**2. Definition of a Zero-Dose Child**

The term "zero-dose child" refers to a child who has failed to receive any routine immunizations. For monitoring purposes, it is a measure of whether a surviving infant has received at least one dose of the DTP vaccine. The focus on routine immunization as opposed to doses received through immunization campaigns is intentional, as the indicator aims to measure the reliable reach of immunization services, extended sustainably to reach all communities to achieve universal coverage. The choice of the lack of DTP as the indicator is a pragmatic one. While surveys can measure whether a child has received no doses of any vaccines, most administrative data systems report aggregated data that do not allow for the joint measurement of the receipt (or lack of receipt) of different vaccines. Vaccines other than DTP could also be considered as proxy indicators. DTP is preferred for global IA2030 monitoring, as the measurement of measles and polio vaccine coverage through household surveys may contain a mix of routine and campaign-delivered doses, and the BCG vaccine is not in every country's national schedule and is delivered through diverse platforms. At the population level, low coverage of the DTP, BCG, MCV, or polio vaccine tends to be highly correlated with the prevalence of children who have received no immunizations, and therefore, the choice of the metric is less important than the programmatic aim of identifying and reaching missed communities with dependable immunization services [6].
