Zero- or missed-dose children in Nigeria: Contributing factors and interventions to overcome immunization service delivery challenges.

'Zero-dose' refers to a person who does not receive a single dose of any vaccine in the routine national immunization schedule, while 'missed dose' refers to a person who does not complete the schedule. These peopleremain vulnerable to vaccine-preventable diseases, and are often already disadvantaged due to poverty, conflict, and lack of access to basic health services. Globally, more 22.7 million children are estimated to be zero- or missed-dose, of which an estimated 3.1 million (∼14 %) reside in Nigeria.We conducted a scoping review tosynthesize recent literature on risk factors and interventions for zero- and missed-dosechildren in Nigeria. Our search identified 127 papers, including research into risk factors only (n = 66); interventions only (n = 34); both risk factors and interventions (n = 18); and publications that made recommendations only (n = 9). The most frequently reported factors influencing childhood vaccine uptake were maternal factors (n = 77), particularly maternal education (n = 22) and access to ante- and perinatal care (n = 19); heterogeneity between different types of communities - including location, region, wealth, religion, population composition, and other challenges (n = 50); access to vaccination, i.e., proximity of facilities with vaccines and vaccinators (n = 37); and awareness about immunization - including safety, efficacy, importance, and schedules (n = 18).Literature assessing implementation of interventions was more scattered, and heavily skewed towards vaccination campaigns and polio eradication efforts. Major evidence gaps exist in how to deliver effective and sustainable routine childhood immunization. Overall, further work is needed to operationalise the learnings from these studies, e.g. through applying findings to Nigeria's next review of vaccination plans, and using this summary as a basis for further investigation and specific recommendations on effective interventions.

Introduction

Immunization is an essential, cost-effective strategy to reduce childhood morbidity and mortality which saves an estimated 2 to 3 million lives each year [1]. However, vaccine coverage in many lower and middle-income countries (LMIC) falls short of the World Health Organization’s (WHO) target of 90 % national coverage with three doses of diphtheria-tetanus-pertussis containing vaccines (DTP3). Among these are ‘zero-dose’ children, which refers to children who do not receive a single dose of any vaccine in the routine national immunization schedule, and ‘missed dose’ children who do not receive the complete schedule. Children who do not receive DTP1 are often used as a proxy indicator for zero-dose children, while completing three doses of DTP-containing vaccine before 12 months of age is WHO’s proxy indicator for routine immunization programme performance [2], [3]. Unvaccinated children remain vulnerable to vaccine-preventable diseases and are often already disadvantaged due to poverty, conflict, and lack of access to basic health services. Globally, 22.7 million children are estimated to be zero- or missed-dose, and of these, 3.1 million (∼14 %) reside in Nigeria [4].

It is critical that children receive their first vaccine doses, as 80 % of children who receive at least one dose of any vaccine will receive at least one further vaccine, and two-thirds will complete the full schedule [1], [5]. Nigeria’s 2020 childhood routine immunization schedule consists of:

Bacillus Calmette-Guérin (BCG), oral polio vaccine (OPV) and hepatitis B at birth

Three series of OPV, pentavalent (diphtheria, pertussis, tetanus, and hepatitis B and Haemophilus influenzae type B) and Streptococcus pneumoniae conjugate vaccine (PCV) at 6, 10 and 14 weeks; inactivated polio virus (IPV) is also administered at 14 weeks

Neisseria meningitidis meningitis, first dose of measles containing vaccine (MCV) and yellow fever vaccinations at 9 months, with a second MCV dose at 15 months [1], [6], [7].

Vaccines are typically administered in primary healthcare (PHC) centres or through campaigns for hard-to-reach communities, and are also provided by the private sector [8], [9], [10].

Achieving high childhood immunization coverage remains a significant challenge in Nigeria, with childhood immunization coverage rates for DTP1 and DTP3 estimated at 65 % and 57 % in 2020 respectively; and 54–67 % coverage estimated for other routine vaccinations [11]. In 2020, WHO / UNICEF (WUENIC) estimated the number of zero- or missed-dose children in Nigeria also increased to 3.1 million (from 3.0 million in 2019)[4]. However, the Government of Nigeria is making efforts to improve routine immunization coverage. In 2017, routine immunization was declared a Public Health Concern, which led to operationalizing the National Emergency Routine Immunization Coordination Centres (NERICC) in low performing states and LGAs. In their 2018 Strategy for Immunization and PHC System Strengthening, Nigeria committed to investing in strengthening immunization service delivery, expanding cold chain capacity, improving data quality, introducing new vaccines and addressing significant risk of vaccine-preventable diseases including measles, yellow fever and meningitis over the Gavi transition period 2018 – 2028. During this transition period, Gavi’s support is gradually phased out and Nigeria is expected to become fully self-sufficient in its procurement and delivery of vaccines [12], [13]. This aligns with global targets such as the Immunization Agenda 2030 (IA2030) [14] and Gavi’s 5.0 strategy [15].

There are many studies available on risk factors and interventions for zero-dose children, however, few articles that comprehensively synthesize study findings. This scoping review aims to summarize all identified risk factors and interventions specific to zero or missed-dose children in Nigeria. It takes a broad view of the available literature, regardless of quality, and synthesizes findings to enhance understanding of the issues; identify gaps; and where possible, make recommendations.

Methods

Literature search

To conduct the scoping review of risk factors affecting, and interventions addressing zero or missed-dose children in Nigeria, we searched the PubMed / MEDLINE and Embase databases for relevant papers published between April 2011 and April 2021. We used the keyword search string (“zero-dose” OR “immunization coverage” OR “under-immunization” OR “under-vaccinated” OR “equitable uptake of vaccines”) AND (“children” OR “child”), and reviewed references to identify additional articles. We excluded studies that were: (i) published before April 2011; (ii) not focused on Nigeria; (iii) not peer-reviewed; (iv) not explicitly covering risk factors or interventions for childhood immunization; and (v) not available in English. We included articles that assessed risk factors or interventions for all childhood vaccines given between 0 and 5 years of age, and articles that assessed ‘missed-dose’ children. We screened titles, and then abstracts to refine our search before conducting full-text review (see Fig. 1).

Fig. 1

Summary of search strategy.

Data extraction and evaluation, and draft development

Eligible papers were randomly assigned to two independent reviewers who extracted the following data: study design, sample size, age of participants, study population (e.g., caregivers, ethnic group, etc), study location (e.g., national, regional), vaccination assessed (e.g., DTP), risk factors and interventions assessed. Studies that assessed determinants for being ‘fully vaccinated’, as opposed to assessing specific vaccinations, were also included, accepting the studies definition of ‘fully vaccinated’.

Both qualitative and quantitative studies, and studies based in communities and health facilities, were included in our analysis. The reviewers did not assess study quality. Reviewers compared extracted data; if a disagreement was identified it was discussed until consensus was found. Extracted data was categorized according to the type of risk factor or intervention. Initial findings were shared and discussed through two virtual consultations among experts and stakeholders at WHO headquarters, WHO Regional Office for Africa, and WHO Nigeria Country Office, for guidance and advice. Information was progressively consolidated into a draft manuscript, which was circulated among all authors for further analysis, review, and feedback, and eventually finalized.

Results

Summary of papers identified in scoping review

The literature search identified 19,015 articles (Fig. 1). Of these, 5,435 articles were duplicates, and 13,436 were excluded based on title and abstract review. The remaining 228 articles underwent full text review, of which 103 did not meet the inclusion criteria: 19 did not cover Nigeria; 55 assessed global coverage rates (not risk factors or interventions); and 29 did not specifically assess vaccines or immunization.

This resulted in inclusion of 127 papers relevant to risk factors and interventions for zero- or missed-dose children in Nigeria, summarised in Appendix 1. Most papers (n = 71, 56 %) focused on ‘childhood immunization’ in general, with varying definitions. The most common vaccine studied was polio (n = 26, 21 % of papers), followed by measles (n = 15, 12 %), DTP (n = 5, 4 %), tetanus (n = 4, 3 %), hepatitis B (n = 3, 2 %), pertussis (n = 2, 2 %), and meningitis (n = 1, 1 %).

Among the 127 studies assessed, two-thirds 68 % (n = 86) focused on regional areas in Nigeria, and 26 % (n = 33) were national, while the remaining studies included Nigeria as part of a broader study focused globally (n = 5, 4 %) or on Africa (n = 3, 2 %).

Most studies (n = 66, 52 %) assessed risk factors only, followed by interventions only (n = 34, 27 %), both risk factors and interventions (n = 18, 14 %), and recommendations only (n = 9, 7 %). Of the studies that assessed interventions (n = 52), 62 % (n = 32) focused on vaccination campaigns, and 31 % (n = 16) focused on interventions for polio.2

Summary of risk factors for zero- or missed-dose children in Nigeria.

Risk factors for zero-or missed-dose children are categorised in three groups: (i) individual factors; (ii) community factors; and (iii) health system factors (see Table 1). They are prioritised by frequency of inclusion in the studies considered, referred to as ‘n’ (see also Fig. 2). In summary, the most frequently identified factors for determining childhood vaccine uptake were: maternal education and literacy (n = 22); access to health facilities (n = 17); socioeconomic status (n = 16); fears and misconceptions about vaccination (n = 14); vaccine availability (n = 13); and giving birth in a health facility (n = 12).

Table 1

Summary of risk factors for zero- or missed-dose children in Nigeria.

Factor	Frequency	Note	Reference
Individual factors
Maternal factors
Maternal literacy and education	22	High maternal education consistently associated with higher vaccination rates, although one national study found that education level was not significant when adjusted for literacy [32].	[32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]
Maternal care (health facility birth)	12	Giving birth in a health facility was significantly associated with vaccination, particularly for birth doses. However, delivery on weekends or outside routine vaccination days, and prematurity or low birth weight, were reasons for delayed or missed birth dose vaccinations [54].	[33], [37], [40], [48], [49], [54], [55], [56], [57], [58], [59], [60]
Maternal age	9	Older mothers (variously defined; at minimum mothers were > 20 years of age) were more likely to have fully vaccinated children.	[37], [38], [39], [40], [48], [51], [61], [62]
Maternal awareness of immunization (benefits, safety, schedule, how / where)	8	Consistently associated with higher vaccination rates; however also higher in educated mothers, mothers who received antenatal care, and mothers with more children.	[36], [41], [47], [49], [56], [57], [61], [62]
Maternal employment	7	Maternal employment positively correlated with childhood vaccination, although one national study did not find this significant [48].	[35], [37], [39], [41], [48], [53], [60]
Maternal care (ante-natal)	7	Receiving ante-natal care was consistently associated with higher vaccination rates.	[34], [43], [54], [58], [59], [63], [64]
Maternal time demands	4	Attending social engagements, maternal or child ill health, and other competing demands on time (e.g., farm work, care for other children) were identified as barriers to children completing childhood vaccinations.	[36], [45], [56], [65]
Maternal discretion to make vaccination decisions without husband’s permission	3	More likely to have fully vaccinated children.	[40], [46], [56]
Maternal marital status (being married)	3	Associated with vaccination completion, particularly in urban slums. One sub-national study suggested this may be caused by social stigma for unmarried mothers to attend clinics, and / or less access to caregivers [57].	[48], [51], [57]
Maternal discretion to spend household income	2	More likely to have fully vaccinated children.	[48], [56]
Socioeconomic factors
Socioeconomic status	16	Vaccination rates consistently increased as household wealth increased.	[7], [32], [35], [37], [38], [39], [44], [45], [48], [50], [51], [53], [55], [59], [66], [67]
Paternal / caregiver factors
Paternal / caregiver support for vaccination	8	Higher paternal or parental / caregiver education, employment, awareness, and support for vaccination was also associated with higher childhood vaccination coverage.	[45], [51], [52], [56], [68], [69], [70], [71], [72]
Paternal permission for vaccination	6	Coverage was lower where caregivers, particularly fathers, refused permission for vaccination.	[33], [45], [62], [69], [73], [74]
Number of children	3	Vaccination rates were higher in families with fewer children (e.g., less than three).	[41], [43], [51]
Access to media	2	Homes with access to media (e.g., television, radio) had higher childhood vaccination rates, particularly in rural areas. One study suggested this was due to the media providing information on the benefits of vaccination, health activities, and location of health facilities.	[39], [48]
Child factors
Child age or birth order	9	Vaccination rates were higher in children with higher birth order, and in children that were older.	[38], [39], [40], [48], [57], [58], [75], [76], [36]
Community factors
Perception of vaccination
Fears and misconceptions	14	Fears and misconceptions over vaccine safety and efficacy were associated with vaccination refusal and missed or incomplete schedules.	[36], [37], [40], [56], [65], [66], [74], [77], [78], [79], [80], [81], [82], [83]
Community disapproval of vaccination	6	Vaccination refusal often clustered in settlements with similar ethnic and religious profiles.	[45], [53], [62], [66], [84], [85]
Geographical factors
Region (North vs South)	7	Northern Nigerian communities were less likely to be vaccinated compared to Southern Nigerian communities. Studies suggested this was driven by higher levels of education and wealth in the South, higher proportions of Muslim households in the North, and greater insecurity in the North.	[39], [48], [53], [59], [60], [68], [86]
Setting (rural vs urban)	5	Rural areas typically had lower vaccination rates, and higher drop-out rates, compared to urban areas, although one sub-national study found the opposite [78].	[37], [39], [46], [48], [59], [78]
Cultural factors
Religious affiliation	9	Muslim households were less likely to be vaccinated compared to Christian households.	[33], [38], [39], [44], [46], [48], [51], [52], [59]
Ethnicity	3	Hausa / Fulani ethnic groups (often nomadic or semi-nomadic pastoralists) were less likely to be vaccinated compared to Igbo or Yoruba ethnic groups	[59], [87], [88], [89]
Conflict and displacement
Insurgency and conflict	7	Insurgency and conflict caused internal displacement and prevented access to settlements by vaccination and / or surveillance teams, particularly in Northern Nigeria, resulting in lower vaccination rates.	[90], [91], [92], [93], [94], [95], [96]
Displacement / migration	3	Rural-to-urban migrants, displaced persons, and recent migrants were less likely to be vaccinated e.g., due to disruption of return visits and challenges to follow-up missed children.	[45], [78], [97]
Health system factors
Access to vaccination
Access to health facilities	17	Long travel times, difficult terrain, poor or non-existent roads, and perception of the facility being ‘far’ were associated with no or incomplete vaccination, particularly for rural and low-income populations.	[37], [41], [45], [46], [48], [49], [56], [65], [67], [69], [74], [83], [98], [99], [100], [101], [102]
Vaccine availability	13	Lack of vaccines, e.g., due to stock-outs and procurement delays (for vaccines or related supplied such as adverse event following immunization (AEFI) kits) where associated with lower vaccination rates.	[33], [36], [41], [54], [56], [64], [65], [73], [79], [83], [85], [100], [103]
Vaccinator availability	11	Absence of vaccinators at health centres, maternity wards were associated with lower vaccination rates. Reasons included insufficient resources, industrial access, and refusal to offer vaccines. One study also reported vaccination teams not visiting homes during campaigns.	[41], [45], [59], [65], [69], [74], [78], [83], [104], [105]
Cost of vaccination	6	Cost of vaccination was a barrier to receiving vaccination, including health facilities demanding payment or bribes for ‘free’ vaccines (n = 5); and the perceived cost of reaching facilities (n = 1).	[46], [56], [57], [59], [105], [106]
Queues at health facilities	4	Long queues at health facilities negatively impacted vaccine uptake.	[45], [65], [102], [107]
Awareness of vaccination
Awareness of need for vaccination	11	Low awareness about the need for vaccination was associated with low coverage.	[37], [49], [65], [73], [80], [83], [88], [89], [99], [108], [109]
Awareness of vaccination schedules	10	Not knowing the need for or timing of subsequent doses was associated with low coverage.	[40], [41], [45], [49], [74], [80], [83], [102], [110]
Vaccination record keeping	3	Retention of home-based records/vaccination cards was associated with vaccination completion. Some studies suggested that retaining cards may indicate greater belief in the importance of vaccination and increase awareness about the vaccination schedule.	[40], [51], [58]
Other factors
Health system governance	5	Poor governance within the Nigerian health system resulted in lower coverage. Drivers included ineffective management; inadequate health workforce; lack of quality data; inequitable distribution of funding and resources; and limited funding.	[36], [69], [80], [68], [95]
Vaccination campaign planning	5	Poor vaccination campaign planning, including outdated population estimates and maps; limited community engagement and use of data led to lower coverage rates.	[33], [79], [91], [99], [111]
Campaign worker factors	3	Campaign worker factors, including finger marking without vaccination, inflating the number of children vaccinated, not adhering to microplans, and non-screening of home-based records/vaccination cards led to lower coverage in vaccination campaigns.	[104], [112], [113]
Healthcare worker factors	2	Healthcare worker factors, including poor interpersonal skills; lack of motivation; community resistance to healthcare workers; and clinical environments not conducive to health education on vaccination were identified as reasons for lower coverage	[102], [107]
COVID-19 pandemic	2	COVID-19 hindered vaccination efforts due to health system disruption; closures of facilities and transport; stay at home orders and fears of contracting disease	[80], [85]
Community engagement	1	Poor community engagement, sensitisation and mobilisation created barriers to vaccine uptake	[68]

Fig. 2

Frequency of risk factors for zero- or missed dose children in Nigeria identified in the scoping review.

Summary of interventions to improve immunization coverage for children in Nigeria

Most interventions were assessed as part of a multi-factorial approach to increase childhood vaccination coverage and were identified as effective by the respective study authors. Specific interventions are categorised in four groups: (i) community engagement, sensitisation, and mobilisation; (ii) sustaining vaccination coverage; (iii) targeting zero- or missed-dose children; and (iv) vaccination campaigns (see Table 2). They are prioritised by frequency in the included studies, referred to below as ‘n’ (see also Fig. 3). Nine publications identified potential interventions for recommendation, however they were not evaluated to determine their effectiveness (n = 9) [16], [17], [18], [19], [20], [21], [22], [23], [24]. In summary, the most frequently mentioned interventions involved training community members as vaccination advocates (n = 8); engaging hard-to-reach communities (n = 8); reminders for follow-up appointments (n = 5); and health education to increase vaccine awareness (n = 4).

Table 2

Summary of interventions to improve immunization coverage for children in Nigeria.

Intervention	Frequency	Note	Reference
Community engagement, sensitisation, and mobilisation
Training community members to advocate for vaccination	8	Training volunteer community mobilisers, traditional and religious leaders (e.g., traditional barbers), schoolteachers, and other community leaders to advocate for vaccination successfully improved vaccination acceptance and uptake, particularly in traditional Muslim societies [95]. This included ‘baby tracking’ by volunteers to remind new mothers to vaccinate new-borns; and involving women and youth to help identify missed children [114].	[95], [107], [112], [114], [115], [116], [117], [118]
Engaging hard-to-reach communities	8	Engaging youth groups, leveraging existing structures, and using multi-pronged approaches to extend vaccination services, including screening tools for healthcare workers to identify vaccination needs and mobilising communities, increased coverage in hard-to-reach and security-challenged communities	[90], [92], [95], [113], [119], [120], [121], [122]
Health education interventions for caregivers to increase vaccine awareness	4	Focused, short (5 min) health education sessions were more effective than longer (10–15 min), generic health promotion messages, and frequent vaccination messaging was preferred (i.e., not just during vaccination campaigns). Successful interventions included group health education for parents and pregnant women attending vaccination or antenatal clinics; and participatory learning on infant vaccination for older women.	[68], [95], [107], [114]
Tailoring communication to preferred channels	3	Healthcare workers and media are the most common sources of information, with radio and town announcers preferred in rural settings and media preferred in urban and rural settings. Radio / television and home visits were preferred by Muslim women unable to leave their homes due to Purdah system.	[95], [107], [114]
Building awareness through media campaigns	3	This included media, sensitisation and enlightenment campaigns, statements from religious leaders, road shows, etc to increase vaccine awareness.	[46], [47], [122]
Engaging community liaisons	1	Engaging nomadic ‘Ardos’a, who know migratory patterns of their communities, to act as a community liaison and provide dates and times of vaccination visits helped to engage nomadic communities.	[122]
Healthcare worker training	1	Training of healthcare workers in interpersonal communication to increase successful interactions with caregivers; and home visits by healthcare workers helped to increase coverage.	[114]
Language support	1	Providing interpreters for native languages in PHC facilities enabled better communication around vaccination.	[114]
Sustaining vaccination coverage
Vaccination reminders
Reminders for follow-up appointments	5	Reminders (e.g., phone call or SMS texts) for appointments successfully increased return rates for appointments. Text messages were often preferred by recipients, however in some instances, cost of sending texts was personally borne by healthcare workers.	[95], [107], [114], [123], [124]
Vaccination reminder bands for infants	1	Training healthcare workers and traditional birth attendants to secure vaccination indicator reminder bands on children improved demand for vaccination, although studies did not assess effectiveness at increasing compliance.	[118]
‘Reminder’ vaccination cards	1	Providing redesigned ‘reminder-type’ home-based records/vaccination cards was successful in increasing visits.	[95]
Other interventions
Combining childhood vaccination with other human or animal health initiatives	3	Providing cattle vaccinations in combination with childhood vaccination for Fulani ethnic groups increased participation and improved coverage. Broader integration of routine immunization with other health initiatives, such as supplemental vaccination campaigns, boosted coverage and was particularly successful where the connected program was of public interest (e.g., malaria prevention).	[90]
Using performance data	2	Using data improved and sustained coverage and performance, including through routine community surveys and performance-based financing for vaccination service delivery.	[125], [126]
Political engagement and governance	2	Support uptake, with specific examples including: i) clear leadership from Ministry of Health and engagement by political leaders to mobilise other stakeholders, including media, pharmaceutical companies, and healthcare workers successfully enabled the switch from trivalent OPV to bivalent OPV vaccine; and ii) establishing a national technical coordination committee, with government and development partners setting a mandate to reach 95 % vaccination coverage, and visibility of state-by-state progress.	[99], [122]
Access to maternal care	1	Improving uptake of maternal care interventions e.g., by using community health extension workers to increase ante-natal care and facility-based delivery increased uptake.	[127]
Leveraging existing infrastructure	1	Transitioning existing capacity and infrastructure of vaccination teams built through polio programs (e.g., NSTOP) in support of measles vaccination campaigns increased uptake.	[128]
Targeting zero-dose or missed-dose children
Providing vaccination at strategic points	4	Providing vaccinations at strategic points e.g., markets and transit areas (e.g., borders, motor parks) to reach nomadic, rural, and migrant populations; and at the entry to camps for internally displaced persons increased coverage for high-risk groups.	[88], [113], [129], [93]
Deploying mobile vaccination teams	3	Deploying dedicated mobile teams combining PHC with vaccination targeting zero-dosed children increased uptake.	[130], [131], [132]
Vaccination campaigns
Offering incentives for vaccination	8	Offering incentives for vaccination, including cash transfers (conditional or unconditional) (n = 3) and non-financial incentives e.g., noodles, soap, sugar, or add-on health services (n = 5) contributed to higher coverage rates during campaigns. However, some studies noted that incentives, where they are primary drivers of vaccination uptake, can negatively impact on coverage if removed.	[80], [82], [97], [113], [117], [118], [120], [125]
Using supplementary vaccination campaigns	5	Using supplementary vaccination campaigns (including fixed and temporary posts in strategic areas, and ‘in-between’ round campaigns successfully increased ‘last-mile’ coverage (n = 4) while one study was critical.	[112], [113], [131], [133], [134]
Using GIS and satellite data	4	Using GIS and satellite data to inform microplanning and team assignments improved identification and coverage of target areas vs hand-drawn maps.	[91], [113], [129], [135]
Using real-time performance data	4	Using real-time performance data improved coverage of vaccination campaigns (e.g., leveraging existing polio reporting structures; analyzing and report daily campaign data; providing feedback).	[99], [129], [136], [137]
Coordinating campaign group members	1	Using WhatsApp for campaign group members improved planning, coordination, and data sharing.	[79]

Ardo are leaders in Fulani (typically nomad) sociocultural settings.

Fig. 3

Frequency of implemented and assessed interventions for zero- or missed dose children in Nigeria identified in the scoping review.

Discussion

This study synthesized recent literature on risk factors and interventions for zero- and missed-dosed children in Nigeria.

Risk factors

Literature on reported risk factors within Nigeria was abundant and largely consistent. These findings also align with key risk factors reported in other parts of the world, including knowledge of vaccines, parental education, misinformation, and lack of access to vaccines and healthcare services. Missed opportunities for vaccination were greater in low- and middle-income settings [25], [26], [27].

There is also clear overlap between risk factors identified at the individual, community, and health system level. For example, higher maternal education and literacy was most frequently associated with higher vaccination coverage (n = 22), followed by access to health facilities (n = 17) and high socioeconomic status (n = 16). A limitation of this study is that it does not assess associations between factors, hence it’s difficult to disentangle cause and effect – however the relationship between education, wealth, and better health outcomes is well documented in other settings [28]. Other studies, beyond the scope of our review, investigate these relationships in more detail – e.g., assessing the relationship between access to health clinics and child morbidity / mortality [29].

At the community level, heterogeneities between and within communities, including region (North vs South Nigeria), setting (rural vs urban), population composition (e.g., religious affiliations, ethnic groups, displaced peoples, and migrants, etc) were frequently identified as determinants of vaccination uptake. These highlight the specific challenges that Nigeria faces in improving vaccination coverage overall. Again, the relationship and compounding effects between these factors, and individual wealth, education and empowerment of women, and access to health services is possible to infer, but hard to disentangle without further assessment and / or research. At the health system level, factors that supported uptake included vaccines that are available, affordable, and easy to access, and populations that know they are needed, and when and how to get them.

Overwhelmingly, this information is not novel. Rather, it highlights that the systemic, long-standing, and population-level challenges faced to improve access to childhood vaccination in Nigeria are well known and widely studied - and begs the question of how Nigeria can operationalize this knowledge to meet its goal of sustainably improving immunization coverage and strengthening PHC systems [12].

Interventions

Literature assessing implementation of interventions was more scattered, and heavily skewed towards vaccination campaigns and polio activities, reflecting the historically polio- and campaign-centric strategy used in Nigeria; and where there was funding to conduct operational research or evaluations. Other studies typically attributed success to a swathe of multi-factorial interventions, although some assessed the impact of individual activities such as training courses for a particular cohort or providing vaccination reminders.

Only one study was identified that assessed the systemic enablers of an effective and sustainable routine immunization program e.g., covering financing mechanisms, program delivery and governance, and measuring or driving routine program performance. The study focused on a demand-side performance-based financing scheme which led to increased utilization of key maternal and child health services (antenatal care and skilled delivery) but had no significant effect on completion of child immunization using measles as a proxy indicator [30]. In addition, there were few studies identified that assessed effective vaccine management capacities in country (i.e., to prevent stock-outs); or that assessed “special interventions” to improve last mile distribution outside of supplementary vaccination campaigns, including revision of house based microplans, use of mobile health teams, scaling up of transit vaccination, scaling up of youth engagement, intensified supportive supervision, scaling up of Directly Observed Polio Vaccination (DOPV), establishing temporary health camps offering treatment for minor ailments for all, and in-between rounds vaccination [64], [95], [104], [114], [117].

Limitations of this study

Comprehensively synthesising available literature is informative, however has limitations. We included all relevant papers without assessment or exclusion based on quality does not allow for deeper analysis or representation of effect sizes and their significance or interrelationships. We prioritised information based on frequency reflects what is most studied and is not necessarily equivalent to what is most important (for risk factors) or effective / cost-effective (for interventions). The frequency of finding an association with a given risk factor and vaccination may therefore reflect investigator preconceptions, rather than their significance. Furthermore, a scoping review precludes delving deeper into root causes. For example, maternal education was identified as a major determinant for childhood vaccination, yet one study indicated that this was not significant if adjusted for literacy. A scoping review can identify this, but not draw a conclusion. Similarly, the summary nature of this study precluded distinguishing between cause, effect, and other relationships between factors at the individual, community, and health system level; or from exploring broader potential drivers (e.g., frequency of vaccination campaigns influencing differences in vaccination coverage in North vs South Nigeria) [31]. Additionally, our search criteria, although broad, did not include specific terms (such as “missed opportunities”, “barriers to vaccination”, “low-vaccination”, or “partial-vaccination”) that may potentially have identified further studies relevant to understanding risk factors and interventions for zero- or missed-dosed children. Similarly, searching for interventions to address specific risk factors (e.g., ‘preventing stock-outs’) may have yielded additional results that focus on addressing those risk factors more generally.

Conclusion

Despite these limitations, our findings highlight that: (i) risk factors for zero- or missed-dosed children in Nigeria are well-known and widely studied; and (ii) interventions implemented and assessed to improve vaccination coverage in Nigeria are campaign- and polio-centric, with major gaps in investigating how to deliver effective, sustainable routine childhood immunization programs.

This knowledge could be operationalised by using this summary to inform Nigeria’s next review of existing vaccination plans; and as a basis to further investigate risk factors and successful interventions – e.g., to better understand root-causes, and /or identify specific measures and recommendations for future action.

Overall, improving childhood vaccination coverage in Nigeria will require substantial and sustained commitment for implementation of routine childhood vaccination programs. Such efforts will need to be adaptive to local circumstances, mindful of the needs of Nigeria’s heterogenous population, and agile in the face of inevitable future challenges that may distract focus and resourcing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.