Introduction

The World Health Organization (WHO) highlights childhood immunization as one of the most effective public health interventions to reduce the risk of major infectious diseases in children [1]. Early childhood immunization strengthens immunity against VPDs, reduces mortality, and supports developmental milestones [2,3]. To achieve full immunization, children should receive one dose of Bacillus Calmette-Guerin (BCG) at birth; three doses of polio and pentavalent vaccines at 6, 10, and 14 weeks; and one dose of the measles vaccine at 9 months [4]. The primary determinants of full immunization coverage include demographic factors such as age, education, and marital status, alongside socioeconomic factors like unemployment, poverty, long distances to health centers, and lack of insurance coverage [5–8].

In 2023, the WHO reported that global immunization coverage for fully immunized children stood at 83%, representing the proportion of children who received all recommended vaccine doses within their first year of life [9]. However, coverage varies significantly across continents and countries, particularly in low- and middle-income settings. In sub-Saharan Africa (SSA), results from a study published in 2022 showed that only 44% of children were fully immunized, with coverage ranging from 0% to 78% across countries [10]. These disparities may reflect differences in immunization schedules and health system performance. Inequalities in immunization coverage are often more pronounced in countries experiencing conflict or with a recent history of war, such as Sierra Leone [11].

Over the years, Sierra Leone has made significant progress in improving full immunization coverage, partly due to the establishment of the Expanded Program on Immunization in 1978 [13]. Other interventions include the launch of the Free Healthcare Initiative in 2010, which provides free access to immunization services for children under five, lactating mothers, and pregnant women, as well as mass routine immunization campaigns [14,15]. Between 2008 and 2019, the proportion of children aged 12–23 months who received all essential vaccines increased from 40% to 56%, while the proportion of children who did not receive any vaccines decreased from 16% to 2% [12]. A 2021 cross-sectional study assessing full immunization coverage among children aged 10–23 months reported that 66% were fully immunized within their first year of life [13]. Despite these improvements, the 2019 Sierra Leone Demographic and Health Survey (SLDHS) revealed significant disparities in immunization coverage, favoring children of mothers with primary education or higher and those residing in Bo District [12]. These disparities highlight the need for this study.

Although previous studies have explored routine immunization coverage and its determinants in Sierra Leone [5,11,13,15–20], none have specifically examined the socioeconomic and geographic inequalities in full immunization coverage among two-year-olds. Understanding these trends and inequalities is crucial for designing targeted public health interventions to improve routine immunization uptake and reduce the burden of VPDs in Sierra Leone. This study aims to examine the trends and investigate the socioeconomic and geographic inequalities in full immunization coverage among two-year-olds in Sierra Leone.

Method

Results

Trends in full immunization coverage among two-year-olds in Sierra Leone, 2008–2019

Fig 1 illustrates the trends in full immunization coverage among two-year-olds in Sierra Leone from 2008 to 2019. The figure shows an increase in full immunization coverage among two-year-olds in Sierra Leone from 41.1% in 2008 to 73.1% in 2013, indicating successful immunization interventions. However, the sharp decline in full immunization coverage among two-year-olds in Sierra Leone to 50.4% in 2019 suggests the need for continued sustained immunization efforts to improve immunization activities.

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Fig 1. Trends in full immunization coverage among two-year-olds in Sierra Leone, 2008-2019.

https://doi.org/10.1371/journal.pone.0329572.g001

Table 1 illustrates the trends in full immunization coverage among two-year-olds in Sierra Leone during the study period disaggregated by the various subgroups of the stratefiers. In 2008, coverage remained notably low, particularly among children from the poorest quintile (33.8%) and those living in the Western province (36.2%). By 2013, coverage was markedly increased across all subgroups, with the poorest quintile achieving 73.6% and children in the Western Province reaching 63.8%, reflecting enhanced efforts in routine immunization programs. The most notable progress was recorded in 2013, with full immunization coverage reaching 73.6% among children in the poorest quintile, 73.2% for those whose mothers had no formal education, and 72.8% for those residing in rural areas. However, by 2019, coverage declined across all dimensions, largely due to the Ebola outbreak during 2014–2016. There was no significant difference in coverage between male and female children. Geographically, all provinces experienced increased coverage except for the northern province, which declined to 36.6%.

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Table 1. Trends in full immunization coverage among two-year-olds in Sierra Leone, 2008-2019.

https://doi.org/10.1371/journal.pone.0329572.t001

Inequalities in full immunization coverage among two-year-olds in Sierra Leone, 2008–2019

Table 2 presents results of the magnitude of inequality based on the various measures considered in this study (D, R, PAF, PAR). Despite differences in inequality across age of the mother, economic status, educational level, place of residence, and sex of the child, between groups and over time, none of the results were significant. At the sub-national level, the coverage difference between children in the southern region and those in the northern region increased from 13.0 percentage points in 2008 to 23.9 percentage points in 2019, with the confidence intervals indicating that this difference was statistically significant. The ratio of children in the southern region compared to those in the northern region also increased from 1.4 percentage points in 2008 to 1.7 percentage points in 2019, with confidence intervals suggesting the presence of inequality between these two groups.

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Table 2. Inequalities in full immunization coverage among two-year-olds in Sierra Leone, 2008-2019.

https://doi.org/10.1371/journal.pone.0329572.t002

Discussion

This study explored the trends and socioeconomic and geographical inequalities in full immunization coverage among two-year-olds in Sierra Leone. It revealed an increase in fully immunization coverage from 41.1% in 2008 to 73.1% in 2013, followed by a sharp decline to 50.4% in 2019. Coverage trends improved across various dimensions, including age, economic status, educational levels, place of residence, and sex. However, at the provincial level, all provinces saw increased coverage except for the Northern region, which decreased. Notably, there was better coverage among the poorest populations and those living in rural areas, with inequality primarily existing at the subnational level.

The increase in full immunization coverage among two-year-olds from 2008 to 2013 is encouraging and can be attributed to several government interventions to enhance healthcare services, including routine immunization. Key initiatives included the launch of Free Healthcare Policy (FHP) in 2010 [14] and two nationwide immunization campaigns conducted in 2011 and 2012 [21,22]. This finding aligns with a study on measles immunization coverage inequalities in Sierra Leone, which stated similar reasons for the increase in coverage [23]. The decline in coverage observed in 2019 is likely linked to the Ebola outbreak from 2014 through 2016 [23,24], which disrupted routine immunization services and uptake, consistent with findings from other studies in Sierra Leone [16,25]. Overall, routine immunization coverage rates over the years have remained significantly below the expanded program on immunization target of ≥ 95% [26], indicating that many children remain at increased risk of contracting vaccine-preventable diseases (VPDs). Furthermore, the emergence of the COVID-19 pandemic in Sierra Leone may have further adversely affected routine immunization coverage, increasing the risk of VPDs among children [15]. As Sierra Leone works to rebuild a resilient health system after multiple shocks in recent decades, an outbreak of VPDs would impose an additional burden on healthcare systems, hindering efforts to meet the immunization agenda [27] and achieve Sustainable Development Goal 3 by 2030 [28]. To address these challenges, stakeholders are urged to enhance government healthcare spending and ensure adequate resources are allocated for immunization activities. Additionally, it is recommended that community healthcare workers be integrated into immunization efforts and provide incentives to support vaccine delivery, particularly during campaigns in hard-to-reach areas. Lastly, stakeholders should engage community structures, including women’s support groups, youth groups, and facility management committees, to promote vaccine uptake initiatives, especially during campaigns.

This study also revealed that children in the poorest quintile had relatively higher full immunization coverage than those in the richest quintile, albeit with the least increase. This contrasts with a multi-country analysis conducted by WHO, which included ten low- and middle-income countries; in that analysis, 9 out of the ten countries showed economic disparities in full immunization coverage for DTP 3 vaccines favoring children in the richest quintile [29]. In Sierra Leone, the higher coverage of full immunization among children from the poorest quintile can largely be attributed to the FHP introduced in 2010. This program, which provides free healthcare services to pregnant women, children under five, and lactating mothers, has significantly boosted the utilization of healthcare services [14]. Another main reason for the observed disparities may be explained by the fact that caregivers of children in the poorest quintile perceive their children to be at a higher risk of VPDs. Due to their limited financial resources to cover medical expenses, they are particularly concerned about their children contracting VPDs, which could worsen their health. This concern may have influenced their health-seeking behaviour regarding vaccine uptake, resulting in improved coverage. To address this disparity, stakeholders are encouraged to enhance ongoing awareness-raising activities through various platforms, including electronic and print media and social media.

Furthermore, children of mothers with no education showed improved coverage compared to those with primary education or higher over time; however, by 2019, the coverage levels were nearly the same. This finding differs from a multilevel analysis involving three Sub-Saharan African countries (Ghana, Kenya, and Côte d’Ivoire) where the authors reported persistent disparities with increased coverage over time for children of educated mothers [30]. In Sierra Leone, the improved coverage among children of uneducated mothers and the minimal difference observed in 2019 may be attributed to regular sensitization activities aimed at women without formal education. For example, the health education units in every district of Sierra Leone carried out routine health education initiatives aimed at pregnant women and nursing mothers during antenatal care sessions. Additionally, the (health education unit) frequently conduct routine awareness campaigns via local community radio stations and community engagement initiatives. These efforts collectively may have contributed to increased awareness of the benefits of immunization uptake, particularly among uneducated women, thereby resulting in higher observed coverage.

Regarding place of residence, children living in rural areas exhibited relatively higher immunization coverage than those in urban areas, except in 2008, when the coverage rates were equal. This finding aligns with studies conducted in The Gambia [31] and a multi-country study involving The Gambia, The Kyrgyz Republic, and Namibia [32]. The authors attributed this difference favoring rural children to the close relationships these children’s mothers have with healthcare workers, which likely enhances their trust and confidence in vaccine uptake, resulting in higher coverage. Just as observed with economic status, where immunization rates were higher among the poorest quintile, the rise in full immunization coverage in rural areas appears to result from targeted immunization efforts, focusing on marginalized populations. For example, the Ministry of Health in Sierra Leone initiated weekly outreach sessions, during which healthcare workers delivered immunization services with a specific emphasis on underserved communities. This approach likely played a key role in the increased full immunization coverage in rural areas observed in this study. It is therefore recommended that other low- and middle-income countries facing challenges in improving full immunization coverage and reducing inequality gaps adopt similar targeted approaches.

Finally, this study revealed inequalities in full immunization coverage among two-year-olds at the provincial level; the Southern region recorded the highest coverage, while the Northern region decreased over time. The observed regional disparities in full immunization coverage in Sierra Leone can be attributed to several factors. The Southern region, often considered more developed, may have better access to healthcare facilities, a higher density of health workers, and improved infrastructure, all of which contribute to higher immunization rates. In contrast, the Northern region, often characterized by rural areas, may face challenges such as poor road networks, limited healthcare access, and lower health worker density. Additionally, cultural beliefs, misconceptions about vaccines, and socioeconomic factors like poverty and low education levels can influence immunization uptake in specific regions. These factors can collectively hinder immunization efforts in the Northern region, leading to lower coverage rates than in the Southern region. Consequently, this study recommends conducting a mixed-methods or qualitative study to understand the reasons for the regional heterogeneity in full immunization coverage among two-year-olds in Sierra Leone.

Conclusion

The study highlights a complex picture of immunization coverage in Sierra Leone. While there was significant progress between 2008 and 2013, the subsequent decline in 2019 underscores the need for sustained and targeted interventions. While inequalities in coverage based on maternal age, place of residence, and child sex were not statistically significant, a substantial disparity emerged between the southern and northern regions. This suggests that future policies and practices should prioritize strengthening immunization services in the northern region, mainly focusing on addressing the underlying factors contributing to the lower coverage. Additionally, continuous monitoring and evaluation of immunization programs are crucial to identify and address emerging challenges and maintain the progress achieved. Furthermore, future research should consider utilizing raw datasets to implement age standardization, thereby minimizing potential biases within the given population.