaccination coverage remains suboptimal, particularly for second and subsequent vaccine doses. In 2024, global coverage of the second measles-containing vaccine (MCV2) reached 76%, falling below the 95% coverage threshold recommended to prevent measles outbreaks.^1,4 Similar drops in coverage have been observed across other multi-dose childhood vaccines, highlighting a persistent challenge in vaccine completion beyond the first dose and undermining population-level immunity.⁵

Parents and guardians play a crucial role in childhood vaccination uptake, positioning parent-level factors as critical targets for public health intervention. A myriad of parent-level factors shape vaccine uptake, influencing whether parents consciously decline certain doses or unintentionally miss them, with some accepting selected vaccines while not completing others.^6,7 Existing research has identified both modifiable and non-modifiable parental determinants of childhood vaccination. Identifying these determinants is important for identifying high-risk populations and targeting modifiable factors, both of which can be addressed through vaccination intervention programmes.

A global umbrella review by Sacre et al. found mixed evidence regarding the association between socioeconomic status (SES) and initiation and/or completion of childhood vaccination uptake, with both lower and higher SES associated with lower and higher uptake, respectively, across different contexts.⁸ An earlier systematic review similarly reported inconsistent findings regarding SES, but identified associations between low maternal education and high birth order and incomplete or delayed vaccination.⁹ More recently, Adisu et al. conducted a systematic review examining determinants of MCV2 uptake among children under five years of age in Africa.¹⁰ Consistent with previous evidence, significant determinants included high maternal education, attendance to antenatal care follow-ups, and use of postnatal care services. The review also identified reduced waiting times at vaccination sites, greater awareness, and positive perceptions of the second dose vaccine as significant predictors of MCV2 uptake.¹⁰

Beyond non-modifiable risk factors, a substantial body of the literature focuses on modifiable parental determinants of vaccine uptake. A global umbrella review by Kaufman et al. synthesised evidence on modifiable parent-level determinants of vaccination uptake, identifying key access- and acceptance-related factors. These determinants were grouped into six categories: access, clinic or health system barriers, concerns and beliefs, health perceptions and experiences, knowledge and information, and social or influence.¹¹

However, existing literature typically examines non-modifiable and modifiable parental determinants of childhood vaccine uptake using binary outcomes, such as completion versus non-completion of the entire childhood vaccination schedule. What remains unclear is how parent-level barriers and facilitators of vaccine uptake may vary by vaccine dose. As vaccinations are delivered over extended periods and at different stages of a child’s development, second or subsequent doses are typically administered at older ages, when parental circumstances and healthcare engagement and experiences likely differ from the early infancy period.

As a result, parents may encounter distinct barriers and facilitators across vaccine doses. For example, parents participating in a qualitative study in the United Kingdom reported differential barriers to the second dose of measles-containing vaccine and diphtheria, tetanus, pertussis and polio (dTaP) booster dose compared with initial doses. Parents reported reduced availability of information and less contact with healthcare professionals regarding subsequent doses. In addition, parents reported a more difficult emotional experience, characterised by increased apprehension and guilt when taking an older child for subsequent doses compared with taking younger infants for their primary vaccination.¹² Similarly, parents from a study in Ethiopia identified lack of information about when to return for the second dose as a significant barrier to MCV2 uptake.¹³ This evidence suggests a waning of informational and other supports following initial vaccine doses.

To the best of our knowledge, no review has synthesised global evidence on parent-level barriers and facilitators to second or subsequent dose uptake in childhood vaccination. While most studies examine vaccination uptake at the level of the overall childhood schedule, which varies across countries, limited attention has been paid to parent-level barriers and facilitators specific to second or subsequent doses within multi-dose vaccine series. Given the substantial variation in study methodologies, a mixed-methods systematic review is the most appropriate approach to address this research question and systematically synthesise the existing evidence base.

Therefore, the objective of this mixed-methods systematic review is to identify and integrate the literature on parent-level barriers and facilitators to second or subsequent childhood vaccine dose uptake. In this review, parent-level barriers and facilitators are defined as both modifiable and non-modifiable determinants of second or subsequent dose childhood vaccination, which are directly relevant to parents. This review is vital to inform appropriate vaccination schedules and interventions, and supports the overarching goals of the WHO Immunisation Agenda.¹

Review question

What are the parent-level barriers and facilitators to second or subsequent dose uptake in childhood vaccination?

Inclusion criteria

Methods

The proposed mixed-methods systematic review was developed following the methodological guidance from the Joanna Briggs Institute (JBI) Manual for Evidence Synthesis guidelines.¹⁵ The review will report in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses.¹⁶

Information sources

Discussion

Childhood vaccination coverage remains suboptimal, particularly for second and subsequent vaccine doses administered across early and later childhood.^1,4 Poor vaccination coverage increases the risk of vaccine-preventable disease outbreaks and poses an ongoing threat to population health. This mixed-methods systematic review will integrate the existing literature on parent-level barriers and facilitators to second or subsequent dose uptake in childhood vaccination. Examining both modifiable and non-modifiable parent-level factors, this review will help to identify populations who may be at a higher risk of under-vaccination, as well as actionable barriers to vaccine uptake that can be addressed through tailored vaccination interventions. The findings of this review are expected to inform service providers and policy makers, and to contribute to the development of appropriate strategies to improve childhood vaccination coverage.