Background This umbrella review aimed to synthesise global socioeconomic inequalities in the uptake of routine vaccinations and identify any mechanisms that may contribute to the association. Existing literature suggests that vaccination uptake is multifaceted, varying over space, time, context, and by vaccine. To our knowledge, no attempt has been made to synthesise the global body of systematic reviews across a variety of vaccines, geographical locations, and measures of socioeconomic status (SES).

Methods The inclusion criteria were as follows: studies assessing vaccination uptake according to education, income, occupation/employment, or area-level deprivation; any country (general populations), or universally recommended routine vaccination (according to the World Health Organization); and synthesising qualitative or quantitative studies published after 2011. The search strategy was piloted in Medline, and then executed in seven additional databases. The results were deduplicated, screened by title and abstract, and then assessed for eligibility using the full text. Each stage was performed by reviewer one, and a sample of 10% checked by reviewer two for consistency. Included reviews underwent data extraction, quality assessment (AMSTAR-2), and narrative synthesis according to country-context. The extracted mechanisms were mapped onto a framework of patient-centred access to vaccination.

Results After deduplication, 9,163 reports underwent title and abstract screening, leaving 119 full texts to be assessed for eligibility. Overall, 26 studies were included in the umbrella review. Evidence for lower uptake amongst lower SES individuals was found in all 26 reviews. However, the majority of these (n=17) showed mixed results, where inverse associations were also identified (lower uptake amongst higher SES groups, and/or higher uptake for lower SES groups). Those that explored high-income countries had a greater prevalence of mixed findings than those focusing on low/middle-income countries. The two most frequently cited mechanisms were individual-level determinants related to access or ability to understand vaccination information, and trust or confidence in vaccination or vaccination providers. These mechanisms were often understood by review authors as varying by level of education.

Conclusion We find socioeconomic differences in routine vaccination uptake, but the association did not always follow a clear gradient. Whilst education may be associated with uptake globally, our study indicates that its role appears to vary by country-context. Testing the mechanisms which link SES and vaccination uptake is required to fully understand the differences. A limitation of this study is the overlap of included systematic reviews, where a primary study has been analysed on more than one occasion.