**3. The Special Issue: Monitoring Inequalities and Understanding Drivers; Sharing Experiences and Impact of Equity-Focused Interventions**

In this Special Issue, we bring together research and evaluation on Inequality in Immunization to contribute to growing evidence and insights on monitoring immunization inequalities and understanding drivers of coverage, as well as pathways towards enhancing and sustaining equity in immunization. The Special Issue features research, reviews, and commentaries that span a range of immunization topics and populations. While there is an emphasis on childhood vaccinations [15–18]—exploring inequalities in DTP and measles-containing vaccine (MCV) coverage [19–23] and patterns of inequality in unvaccinated or zero-dose children [24–29]—contributions also cover inequalities in adult immunization [30], including protection of pregnant women and their newborns against tetanus [31] and COVID-19 vaccination [32,33].

An encouraging observation while putting together this Special Issue has been the use of a variety of data sources to assess immunization inequalities. Studies have made use of traditional sources of immunization data like administrative data [19,23,32] and population surveys [18,21,22,27,29,30] (including Demographic and Health Surveys and/or Multiple Indicator Cluster Surveys [15,20,31]), while several other studies explored the potential of novel sources such as geospatial data [24,25], electronic immunization registries [34], dialogues [16], country appraisals and reports [35], and funding proposals [26]. Three review studies relied on synthesis and structured analyses drawing from a multitude of existing studies [17,33,36]. Indeed, the diversity of data sources represented across the articles of this Special Issue points to greater availability of data, and, critically, the innovative use of these data to delve more deeply into inequality analysis and inference. This is a practice that is welcome and will be key to generating new insights into immunization inequalities and progress in this area.

This collection of articles makes important contributions to understanding dimensions of immunization inequality—that is, the diverse demographic, socioeconomic, or geographical characteristics that define populations who are advantaged and disadvantaged, while also highlighting the frequent co-occurrence and compounding of multiple deprivations. As dimensions of inequality present themselves and intersect in dynamic ways, our modes of understanding must keep up. Several studies in this Special Issue examined multiple dimensions of immunization inequality [18,19,21,27,29,31,33,36], while others focused on specific dimensions, such as gender barriers [20,34] or socioeconomic status [15,30,32].

There is an established and growing evidence base on exemplars of action on immunization equity, particularly among Gavi-supported countries, but also in other contexts with successful immunization programmes [26,35,37]. This research offers important insights into what strategies are being deployed to reduce inequalities ("the what") [35,37,38], while starting to shed light on how gains in immunization equity were achieved ("the how") [39,40]. There is, admittedly, a long way to go in expanding the evidence base in this latter "how" category and what is required to feasibly implement these strategies, including costs and drivers of sustained change.

Taken together, the articles in this Special Issue spotlight some of the most current and pressing areas of interest in the topic of inequality in immunization, though the absence of certain themes is notable. For instance, analyses pertaining to conflict or fragile state contexts were lacking. Several of the contributions to this Special Issue acknowledge the need for greater reliance on qualitative methodologies and longer-term engagement with affected populations. These approaches are vital to developing contextually tailored monitoring and planning mechanisms that foreground equity in the face of changing or worsening relationships of security or trust.

Our Special Issue launch in April 2023 is timed to coincide with the 2023 World Immunization Week, which this year focuses on the theme of 'The Big Catch-Up' [41]. This initiative calls for the year 2023 to be a coordinated, intensified period of vaccination catch-up—to close immunity gaps among persons missed during the pandemic—involving recovery and strengthening of immunization services. "The Big Catch-Up" is a concerted effort intended to be driven by communities and countries, regions working in partnership with IA2030 institutions and structures, to which equity is integral [42]. This requires vigilance to change local realities with more sensitive and flexible metrics and methods to understand the complex, intersectional and dynamic nature of inequities, alongside concerted collaboration, context-tailored, and community-driven responses that chip away at inequities. In short, it is crucial that we hold on to equity in immunization in our efforts to catch up on the IA2030 goals to realize the vision of a world where everyone, everywhere, at every age, fully benefits from vaccines to improve health and well-being [9].

**Funding:** The Special Issue was funded in part by Gavi, The Vaccine Alliance. Beyond the individual contribution of HLJ, who is a Gavi employee, the funder had no role in the writing of the editorial.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest. The authors alone are responsible for the views expressed in this publication and they do not necessarily represent the views, decisions or policies of their institutions.

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