Sustained population immunity is required to prevent the emergence and spread of circulating vaccine-derived polioviruses. Routine immunisation protects successive birth cohorts through scheduled oral and inactivated poliovirus vaccine doses, while supplementary immunisation activities provide additional doses to children within targeted age groups irrespective of previous vaccination status. The two delivery platforms are complementary: routine services maintain immunity over time, whereas campaigns rapidly close immunity gaps and support outbreak control [1].

Routine-immunisation performance cannot be judged by a single administrative coverage estimate.

It also depends on completion of multidose schedules, implementation of fixed and outreach sessions, vaccine availability, reporting quality and the extent to which zero-dose and under-immunised children are reached.

Administrative coverage may be distorted by inaccurate target-population denominators, movement across administrative boundaries, duplicate reporting and weak reconciliation of doses. Consequently, values above 100% may coexist with communities in which substantial numbers of children remain unvaccinated [2,3].

Supplementary immunisation activity performance is similarly multidimensional. Administrative coverage is supplemented by independent monitoring, lot quality assurance sampling, missed-child data, household non-compliance, settlement accessibility and the timeliness of outbreak-response rounds. These measures are important because favourable state averages can conceal settlements, households or population groups that vaccination teams did not reach [4–8].

Northern Nigeria has implemented several approaches to strengthen immunisation, including mobile teams, household-based microplanning, community mobilisation, engagement with resistant households and systematic use of campaign-monitoring data [4,8–12]. These interventions have improved access in high-risk settings, but insecurity, population mobility, vaccine hesitancy, inaccurate microplans, service interruptions and weak data quality continue to create geographically concentrated immunity gaps [2,3,13,14].

Kano and Katsina States contain 78 local government areas with heterogeneous urban, rural, mobile, border and hard-to-reach populations. Although both states have implemented routine and supplementary poliovirus immunisation activities, evidence comparing performance across states, years and local government areas during 2020–2024 has been limited. This study therefore assessed routine-immunisation coverage, service delivery, dropout, stock-outs, reporting, zero-dose and under-immunised prevalence, supplementary-immunisation quality and outbreak-response timeliness in Kano and Katsina States.

Materials and Methods

Study design and setting

A retrospective longitudinal ecological design was used. The study covered Kano and Katsina States in Northwest Nigeria from 1 January 2020 to 31 December 2024. Kano State has 44 local government areas and Katsina State has 34, giving 78 local government areas. The local government area-year was the unit for routine-immunisation analyses, while state-year and local government area-round records were used for supplementary-immunisation analyses.

Study population and sampling

The study population comprised eligible routine-immunisation and supplementary-immunisation programme records generated in the 78 local government areas during the study period. A total-population sampling approach was used, and all records meeting the eligibility criteria were included. Records were eligible when the state, local government area, year or campaign round and the variables required for the relevant indicator could be established. Duplicate records, records outside the two states or study period and records that could not be reconciled to a valid reporting unit were excluded.

Data sources and extraction

Data were obtained from District Health Information System 2 routine-immunisation reports, vaccine-utilisation and stock records, fixed- and outreach-session reports, supplementary-immunisation activity microplans, campaign-preparedness records, independent-monitoring reports, lot quality assurance sampling results, missed-child registers, non-compliance reports and outbreak-response assessments. A structured programme-data extraction form captured state, local government area, reporting period, target population, vaccine doses administered, service-delivery indicators, vaccine availability, reporting indicators, campaign-monitoring results and outbreak-response intervals.

Data quality procedures included range and consistency checks, review of duplicate identifiers, reconciliation of conflicting totals and comparison of state summaries with local-government records. Administrative coverage estimates above 100% and negative dropout values were retained as reported because they constituted findings relevant to denominator and reporting quality rather than values to be automatically corrected.

Study indicators

Routine-immunisation indicators included OPV1, OPV3, IPV1, IPV2, Penta1 and Penta3 administrative coverage; pentavalent-vaccine dropout; implementation of planned fixed and outreach sessions; vaccine stock-out rate; reporting completeness and timeliness; estimated zero-dose prevalence; estimated under-immunised prevalence; and the number of local government areas with OPV3 or Penta3 coverage above 100%. Penta dropout was calculated from the difference between first- and third-dose coverage relative to first-dose coverage.

Supplementary-immunisation indicators included the number of campaign rounds, administrative coverage, independent-monitoring coverage, missed-child proportion, contribution of non-compliance to missed children, inaccessible-settlement rate and lot quality assurance sampling outcomes. Outbreak-response timeliness was measured from Day 0 to the rapid-response round, first large-scale round and second large-scale round, using the prescribed response periods in the applicable programme procedures [15].

Statistical analysis

The analytical datasets were processed in Python. Frequencies, percentages and means were used to summarise performance by state, year and local government area. Five-year local-government summaries were obtained by averaging annual indicator values, while state-year summaries were calculated from the programme records. No composite immunisation-performance index was created because combining indicators could conceal failure in a specific programme component.

Mean performance in Kano and Katsina was compared using Welch’s independent-samples t test because equality of variances was not assumed. Temporal and state differences in Penta3 administrative coverage were examined using a linear mixed-effects model containing a random intercept for local government area and fixed effects for state and year. The state coefficient is presented as the adjusted percentage-point difference, while the year coefficient represents the annual percentage-point change. All tests were two-sided, statistical significance was set at p<0.05 and estimates are reported with 95% confidence intervals.

Ethical considerations

Ethical approval was obtained from the Research Ethics Committee of Nasarawa State University, Keffi. Administrative permission to access the required programme records was obtained from the relevant federal and state health authorities. The study involved retrospective review of programme records without direct contact with children or caregivers. Personal identifiers were excluded, and findings were reported in aggregate form.

Results

Routine-immunisation performance by state and year

Routine-immunisation performance generally improved between 2020 and 2024. In Katsina, OPV3 coverage increased from 89.29% to 100.88%, while Penta3 coverage increased from 89.22% to 103.09%. In Kano, OPV3 increased from 83.10% to 99.27% and Penta3 from 81.20% to 100.00%. IPV1 coverage also increased in both states. Penta dropout declined from 9.07% to 6.04% in Katsina and from 12.89% to 6.84% in Kano. Vaccine stock-out rates and estimated zero-dose prevalence generally declined, while reporting completeness improved (Table 1).

Table 1. Routine-immunisation performance indicators by state and year

Panel A: vaccine coverage and dropout indicators. Administrative coverage above 100% was retained as reported.

Panel B: service delivery, vaccine stock, reporting and immunisation-gap indicators.

Fixed-session implementation increased in both states. Outreach-session implementation improved in Kano but fell sharply in Katsina to 59.00% in 2024. Administrative coverage above 100% occurred increasingly in later years. By 2024, 15 Katsina local government areas and 21 Kano local government areas reported OPV3 coverage above 100%, while 17 Katsina and 24 Kano local government areas reported Penta3 coverage above 100%.

Local-government variation in routine immunisation

Substantial variation was observed across the 78 local government areas. In Katsina, mean OPV3 coverage ranged from 53.9% in Zango to 182.8% in Danja, and mean Penta3 coverage ranged from 55.3% to 176.0% in the same local government areas. Bindawa had the highest mean Penta dropout at 21.5%, while Daura, Ingawa, Zango and Sandamu recorded mean zero-dose prevalence above 25%. Outreach implementation was lowest in Ingawa at 67.9%.

In Kano, mean OPV3 coverage ranged from 74.0% in Bichi to 110.3% in Kibiya, while mean Penta3 coverage ranged from 73.4% in Bichi to 109.4% in Kibiya. Karaye recorded the highest mean Penta dropout at 15.0%. Bichi had the highest mean zero-dose prevalence at 17.3%, and Karaye had the highest under-immunised prevalence at 17.1%. Outreach implementation ranged from 79.9% in Madobi to 88.0% in Sumaila (Table 2).

Table 2. Five-year local-government variation in routine-immunisation indicators

Supplementary-immunisation performance improved in both states. Independent-monitoring coverage increased from 86.94% to 91.62% in Katsina and from 88.41% to 94.36% in Kano. The missed-child proportion declined from 13.06% to 8.38% in Katsina and from 11.59% to 5.64% in Kano. Non-compliance continued to account for approximately 28%–31% of missed children, while inaccessible-settlement rates remained higher in Katsina.

Lot quality assurance sampling performance improved but remained suboptimal. The proportion of lots passing increased from 22.06% to 36.27% in Katsina and from 14.77% to 59.09% in Kano. Despite this progress, substantial numbers of lots remained in warning or failure categories, particularly in Katsina (Table 3).

Table 3. Supplementary-immunisation activity performance by state and year

LQAS, lot quality assurance sampling. Administrative coverage above 100% was retained as reported.

Timeliness of outbreak-response immunisation

Outbreak-response timeliness varied by state and year. In Katsina, the rapid-response and first large-scale rounds were delayed in 2021 and 2022, while all three stages were timely in 2024. Kano implemented all three stages within the prescribed periods in 2021 and 2022. The rapid-response round was delayed in Kano in 2023, and the first large-scale round was delayed in 2024. The second large-scale round was timely in every recorded outbreak year (Table 4).

Table 4. Timeliness of cVDPV outbreak-response immunisation activities

1 = implemented within the prescribed period; 0 = not implemented within the prescribed period.

State comparisons and temporal change

Katsina had significantly higher mean OPV3 coverage than Kano (96.70% versus 91.85%; p=0.029), higher IPV1 coverage (103.35% versus 88.25%; p<0.001) and higher Penta3 coverage (97.41% versus 91.11%; p=0.004). Kano had a higher mean Penta dropout rate (9.91% versus 7.68%; p<0.001). Kano also recorded higher supplementary-immunisation independent-monitoring coverage (91.81% versus 89.91%; p<0.001) and a lower missed-child proportion (8.19% versus 10.09%; p<0.001) (Table 5).

In the linear mixed-effects model, the adjusted difference in Penta3 coverage between Kano and Katsina was not statistically significant (β=-6.300 percentage points; 95% CI: -14.277 to 1.678; p=0.122). Penta3 coverage increased significantly by 4.136 percentage points for each successive year (95% CI: 3.721–4.551; p<0.001) (Table 5).

Table 5. State comparison and temporal model of immunisation performance

Panel A: Welch’s independent-samples t tests.

Panel B: linear mixed-effects regression with a local-government random intercept. β values are percentage points. CI, confidence interval