Childhood immunization is considered one of the most proven, successful, and cost-effective child survival public health interventions (1). Globally, significant progress has been made in vaccination coverage since 1974 after the initiation of the Expanded Program on Immunization (EPI), which dramatically reduced 2–3 million annual avertable child deaths from vaccine-preventable diseases (VPDs) (2). Yet, according to the World Health Organization (WHO), an estimated 1.5 million children die every year from VPDs, mostly in low- and lower-middle income countries (LMICs) of Africa and Asia (3), indicating that compliance with the recommended vaccination schedule is a challenge for fragile healthcare systems. Hence, the Global Vaccine Action Plan (GVAP) 2011–20 set a target to reach 90% coverage of EPI vaccines at the national level by 2020, following which the Immunization Agenda 2030 (IA2030) also aims to leave no one behind routine immunization in each country by 2030 (4). Despite these commitments, various persistent and new challenges, such as lack of infrastructure and/or required human resources, high dropouts, shortage of supplies, vaccine stock out, cold chain breakages, lack of immunization culture, and even the emergence of coronavirus disease (COVID-19) (5–7), result in delayed, incomplete, and low vaccination coverage, especially in LMICs. Thus, to evaluate the progress towards achieving these targets, it is important to clearly understand the coverage gaps both before and during the onset of COVID-19.

Age-appropriate vaccines have, thus, been identified as an essential health intervention for child protection against mortality and morbidity, which has been closely looked at in many countries in recent years (8–12). The design of its schedule is based on when the immune system can safely respond to the vaccine and age-related diseases that can impact child morbidity and mortality, through which it can reduce the risk of infection of VPDs among infants and children in early life (13). Thus, incorrectly timed vaccinations may partially explain the persistence and reemergence of VPDs (11, 14). However, some reviews reported lack of concordance in the definition of age-appropriate vaccination (10, 11). Previous studies suggest that only a few children receive timely vaccines in their recommended age, even with high vaccination coverage (11, 12, 15–18). In addition, a secondary analysis of Demographic and Health Surveys (DHS) and Multiple Indicator Cluster Surveys (MICS) also indicated maternal education, place of residence and socioeconomic status as most common factors associated with lower vaccination coverage (12, 19). However, studies revealing coverage trends combined with projections in coverage among different LMICs are scarce.

To date, no study has assessed the progress towards age-appropriate vaccinations in LMICs at regional and national levels, as well as by place of residence and wealth index. Since the emergence of COVID-19, national immunization programs in most LMICs have been affected, resulting in an additional burden on the health systems, halting their progress, and putting the provision of age-appropriate vaccination coverage at risk. Moreover, owing to economic constraints, health systems in LMICs are expected to experience a reduction in development assistance for health (DAH) and health worker density to some extent. Therefore, the objective of this study is to estimate recent trends in the coverage of age-appropriate vaccinations and to derive projections of these indicators up to 2030 for 41 LMICs at regional and national levels, as well as by place of residence and wealth index. Second, we evaluated the impact of decreasing DAH and skilled health workforce (SHW) due to COVID-19 on age-appropriate vaccination coverage.

This is the first study to provide a comprehensive assessment of age-appropriate vaccination coverage among children under 5 years in LMICs. The findings suggest that the coverage of age-appropriate vaccinations among children has increased substantially at the regional and national levels, as well as by area of residence and socioeconomic status in most countries between 2000 and 2030. The largest increase in vaccination coverage was observed in East Africa, followed by South and Southeast Asia. However, of the 41 LMICs, only 10 countries would achieve 90% coverage of the BCG vaccine by 2030, five of DTP3, three of polio3, and none of MCV vaccines. Wider coverage gaps between urban–rural and poorest-richest groups were observed in several African countries. However, a narrower coverage gap was found in Burundi, Cambodia, Rwanda, Sao Tome and Principe, as well as Sierra Leone.

Our study suggests that age-appropriate vaccination coverage is predicted to increase at national and regional levels between 2000 and 2030, corresponding to the initiation of EPI by the WHO and GVAP, as well as substantial financial support for vaccination programming generated by the introduction of Gavi, the Alliance vaccine, that helped in reducing VPDs and under-five mortality (25). Despite these investments and policies, our study predicted several countries from Africa and South Asia, to have relatively slower progress in age-appropriate vaccine coverage during 2020–2030. This finding is similar to WHO and UNICEF estimates of national immunization coverage (WUENIC) 2020, which showed that more than 60% of unvaccinated children live in 10 countries in the same regions (26). One possible reason could be the availability and acceptability dimensions, such as long distance to health facilities, immunization administration outside a health facility, vaccine hesitancy or refusal, and shortage of health workforce (27). Another reason for barriers to accessing vaccination in the listed countries within an appropriate time frame could be civil war, political instability, and ethnic conflict (28, 29). Hence, safe and better access to health facilities, greater provision of childhood immunization services, and improved attitudes and knowledge of mothers regarding immunization can enhance coverage and utilization of childhood immunization services in LMICs. Therefore, effective strategies must be adopted by stakeholders and policymakers to expand the coverage of age-appropriate vaccines, particularly for those living in rural and hard-to-reach areas.

Conversely, a high coverage of age-appropriate vaccines with relatively fewer inequalities has been observed in several countries, such as Burundi, Cambodia, Rwanda, Sao Tome and Principe, and Zimbabwe. In a short period of time, Bangladesh, Cambodia, Nepal, Rwanda, Gambia, and Zimbabwe have had successful vaccination coverage, which is attributable to a combination of different strategies such as multi-year national immunization plans, advocacy for better domestic funding for immunization, and evaluation of EPI activities including joint field missions, sensitization of health workers on VPD surveillance, tracking vaccination defaulters, and data quality improvement (30, 31). Cambodia, for instance, introduced a catch-up campaign aimed at children who had missed or had not received doses of vaccines for any reason (32). Thus, concerted efforts to mitigate accessibility through intensive community mobilization and expanding basic health services will effectively help narrow the inequality gaps in resource-constrained settings.

Immunization programs are highly vulnerable to unprecedented public health emergencies at the subnational, national, and/or global levels, which can hinder efforts by disrupting the normal function of nations and potentially derailing past gains. A recent study indicated that routine immunization services have decreased in early 2020 due to the COVID-19 pandemic (5). Inadequate human resources, shortage of vaccine supply, and strict lockdown may explain why (5, 33). Our scenario-based analysis elucidated that these reductions or disruptions are not projected to downgrade the trends of age-appropriate vaccination by 2030. Because, though the recent outbreak of COVID-19 hampered the immunization schedules, majority of the LMICs are now leveraging pre-existing program strengths and incorporating protocols for safer vaccine administration (5, 33). For instance, health workers in Cambodia were reported of providing “village to village” and “door to door” services to maintain routine immunization service and protect Cambodians from COVID-19 (34). Therefore, international organizations and national government must increase advocacy for ensuring timeliness of childhood vaccinations by overcoming resource mobilization, staff displacement, low health-seeking behavior of the public due to fear of getting the infection by visiting health facilities, etc.

This study has several strengths. First, this is the first comprehensive assessment of the trends and projections of age-appropriate vaccination coverage at the regional and national levels, as well as place of residence and wealth index in LMICs. Second, this study estimates the required rate of increase for a country to achieve a 90% target by 2030. Third, it used a large number of nationally representative surveys, which improved the model validity of the indicators. Lastly, it used robust statistical tools, such as the Bayesian model, which allowed better use of available data and provided better estimates, especially for low data points. The study also has some limitations, most information was from home-based records which can introduce several biases. Moreover, this study excluded children without vaccination card and older children have less percentage of card seen. There might be a possibility of selection bias. Thus, this study was unable to adjust for these biases. Previous studies have shown that the direction and magnitude of these biases vary substantially (35, 36). The projections were made on the assumption that there would be no major change in the existing health system, which may be too strict as countries might introduce new strategies or other forms of health financing to increase vaccination coverage. Therefore, these projections cannot account for the unknown forthcoming impact of national or international health policies or other emergencies. Lastly, this study focused only on the LMICs of South Asia, Southeast Asia, and Sub-Saharan Africa and is not representative of all countries in these regions due to the lack of information on the date of birth and vaccination. It also did not allow the model to adjust for the COVID-19 pandemic, ethnic conflict, and civil war issues due to lack of data, which could potentially affect the predictions.

Substantial improvements have been made in age-appropriate vaccination coverage in LMICs during the past decades. However, progress stalled from 2000 to 2020 in many countries. Except for the measles vaccine, only four countries (Cambodia, Sao Tome and Principe, Rwanda, and Zimbabwe) are predicted to meet 90% vaccination coverage targets for BCG, DTP3, and polio3 by 2030. Wide urban–rural and poor-rich disparities in age-appropriate vaccinations will remain in many countries, especially Ethiopia, Mali, Niger, and Nigeria. These findings will contribute to the improvement of age-appropriate vaccination coverage and support policymakers in developing necessary control strategies with respect to delayed vaccination. Improving vaccine availability, increasing interaction with community health workers, and decreasing the coverage gap in underserved children by introducing appropriate policies and strengthening the health system have the potential to improve child survival through enhanced vaccine coverage and timelines.

Publicly available datasets were analyzed in this study. This data can be found at: the data that support the findings of this study are publicly available from the DHS Program at https://dhsprogram.com/ and MICS at https://mics.unicef.org/. SDI, GSH, and DAH were extracted from the IHME database on the Global Health Data Exchange website (https://ghbx.healthdata.org) and SHW from the WHO Global Health Observatory Data Repository (https://www.int/data/gho). The data was extracted until September 2021. During this period, we extracted 174 databases from DHS and MICS.

This study was a secondary analysis; therefore, ethics approval and consent to participate was not required. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation in this study was provided by the participants' legal guardians/next of kin.

MI: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. MMR: Writing – review & editing, Validation, Methodology, Funding acquisition. MSR: Writing – review & editing, Validation, Methodology. SA: Writing – review & editing. MA: Writing – review & editing, Methodology. MH: Writing – review & editing, Validation, Resources.