**5. Conclusions**

Recognizing where exceptional progress in reducing zero-dose children has occurred is the first step toward better understanding what countries did to attain such improvements. Such insights then can inform strategy adaptions to other settings, and further reinforce successful strategies in places that achieved large reductions historically but still have large populations of unvaccinated children today. Characterizing pathways to greater vaccination equity, as well ensuring mechanisms by which effective knowledge translation and cross-country learning can be supported, will strengthen efforts toward ensuring all children can fully benefit from vaccines.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/vaccines11030647/s1, Table S1. Initial countries considered for the present

analysis but were excluded for not meeting inclusion criteria; Figure S1. National and subnational trends in the prevalence of no-DTP children, 2000-2019, in the Democratic Republic of the Congo (A), Ethiopia (B), India (C), Bangladesh (D), and Burundi (E); Figure S2. Subnational no-DTP prevalence in Nigeria (A) and Ethiopia (B), 2000–2019.

**Author Contributions:** Conceptualization and Methodology: N.F., G.C.C., G.I., D.E.P. and H.W.R. Formal Analysis: N.F. Writing—Original Draft Preparation: N.F.; Writing—Review and Editing: N.F., G.C.C., G.I., D.E.P. and H.W.R. Visualization: N.F. and D.E.P. Supervision: G.I. and D.E.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Vaccine estimates used in this analysis are currently unpublished estimates from the Institute for Health Metrics and Evaluation (IHME). Data may be shared upon request to the corresponding author and IHME.

**Acknowledgments:** We thank Jonathan Mosser, Emily Hauser, Sam Rolfe, and Jason Nguyen from the Institute for Health Metrics and Evaluation for providing the estimates of childhood vaccination used in this study. We thank the following individuals from Gates Ventures, for their programmatic support and assistance: Patrick Y Liu, Jordan-Tate Thomas, Rowan Hussein, and Ryan Fitzgerald.

**Conflicts of Interest:** N.F., G.I. and D.E.P. are paid employees of Gates Ventures, which coordinates the Exemplars in Global Health (EGH) program. N.F., G.I., and D.E.P. previously worked at the Institute for Metrics and Evaluation (IHME): September 2008–2011 and February 2013–June 2022 for N.F.; August 2014–July 2020 for G.I.; and June 2009–February 2019 for D.E.P., G.C.C. and H.W.R. are employed by Gavi, the Vaccine Alliance. They had total freedom to express their views, which do not necessarily reflect those of Gavi.

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**1. Introduction**

Arc welding technology is one of the most widely used welding methods in industrial production at present, having the advantages of energy concentration, high efficiency, easy realization of automation and wide range of material applicability [1,2]. It has already been successfully applied in the joining of steels [3], aluminum alloys [4], nickel alloys [5], copper alloys [6], high entropy alloys [7], and so on. Additionally, industrial automation is an important direction for the future development of the manufacturing industry [8,9]. To meet the demands of productivity, pulsed gas metal arc weld (GMAW) is a good choice in industrial automation and robot welding for the advantages of controllable heat input, all position welding and no spatter [10–12]. In addition, the droplet transfer process, a research hotspot around the world, plays a crucial role in the welding quality. In pulsed GMAW, "one droplet per pulse" (ODPP) has been recognized as the most ideal transfer mode by many researchers [13–15].

The pulse peak current (*I*p) and pulse peak current time (*t*p) are generally regarded as the main parameters that affect ODPP mode. Moreover, the power law relationship (*I* n p*t*p = constant) is applied in lots of studies to determinate the peak current and time [16–18]. Additionally, Wu [15] put forward a six-parameter pulse waveform and obtained a mode of ODPP with lower heat input, proving the important influence of the droplet-detachment current and time on the droplet transfer. In addition, the base current is usually very small, and only plays the role of a stabilizing arc. As a result, a projected or spray type metal transfer at low average current can be obtained in pulsed GMAW.

Additionally, under the condition of short arc and low current, short circuiting transfer is also applied using constant voltage direct-current (DC) welding power source. However, the spatters are inevitable in short circuiting transfer, because of the electric explosion of the liquid bridge before the arcing period [19,20]. Compared with short circuiting transfer, the arc length could be easily adjusted, and no spatter welding seam could be obtained using pulsed current, leading to the advantages that the step of removing spatter on the base metal can be avoided and the manufacturing efficiency can be increased.

However, the base current time (or datum current time) has not been regarded as an influential parameter for ODPP in previous studies, but rather a parameter for adjusting the arc length. As a matter of fact, the arc length directly affects the shape of the arc, as well as the heat transfer and heat dissipation mode of the droplets [21–23]. For example, a certain arc space is required for a droplet from growth to detaching the wire. If the arc length is too short to provide sufficient space, the droplet would contact the molten pool but still on the wire, contributing to a short circuit [24]. However, most of the current research focuses on how to stabilize the arc length, while the study of how to choose the suitable arc length and the effect of different arc length on droplet transfer is ignored.

Therefore, in this work, different arc lengths were obtained by only changing the base time and fixing the other parameters. Then, the weld bead formation and the process of droplet transfer under different arc lengths were investigated. Eventually, the strategy of choosing suitable arc length of pulsed GMAW was found. The research results are helpful for optimizing the welding parameters, improving the quality of weld formation, and promoting the further application of pulsed GMAW in industrial automation production and robot welding field.