The primary intent of the study was to characterise the NIPs of countries within the WHO-Afro experiencing PHEs using immunisation targets outlined for the DoV as proxy indicators. We found that PHEs were endemic during the entire study period and the performance of NIPs in the PHE endemic countries was variable, although mostly suboptimal. Additionally, higher total PHE and armed conflict counts were associated with not achieving the immunisation targets for ≥ 90% national DTP3 coverage and MNT elimination. Higher disaster counts were also associated with not eliminating MNT.
The endemicity of PHEs observed during the study period, agree with other reports on the recurrent and diverse nature of PHEs experienced in the WHO-Afro (4, 38). While armed conflict and disasters were found to have the highest cumulative count in our study, contrary, trends were observed in the WHO-Afro, with a majority of reported PHEs being that of disease outbreaks (3, 38, 39). The discrepancy can be explained by findings from other studies done in similar settings, where disease outbreaks were noted to occasionally go unrecorded as they occur as twin PHEs, in the context of armed conflicts and disasters (2, 40).
Regardless of the immunisation maturity level, the overall performance of NIPs in the PHE endemic countries was variable but predominantly suboptimal. The fluctuating and the low national DTP3 coverage rates below the WHO-Afro DTP3 coverage stagnation point are synonymous with DTP3 coverage reports in other NIPs in the region during PHEs (7, 12, 23). Similarly, the high DTP vaccine drop-out; classified as ≥ 10% (26), recorded in some of the NIPs, concur with findings from a study by Mugali et al. (41) in Afghanistan where high DTP drop-out rates were associated with conflict and insecurity. The introduction of the PCV and the Rotavirus vaccines into the NIPs of the study countries was high compared to the HPV vaccine. These findings are comparable to those in the region as 86%, 76%, and 8% of GAVI eligible countries had introduced the PCV, Rotavirus, and the HPV vaccines respectively by 2017 (42). Delay in HPV vaccine introduction has been attributed elsewhere to the poor adoption of life-course immunisation, vaccine supply constraints, and pricing issues (17, 24) which stand exacerbated by PHEs (7). The outlier countries yet to introduce any of the three selected vaccines are potential key pointers to the contributing role PHEs play in delaying vaccine introductions into NIPs as raised by Grundy et al. in their study (7).
Despite the positive progress in NITAG establishment in the WHO-Afro (43), a non-consistent pattern in their existence and functionality simultaneously existed. Such inconsistent trends have been attributed in previous reports to the lack of political commitment and country ownerships of NIPs and the low NITAG financial investments, in general, and during PHEs (43, 44).
The elimination target for MNT was not met in countries with high counts of PHEs like South Sudan, the Central African Republic, Niger, Guinea, and Angola. The enlisted five countries constitute nearly 50% of the remaining global MNT elimination priority countries (45) and have similar trends with other MNT priority countries like Afghanistan and Yemen, where the presence of protracted PHEs have been blamed on delaying elimination (46).
Having higher armed conflict and total PHE counts were associated with not attaining the target for ≥ 90% national DTP3 coverage. These findings are comparable to those from studies conducted in similar contexts where armed conflicts were reported to be associated with poor immunisation coverage outcomes (7, 12, 30). The findings further show that periods of higher counts of armed conflicts, disasters, and total PHEs were also associated with not eliminating MNT. Elimination of MNT in the WHO-Afro has been evasive to some extent, with most of the countries prioritised for MNT elimination being affected by PHEs (47–49). In other PHE endemic contexts, where MNT elimination remains unattained, low tetanus vaccine coverage among women of reproductive age, unhygienic birth practices, delayed treatment, and inadequate MNT surveillance have been cited to delay progress (48, 50, 51).
Higher counts of all the three types of PHEs and the total PHEs were not associated with meeting the immunisation targets of having functional NITAGs or the introduction of new vaccines. The lack of consistency in the existence and functionality of NITAGs is not uncommon as observed in other PHE prone contexts in the region (43). On the other hand, during PHEs, whereas the functionality of NITAGs may be compromised (44), the WHO recommends humanitarian immunisation teams to utilise the unique expertise and local knowledge of existing NITAG members (19). For the target to introduce new vaccines, a possible caveat may exist in interpreting association, as within the scope of this study, only three vaccines were selected out of a pool of new vaccines. Conversely, we equally recognise that despite PHE endemicity, most of the case study countries had performed relatively well in this target which is also credited as one of the WHO-Afro DoV successes (24, 29) owing to political commitment and strong immunisation investments (49). Arguably, such progressive performance noted in this DoV target, can offer a lesson; that the link between PHEs and immunisation performance may not be always absolute.
Our results should be interpreted in light of certain caveats. First, our study was limited to three broad types of PHEs, that may not be exhaustively representative of all types of PHE experienced in the study countries. Furthermore, the severity of the PHEs could not be accounted for as this level of data was not available. The WHO conventional PHE grade system for grading PHE severity was first availed for use in the WHO-Afro in 2017, which was towards the end of the study period. It should be noted that PHEs often overlap in the real-world context and it may be challenging to identify how a single PHE may have impacted the performance of immunisation programmes. Additionally, we may be unable to determine how previous PHEs experienced before the start of the study period in 2010 may have affected the NIPs in the selected countries.
Secondly, we used country generated reports from the WHO/UNICEF JRF to abstract data on DoV immunisation indicators. It is, possible that potential inaccuracies in the country data may have influenced the outcome of our study. However, this could not be avoided within the scope of this study. Data quality issues from country reports have been constantly highlighted as one of the major concerns by the WHO, SAGE in their annual DoV immunisation reports (24, 28, 29).
Lastly, while associations between the occurrence of PHEs and the performance of immunisation programmes were inferred we cannot rule out with certainty the influence of other determinants of immunisation programme performance. It was also not possible to account for emergency responses aimed at providing immunisation services during PHEs. As such, our estimated results may represent a lower bound for the true effect of PHEs on immunisation.
Future research investigations should address these limitations to broaden the scope of evidence on the interactions that exist between PHEs and immunisation performance. We propose that future studies should further explore 1) how other types and grades of PHEs may variably impact immunisation performance, 2) immunisation performance at the sub-national level using immunisation data from localised communities where PHEs are recorded to occur, and 3) other determinants that may act together with PHEs to influence immunisation performance.