**1. Introduction**

The Sustainable Development Goals (SDGs) place great emphasis on equity with a shared commitment to "leave no one behind". However, surveying the SDG indicators and related targets reveals that they place their measurement focus on national averages rather than disadvantaged or marginalized populations. Reducing child and maternal mortality, ending the epidemics of AIDS, tuberculosis, malaria, and neglected tropical diseases, and raising the coverage of essential services will all require health systems to reach disadvantaged and hard-to-reach populations suffering from a disproportionately high burden of morbidity and mortality. Therefore, direct measures of, and focus on, communities left behind are critical for the design of equitable health programs and for the success of the SDGs.

Immunization has one of the highest coverage levels of any health intervention [1] and therefore can be a pathfinder for other services and interventions. Immunization also provides substantial health and economic benefits, with an estimated 50 million future deaths averted through immunization activities in 2000–2019 [2], and USD 26 in economic benefits through averted costs of illness for every USD 1 spent on immunization between 2011 and 2020 [3] in low- and middle-income countries (LMICs). However, globally, in 2021, over 18 million infants failed to receive even the first dose of the basic diphtheriatetanus-pertussis-containing vaccine (DTP1). These zero-dose children are markers of missed communities facing multiple deprivations, with two-thirds of zero-dose children living below the international poverty line of USD 1.90 per day [4]. Reaching them with

**Citation:** Hogan, D.; Gupta, A. Why Reaching Zero-Dose Children Holds the Key to Achieving the Sustainable Development Goals. *Vaccines* **2023**, *11*, 781. https://doi.org/10.3390/ vaccines11040781

Academic Editor: Eduardo Osuna

Received: 28 February 2023 Revised: 24 March 2023 Accepted: 29 March 2023 Published: 31 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

immunization services can connect them and their families to the health system and other services, and all the health, economic, and social benefits that come with that. This includes poverty reduction (SDG1), better nutrition (SDG2), improved educational outcomes (SDG4), and reductions in inequalities (SDG10). In this Perspective, we explain how a focus on zero-dose children offers a pragmatic entry point for designing and reinvigorating programs and systems to achieve immunization commitments made by countries through the Immunization Agenda 2030 (IA2030) [5], including a 50% reduction in zero-dose children by 2030, and more broadly, to ensure that the aspiration of SDGs to leave no one behind is achieved.
