The conceptual framework for analysing the impact of armed conflict on vaccination coverage is structured around theories. First, the assumption that armed conflicts disrupt health services which is expected to directly impact vaccination coverage. Second, protection of the population from the effects of armed conflicts should ensure the continuity of vaccination program delivery. Finally, it is projected that resource mobilization, allocation and accessibility during armed conflicts will interact with the provision of immunization services. This approach recognizes the complex and multifaceted nature of the question, where the intensity of armed conflict acts as a disruptor to public health infrastructure and service delivery. The intensity of armed conflicts is typically assessed using casualty-based indicators (e.g., conflict-related deaths per 100,000 population), event-based indicators (e.g., frequency and type of violent events), duration-based indicators (e.g., years of conflict exposure), and displacement-based indicators (e.g., number of internally displaced persons). The vaccination coverage is the proportion of the target population that has received the recommended vaccines within a specific period of time and may vary depending on age groups, types of vaccines and geographical areas. Health services are defined by both infrastructure, physical and organizational structures with sufficient supplies and equipment, and human resources, which are characterized by the healthcare workforce involved in the delivery of vaccination programs, in the necessary numbers and with adequate training. Accessibility to immunization services refers to the availability of vaccines in conflict-affected areas, while utilization describes their actual use. The presence of vaccines does not guarantee their administration, as factors such as awareness, acceptance, perception of safety, and service provider availability significantly influence utilization. International organizations and non-governmental organizations (NGOs) provide humanitarian aid to maintain, increase, or develop immunization programs during armed conflicts, with the support varying depending on funding, population access, types of aid programs, and coordination. Those activities are often led or co-founded by national authorities.

The review may explore several variables that combine these key concepts.

This scoping review follows the methodological framework outlined by Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) ² to identify relevant and available research on the topic, summarize research findings and identify gaps in the literature as described by Arksey and O’Malley. ³ The review adheres to established guidelines for scoping reviews to ensure rigor and transparency in the methodology. The research question was defined by using PCC method ⁴ “How armed conflict intensity influence vaccination rates in children under 5 years old? And which factors contribute to the vaccination coverage evolution in armed conflicts’ situations?”. A comprehensive search strategy was developed between February and April 2024 to identify relevant literature using electronic databases including PubMed (MEDLINE), Swisscovery and Google Scholar. A combination of keywords, MeSH terms, and Boolean operators related to armed conflict, vaccination coverage on specific vaccines or all vaccines provided by the country’s Expanded Program on Immunization (EPI), and target population health are being used to retrieve articles. The search strategy was adapted to suit the syntax and capabilities of each database.

In addition, a web search was conducted on various non-governmental and international organizations websites to identify grey literature. Websites from WHO, UNICEF, Global Alliance for Vaccine and Immunisation (GAVI), health ministries and other organisations responsible for and/or dedicated to the immunisation of populations affected by armed conflict were screened. The location of populations affected by conflict was defined based on a combination of the World Bank List of Fragile and Conflict-Affected Situations and the UN Office for the Coordination of Humanitarian Affairs (OCHA) Response Plans with a minimum of 5 consecutive years of being classified as fragile and/or with the presence of a peace-building/peacekeeping mission from 2006 to 2023 and/or humanitarian appeals from 2003 to 2023 ( Table 3).

The detailed search strategy conducted between February and April 2024 is shown in Table 1 and Table 2.

The main inclusion criteria were that the articles had the following: 1) exposure: time (published between 2003 and 2024, studies with data on VR during active conflict), location (conflict affected countries, fragile countries as established in the list available on Table 3) and population (children under 5, affected by armed conflict; including those living in conflict area, IDPs and refugees). The articles should be in a 2) language perfectly understood by the reviewer (either French or English). 3) Types of vaccines for whom the vaccination coverage was described (specific vaccines such as BCG, vaccine containing measles 1st and 2nd dose, DTP 1st to booster, and polio 1st to 3rd dose and/or all vaccines available by the Enlarged Vaccination Programme (EVP) of the country), 4) ethic (validation or not of an ethics committee) and 5) the quality of the methodology (detailed in the article regardless of the study design) were also criteria. Articles that were not relevant to the research question, and lacked sufficient details on vaccination coverage or its connection with conflict were excluded. If the full article wasn’t accessible, authorization to use the document not provided by the author or if appeared twice during the research it wasn’t included. Articles undergo a single reviewer screening process based on titles and abstracts to assess eligibility for full-text review based on predefined eligibility criteria. In that second stage of the research ( Table 2), the previously selected articles were screened again according to the following criteria: 1) research or documents that not specifically focusing on childhood routine vaccination coverage such as COVID-19; 2) that focus on the epidemiology of vaccine-preventable diseases or other infectious diseases; 3) that not specifically address the impact of conflict on vaccination coverage; 4) that focus on vaccination program outcomes; or 5) that do not mention the intensity of the conflict. Some relevant information for the topic that was identified during the literature research process but did not meet the inclusion criteria was still included in the analysis. No ethics approval was required since the study made use exclusively of anonymized data available in open-access mode.

The extraction of data is conducted to capture key findings, including the association between armed conflict intensity and vaccination rates and the mechanisms underlying this relationship. The selected articles were examined through question research and exported to Zotero software ( https://www.zotero.org/). The information, when available, was extracted according to a non-standard data extraction guide and a chart was created specifically for the purposes of this review on Microsoft Excel to record the relevant information from each article. During the charting, an additional 15 documents were deemed ineligible based on the same criteria. A narrative synthesis approach is employed to analyse and interpret the findings, allowing for a comprehensive understanding of the topic based on 35 documents. A flow chart of selection process for studies included in the review is available in the Extended Data.

This scoping review identified a variety of conflict indicators used to gauge the intensity of armed conflicts and their impact on vaccination coverage. Casualty-based indicators measure the human costs of conflicts, such as conflict-related casualties per 1000 population ²⁴ and battle-related deaths per 100,000 population, ²⁰ and provide standardized metrics for comparing conflict intensity across regions and different population sizes. The average number of deaths per year of conflict exposure and the number of conflict-related deaths within a kilometre radius of residence ^{15, 20, 25– 27} are used to estimate conflict intensity’s proximal impact on communities categorized into less, more than median, or high numbers, which influence vaccination service accessibility and utilization. Event-based indicators capture the frequency and regularity of violent incidents. Metrics such as the list and number of conflict events and the conflict event-days rate ⁹ quantify the occurrence of violent activities like destruction of civilian assets ⁵ or airstrikes per 1000, ¹³ serve as critical markers of the direct threats faced by vaccination programs and healthcare providers during conflicts. These events often hinder routine vaccination services and create barriers to accessing healthcare facilities, impacting vaccination coverage rates. Duration-based indicators provide insights into the temporal dimensions of conflict exposure experienced by individuals within a particular area, as well as the long-term health and economic repercussions of extended conflicts. The number of years between the earliest and latest violent events, ²⁸ period, ⁶ and total length of war ²³ in an area reflect prolonged exposure to conflict, which can have cumulative adverse effects on the sustainability of routine vaccination services over time. Database and reporting-based indicators include comprehensive datasets such as UCDP ^{17, 25, 26, 30} indicators and reports from conflict monitoring systems like the Bangsamoro Conflict Monitoring System (BCMS) ¹⁴ and others such as the International Institute for Strategic Studies ¹³ and the Armed Conflict Location & Events Data (ACLED). ²⁶ These sources offer detailed accounts of state conflicts, one-sided violence, and non-state conflicts, enabling a broader understanding of various conflict types and their potential impact on public health. Threshold-based indicators categorize conflict intensity based on specific thresholds of battle-related deaths. The presence of conflict as a binary variable and categorical conflict intensity. ²⁷ The intensity of the conflict was measured based on several criteria, which help to differentiate regions experiencing different levels of conflict intensity, based on the number of battle-related deaths, ^{20, 30, 31} which is crucial for assessing varying impacts on vaccination coverage. Indirect Impact indicators assess the broader effects of conflict on health zones, using “all other damaging war mechanisms” ²⁶ and measures like annual conflict death rates, ⁹ “non-combatant mortality” ³² and monthly insecurity rates. ³³ These indicators reflect both direct and indirect impacts, providing a more nuanced understanding of how conflict intensity affects routine vaccination services.

Key conflict-related factors impacting immunization

As documented by the number of occurrences in the selected literature, this review explained that conflicts tend to have a profound impact on healthcare systems, ^{34, 35} often leading to their fragilization as resources are diverted to finance the war economy. During times of extended, high-intensity conflicts, funding that would typically support healthcare is reallocated to sustain military efforts. This reallocation of resources may transfer the government’s duty of care to other actors, such as NGOs, and further impact the socio-economic status of the affected population and the organization of vaccination programs. ³⁶ In contrast, at lower levels of conflict intensity, governments and international organizations may respond to health crises by increasing the supply of healthcare in affected areas. ^{27, 31} Worldwide, conflicts frequently cause extensive damage, ranging from looting and power outages to the partial or complete destruction of healthcare facilities, ^{37, 38} which can disrupt cold chain management, ³⁹ vaccine supply chains, ⁴⁰ including transportation, ³⁸ thereby immunization services. ^{5, 34} For instance, in areas where moderate to severe insecurity led to non-functional district health services, outbreaks of vaccine preventable disease broke out, ¹¹ compounding the challenges faced by healthcare providers to maintain their performance, particularly in hard-to-reach areas. ⁴¹ More so, the more intense the conflict is, the “more priority is paid on the treatment and management of casualties involved with less attention on preventive care”. ⁴³ The killing, ^{11, 16, 34, 36} abduction ³³ and harassment from both state and non-state armed groups, ^{5, 42} of healthcare personnel are reported, leading to a substantial decrease in the availability and motivation of health workers to reach those conflict-affected populations in certain highly insecure zones. Fear of targeting or collateral damage may cause staff to flee their positions, ^{5, 16} potentially leading to a brain drain in the health system, ⁴¹ while the distribution of health staff in some far-flung and conflict-affected zones of countries remains uneven. ¹⁶ In conflict zones where vaccination rates have improved in spite of these challenges, local indigenous vaccinators, while in some places it is complicated to retain competent staff, ¹⁶ who have long-standing relationships with their communities, play a crucial role. Their acceptance by armed groups allowed them to continue their duties with minimal restrictions. ⁷ In some cases, negotiations and strategic efforts have led to a gradual improvement in immunization in different areas. ⁴³ With human resources retention issues, non-governmental organizations (NGOs) have increasingly filled the gaps, providing critical healthcare services in conflict-affected areas. ¹⁶ Politics in the context of armed conflicts often play an important part in influencing immunization efforts. For instance, governmental endorsements of polio eradication campaigns in areas controlled by armed groups regularly backfired, compromising the security of the vaccination teams. ⁴⁴ Indeed, armed groups had regularly banned vaccinations campaigns, mistrusting those activities for foreign espionage missions. In the past, similar campaigns were tooled by intelligence agency agenda, as well as a protest against attacks and leverage in negotiations for cease-fire. ⁴⁵ Geopolitical actions and international media attention drowned in immunization efforts can severely impact such fragile children’s health initiatives. ³¹ Separating political agendas from religious convictions and motivations is important. Indeed, some religious leaders opposed the ban and officially encouraged immunization. ³¹ In areas of conflict that are not under government control, such as besieged areas, some communities may be subject to discriminatory policies and neglected health care services, as vaccine supplies are blocked or infrastructures are more severely damaged. ^{36, 38} Long lasting violence disrupts the affected populations’ health seeking behaviours and their daily lives ²⁶ and complicates the consistent knowledge and timely use of routine vaccination services. ²³ Research indicates that a population’s likelihood of dropout increases with the duration of conflict. ²³ Even less intense, sporadic attacks hinder reconstruction efforts, exacerbate instability ²⁶ and increase vaccine hesitancy. Several obstacles to vaccination in conflict zones have been identified, including gaps in knowledge and beliefs about vaccines, attitudinal barriers, and logistical challenges. ⁴⁶ These issues are often worsened by the danger and instability of armed conflicts which may cause the population to suspend their travel to health facilities or avoid services all together, fearing being associated with one or another party to the conflict. ⁴⁷ “A greater appreciation for life” and a heightened instinct to protect newborns in the midst of war ⁴⁸ were hypothesis explaining some cases trend of vaccination rate’s increase. The psychological toll of armed conflict remains unarguable. For instance, grieving from a family member triples the risk for children having incomplete immunization ²³ while “home demolitions and land confiscation of land” ³² contribute to psychological stress, further hindering timely healthcare access and vaccination efforts. ³² During crises, the primary concern of affected populations often shifts to securing essential needs like food, shelter, and safety, as well as recovering from personal losses and property damage. ⁴⁹ As a result of economic, social and psychological constraints, the immunization of children is often deprioritized. Conflict-related displacement or forced migration, whether internal or across borders, disrupts access to healthcare and impacts routine vaccination efforts. ^{34, 50} In certain regions, insurgencies force many to seek refuge in temporary camps, making it difficult to maintain vaccination schedules. ¹⁰ This displacement creates hard-to-reach populations, exacerbating public health challenges. ³¹ For instance, ongoing conflicts have led to continuous relocations, preventing planned access to health care, thereby disrupting routine immunizations. ⁴¹ In protracted conflicts, ³¹ despite intensity fluctuations through the years, the ongoing territorial fragmentation, numerous checkpoints, blockades and different types of travel restrictions such as not knowing when travel is authorized, ⁵¹ severely limit medical supply to reach civilians, and impact healthcare spending and access, irrespective of socio-economic status. ⁵² Obstructions to humanitarian access, such as the limited access or denial of entry to aid workers and relief supplies, worsen health inequalities ⁵³ and might cause major disruptions in the health system. ²⁷ Instances in several parts in the world ^{41, 54– 56} illustrate how various armed groups and state actors’ restrictions and barriers on aid agencies’ access and distribution have intensified the Expanded Program of Vaccination (EPI) challenges. Security concerns further complicate aid delivery, making it challenging to establish target populations ⁶ and access certain areas. ^{57, 58} Indeed, the often-sporadic nature of conflict zones creates volatile conditions, limiting the ability of organizations to assess, plan and operate effectively. ⁶ It also poses risks to the safety and well-being of healthcare workers, particularly in regions where working conditions are precarious and resources are scarce and based on gender. ⁶

Other key factors impacting immunization in conflict areas

Studies show that higher education levels, particularly among mothers, improve awareness and adherence to vaccination schedules. ⁵⁹ Educated mothers are more likely to ensure their children receive timely immunizations, even during conflicts. ^{15, 25} It appears that education plays a protective function because, although conflict is more common in educated populations, its negative effects on vaccination coverage are more noticeable in less educated groups. ^{15, 19, 21, 28} Higher household assets and wealth quintiles are associated with increased rates of timely childhood vaccination. ^{25, 27} The positive correlation between income and complete immunization indicates that having stable finances might enable better access to vaccination despite being statistically more exposed to conflicts. ^{19, 28} However, the adverse effects of conflict on immunization can still be significant among wealthier populations, indicating that although wealth might partially mitigate some conflicts’ consequences, it does not entirely shield against all disruptions. ¹⁵

Familial characteristics such as the presence of multiple children, maternal age, marital status, and coming from polygamous households negatively impact a child’s likelihood of full immunization and vaccination adherence. ^{22, 27} Cultural and familial restrictions on women’s autonomy to seek healthcare may further complicate immunization efforts. ²⁶ Community and religious leaders play a pivotal role in shaping health behaviours. Lack of discussion about immunization within churches and communities significantly increases the likelihood of under-immunization. ²² Effective communication and endorsement by influential leaders can enhance vaccination coverage by fostering trust and awareness within communities. Geographical location is another critical factor that significantly affects access to immunization services. Rural areas experience more substantial barriers to timely childhood vaccination compared to urban areas, ²⁵ which despite facing their own set of challenges, generally have better access to healthcare facilities ²² than rural settings. ^{25, 60} Children in rural areas or those born at home are more likely to miss vaccinations compared to their urban counterparts or those born in healthcare facilities. ²³ In some contexts, outreach activities and the frequency of services also impact vaccination rates, with facilities not conducting outreach or offering infrequent vaccination services showing lower rates. ^{7, 13, 22} Perceived quality of services is also a recognized factor influencing health-seeking behaviour. ²⁶ Climate change, in addition to armed conflict, exacerbates health challenges by increasing the frequency of extreme weather events and might cause further difficulties in accessing routine immunization services. ⁶¹ Vaccines are thermolabile and inadequate climate adaptation may compromise their effectiveness. ¹¹ Displacement due to climate-induced events further complicates vaccination efforts as populations will move or settle elsewhere, making it difficult to maintain vaccination schedules. ²²

Inconsistent health financing, as well as inefficient government health coverage or international donors’ programs resulting in catastrophic out-of-pocket expenses, may lead to inadequate service provision and often result in under-immunization. ^{32, 62} Refugees and internally displaced persons face additional barriers, with access to healthcare, frequently disrupted during periods of conflict in spite of the free-of-charge services, if provided. ^{32, 49} Despite the Health Cluster mechanism’s recognition for enhancing service delivery, challenges remain in decision-making, leadership, contingency plans and prioritization of resources, which affect consistent and equitable immunization access. ⁴⁹ Thus, successful health sector interventions necessitate inter-institutional and inter-sectoral coordination, with NGOs and UN agencies collaborating closely at various governmental levels to develop strategies and optimize service delivery in conflict zones. ⁶ Funding of vertical programs such as specific vaccination programs is common as those programs offer rapid and quantifiable results with a concrete impact. However, it may provide false beliefs about the long-term efficiency of EPI. ⁶³ The impact of conflicts on vaccine-seeking behaviours in zones that are already fragile is complex and variable. ^{64, 65} “A human rights approach” ⁶⁶ and efficient partnerships between health actors emerge as crucial strategies to overcome these multi-faced variables. ⁶⁷

Today, the International Committee of the Red Cross (ICRC) recognizes over 120 armed conflicts globally, ⁶⁸ each with unique impacts on health services and children’s routine immunization. “Vaccines are one of the most impactful and cost-effective health and development interventions available today”, declared Professor José Manuel Barroso, Chair of the Gavi Board. ⁶⁹ Furthermore, robust immunization programs and a resilient health system not only lead to sufficient vaccination coverage but also significant economic benefits. ⁷⁰ Humanitarian aid has demonstrated effectiveness at specific conflict intensity levels. However, the literature identifies two major factors that hinder the achievement of acceptable vaccination coverage: each conflict party’s responsibility and obligation to maintain health systems and allow aid to reach the affected population. The legal framework for the protection of children in armed conflict is vast and unambiguous. The Geneva Conventions and Additional Protocols I and II ⁷¹ ensure assistance, medical care, and protection for children in both international and non-international armed conflicts, emphasizing the right to health and preventive care, including immunization. International laws, such as the Convention on the Rights of the Child ⁷² and various UN Security Council Resolutions, ^{73, 74} along with the Rome Statute ⁷⁵ and International Humanitarian Law, ⁷⁶ criminalize attacks on medical facilities and personnel, demanding safe passage and protection for those providing medical and humanitarian aid. Furthermore, states are legally required to disseminate and comply with the child-specific protection concepts of the IHL. ⁷⁷ The combined spirit of these regulations underscores the critical importance of safeguarding children’s rights to health in conflict zones, mandating special protections and ensuring unhindered access to medical and humanitarian aid. ⁷⁸ Nevertheless, there is “still room to strengthen and clarify the existing legal framework”, as “public health protections are a human right”. ^{78, 79} To this day, there is still continuous negotiation and advocacy for the establishment of humanitarian corridors and ceasefires to allow “operational freedom to vaccinate children in hard-to-reach places”. ⁸⁰ Since the onset of the COVID-19 pandemic, there has been a notable increase in research on the intersection of armed conflict and COVID-19 vaccination rates, as evidenced by a surge in publications in scientific databases such as PubMed, particularly in 2022. This trend highlights the critical issue of vaccine inequity in conflict-affected regions. The concepts of the “Last Mile” ⁸¹ and “Leave No One Behind” ¹ are therefore especially relevant in these settings.

The ongoing negotiations for an international pandemic treaty could establish vaccines as global public goods, ensuring they are funded and distributed equitably during pandemics. ⁸² However, defining this economic concept poses legal challenges, requiring a consensus among states on production and distribution principles. Patent restrictions and limited technological capabilities, along with political challenges pose barriers to addressing vaccine inequality. ⁸³ Successful initiatives such as the launch of the MenAfriVac vaccine showcase a model of public and private collaboration that encourages continued innovative approaches. ⁸⁴

Regarding vaccination coverage

The literature reveals significant challenges in accurately measuring vaccination coverage in conflict areas. Key issues include the absence of indicators for service quality, such as proper handling of vaccine storage conditions, ¹¹ and measures for age-appropriate vaccination, like delayed vaccine, ²⁷ timeliness ¹⁵ and comprehensive immunization histories. ²¹ Information on targeted immunization campaigns, ¹³ health sector reforms, ¹⁹ and political changes ^{32, 44} is often incomplete, missing or has an unmeasured impact. Data quality and availability are major concerns during armed conflicts, ^{8, 85} with frequent reporting gaps, ^{18, 59} inaccuracies, ¹⁷ and incomplete records, ⁹ especially from private sector sources. ¹⁹ Additionally, vaccination coverage information is often lacking for specific provinces, ¹⁹ obscuring subnational variations and regional differences, ^{18, 59} and multi-country studies may mask unique national dynamics. ²⁸ Other variables such as urban versus rural settings, ^{7, 59} population density, ²⁶ overlooked qualitative cultural factors, ²⁴ and other socioeconomic variables (including education and poverty) ^{7, 16} may confound the analysis. Factors like political instability, climate change, and human development indices ⁵⁹ complicate the situation further, as they often overlap with other public health emergencies, ¹⁷ making it difficult to isolate the impact of individual events. The implications of humanitarian organizations in such cases might not be captured ²⁶ and therefore may blur the results. The lack of data or their interpretation on migration and the displacement of populations ^{5, 9, 14} add another layer of complexity to an accurate assessment. Survey-based coverage data, ¹⁸ which often rely on caregiver recall, ²¹ can lack robustness ¹² and comparative groups. ⁵ Defining accurate target populations is challenging due to under- or over-reporting and uncertainties in population counts, affecting the denominators used in national estimates. ⁹

Regarding the indicators of conflict

Accurately assessing the impact of conflict on immunization coverage faces several methodological limitations. A major challenge is the inability to establish temporal relationships, ²³ as studies often fail to evaluate long-term effects. ^{24, 32} The measurement of conflict itself lacks standardization, ²² leading to inconsistencies across studies. This can result in the underestimation of conflict effects, ²⁷ particularly in regions without death registries ¹³ or where casualties are undercounted. ^{24, 29} Furthermore, data often exclude smaller or ongoing conflicts, ²⁸ as these do not meet the threshold of 25 deaths required by sources such as the Uppsala Conflict Data Program (UCDP). ²⁶

This exclusion results in a failure to capture all forms of fighting and destruction, ²⁶ and fails to adequately consider different types of violence, ⁶⁵ which may have varying impacts on public health. Additionally, the consequences of violence are not always exogenous, ¹⁴ and the data do not account for blockages, embargoes and restrictions of movement, which significantly affect access to health services and immunization efforts. ^{7, 13, 32} The characteristics of the conflicts may singularly vary within the same region and population during the time captured by the immunization data, making the causality factor difficult to analyse. ⁷ We also need to highlight the fact that not all information regarding the conflict is available in the media and some may remain confidential. ^{7, 14, 29} Indeed, the number of casualties is particularly sensitive for armed groups, either governmental or non-governmental, in times of armed conflict.

The reviewed articles vary significantly in their study designs, complicating direct comparisons. There is a lack of uniformity in methodologies, reflecting the diverse target populations and nature of conflicts and their impact on immunization efforts. This heterogeneity poses challenges for drawing comprehensive conclusions. For instance, each conflict intensity measure has limitations. Casualty-based indicators do not account for displacement and other forms of violence, such as kidnapping and shelter destruction, and they may underestimate the number of missing individuals, unrecovered or unidentified bodies, overburdened health facilities and morgues. Duration-based indicators do not reflect varying intensity levels since the start of the hostilities. Database and reporting-based and threshold-based indicators contain potential biases from armed barriers and political sources involved in conflicts and may be falsified as part of war strategies. It is possible that some retrospective studies included in a scoping review may be prone to data errors or changes in data collection methods for immunization. More so, we often find a generalisation of the concept of armed-conflict with no geographical specificities while routine immunization is seen as part of an overview of health care.

This scoping review highlights the difficulty in comparing studies due to the methodological diversity. Conflicts and contexts differ, and while some populations may experience improved immunization access during conflicts, others face severe disruptions or mixed antigen-specific and drop-out results. Chosen variables and other external factors need to be fit in to truly understand specific dynamics, as the presence of armed conflict and the intensity of insecurity themselves aren’t the sole predictors of vaccination coverage among affected children. Data collected for the studies often reflects medium to high-intensity conflict areas, while low-intensity more protracted-type conflicts, less publicized conflicts, and more frequent events with lower impacts, are less studied.

Practical implications

The presence of aid organizations significantly enhances the delivery of vaccines and health services, demonstrating the international community’s effectiveness in improving vaccination rates amidst conflicts and in previously underserved areas. ^{24, 65} These efforts should be sustained and expanded to ensure continued support, with the cooperation of national governments. ⁵ Countries should invest in developing a stable and distributed public health infrastructure and workforce to reduce reliance on external contracting models and facilitate rapid and effective responses. This approach should strengthen local health systems towards resilience and sustainability. Using context-specific strategies and operational approaches enhances resilience and ensures sustainable health service delivery. ^{16, 28, 29, 86} Policymakers should utilize these successful strategies to improve vaccination programs in both conflict and non-conflict settings. ^{28, 87} Proactive planning in cases of conflict in countries apparently at peace should be implemented. ³⁴ Policy interventions must also address access to healthcare, strongly considering Universal Health Coverage (UHC) and Special Immunization Activities (SIAs) specifically targeted at settings with low coverage, migrant populations, and hard-to-reach subpopulations that routine systems often miss. ^{36, 88} Efforts to reduce childhood mortality must focus on addressing inequities in immunization coverage. ^{12, 19} It is also advisable to more systematically combine immunization programs within broader activities to create a more comprehensive package of care. ⁸⁹ Engaging and involving communities in promoting immunization education can significantly improve vaccination uptake, particularly in areas with limited access to formal education. Community-driven initiatives foster trust and support for vaccination programs. ^{22, 43, 90} It is key for programs to remain relevant and adapt to the needs and barriers linked to the evolution of conflicts. Education is a powerful leverage to reach appropriate and adaptive health seeking behaviour. ⁹¹ Compliance to international treaties and domestic laws supporting the protection of essential health care services should be reinforced and implemented at all times, while the diffusion of their content should be known by everyone. ^{7, 34, 89} As seen, conflicts tend to have a negative impact on several variables conditional to a functioning health information system. ⁹ Technology will support research in this sector, by utilizing robust DHIS for real-time data collection and monitoring to track immunization coverage, timely identify gaps and swiftly respond to needs. A digital health records system might allow for accurate immunization records, even amid displaced populations. Researches, Machine Learning (ML) and Artificial Intelligence (AI) might support efforts in tracking emerging actors and predict vaccine-preventable disease outbreaks in conflict affected areas. ^{92, 93} As part of pre-emptive strategies for future threats, anticipating future epidemics and potential biowarfare scenarios for the development of new vaccines is necessary. ^{94– 96} Advanced vaccine technology, such as the development of adaptive vaccination schedules, single-dose, lifetime-lasting vaccines and innovative vaccine delivery systems such as microneedle patches or inhalable vaccines to simplify delivery, storage, including pre-positioning vaccines and other supplies, and administration, might be valuable to reaching the most remote and affected areas, that are less likely to complete full courses. ^{29, 97– 99} Efforts to expand vaccination coverage during conflicts should extend beyond refugee camps or rural areas where displaced populations often have better access to immunization, to include other marginalized populations living in conflict-affected areas. ^{29, 100} An inclusive approach should ensure that all vulnerable groups receive the necessary immunization. ¹⁰¹ Migration is common during armed conflict, but unfortunately population movements are rarely integrated into planning or strategies to improve immunization. ¹⁶ Host countries should take measures to improve the systematisation of immunization of migrant populations at risk. ^{21, 27} Communication and public health education are often key to improving vaccination coverage, even in armed conflict zones. For instance, emphasizing the societal benefits of children immunization, using all the digital platforms available to build trust, disseminating reliable health information such as vaccination schedules, locations and educational campaigns to counter misinformation, and impacting health-seeking behaviours regarding immunization. ¹⁰² Improving access to vaccination with outreach intervention for the hard-to-reach populations, ³⁶ building trust in health systems, by avoiding assimilation of health workers with parties to the conflict, ⁸⁹ and enhancing their training, punctuality and retention in their place of origin are crucial to maintaining immunization services during and after conflicts. ^{5, 22} Efforts are to be maintained to identify and support actors already working in those areas, avoiding armed forces but including society and individuals from the private sector for transportation and logistics, with the condition that they adhere to humanitarian principles. ⁸⁹ Trustworthy and accessible health services encourage higher immunization rates and all local stakeholders should be engaged to overcome this challenge. Research indicates that local NGOs, though often more cost-efficient and accountable due to their deep community integration and cultural understanding, receive only a small percentage of donor funds—hindered by competition, corruption concerns, and limited funding—prompting advocates to call for increased support to enhance their effectiveness and sustainability in humanitarian efforts. ¹⁰³ Additional efforts, such as financial support and donations, are needed for the early rehabilitation of destroyed health facilities, the vaccine cold chain, and the information health system in conflict areas. ^{11, 13, 36} Vaccination campaigns and catch-up should be prepared to increase coverage. ^{23, 49} International dialogue, sharing experiences across the globe ³⁶ and policy coordination with all parties, including donors, large international organizations, armed groups and political figures, are necessary to negotiate humanitarian access, develop best practices for rebuilding health systems, and address disparities in conflict settings. ^{44, 89} Collaborative international efforts can lead to more effective and equitable health interventions, with a focus on supporting governments in vaccination planning and precise resource allocation. ^{20, 32, 41, 89} Predictability in vaccine procurement from manufacturers and donors, will allow states to better anticipate and plan accordingly the distribution and administration, as armed conflict contexts are quite often hardly predictable. ⁸⁹ The new Gavi strategy brings key elements to fit its support to the needs of fragile and armed-conflict contexts. For instance, the launch and endorsement of the African Vaccine Manufacturing Accelerator (AVMA) is a commitment to support the sustainability and resilience of the immunization system on the African continent. ¹⁰⁴ Those initiatives would eventually lead to decreasing dependency to aid. ³⁶

Future studies recommendations

The literature suggests that research should concentrate on how armed conflicts affect both the supply of immunization services, such as public health expenditure and logistical challenges, and the demand for these services, particularly the socio-economic status of the population and the level of trust in the health system. ^{32, 65} Studies should explore all these mechanisms in cases of population displacement, to understand vaccine take-up during conflicts. ²⁷ A practical evaluation and comparison of National Immunization Programs (NIPs) and UHC performance in armed conflict contexts, would enable researchers to align with global health goals like the Decade of Vaccines (DoV), Sustainable Development Goals (SDG) 3 and 16, and the Immunization Agenda 2030. ^{1, 17, 105} The influence of armed conflicts on health behaviours among the furthest communities, both geographically and socially, including vaccination dropouts and timeliness ¹⁵ remains to be explored. It is essential to monitor health effects at the individual level as conflict, types of violence and armed actors evolve over time, to offer timely recommendations and evaluate program implementation. ^{7, 65} Understanding how prolonged conflicts affect immunization initiatives can inform more resilient and adaptive strategies, to avert delays during heightened conflict periods. ³²

They should consider local or sub-national conflict dynamics and immunization performance ¹⁷ via in-depth qualitative studies ³⁴ and across various health indicators and conflict exposure measures. ²⁵ Understanding why some immunization indicators are more responsive to conflict than others is crucial for developing targeted and adaptable interventions to tailor relevant context-flexible recommendations. As stated in UNICEF’s Immunization Roadmap 2018–2030: “A ‘one-size-fits-all’ approach is no longer appropriate in this changing world”. ¹⁰⁶ The adapted UNICEF’s Immunization Roadmap 2022-2030 highlights the importance of the Nexus approach. ¹⁰⁷ Looking at all the variables on the subject, it is essential to recognize the added value of local experts’ opinions and be open to changing global health practices. ¹⁰⁸