This study aims to describe the epidemiological profile of AFP cases in Niger from 2016 to 2021, measure the performance of the surveillance of AFP through performance indicators including annualized NPAFP, AFP with one faecal specimen within 14 days of paralysis onset, proportion of AFP cases with stool specimens received in good condition, proportion of AFP cases with two adequate stool specimens, reported AFP cases with follow-up exam at least 60 days after paralysis onset, NPEV, Average delivery time of stools samples transport (days), and AFP cases with laboratory results < 28 days of receiving samples at laboratory. Among the 4134 cases, the vast majority of AFP cases were less than 5 years old. The most affected age group is that of 1–4 years with 79.85%. These data confirm that poliomyelitis mainly affects children under 5 years of age [2]. Our findings were similar to those reported in Mauritania with 77% [14], in Côte d'Ivoire with 60% [15] and in Nigeria with 82.4% [1]. In contrast, Indonesia (42.5%) [16], in Spain (47.5%) [17] and Iran (52.6%) [18] reported moderately lower AFP cases. This could be explained by high polio vaccination coverage in the under 5 years. Our study also reported more than a haft of AFP cases (55.90%) was reported males. This findings was similar to Nigeria (56.4%) [1], Spain (56.4%) [17] and Mauritania (55.5%) of AFP male cases [14]. In contrast, a higher AFP cases among males was found in Indonesia with 74% [16].
Our study also revealed that fever at onset was present in 90.13% of AFP cases. Our result was in the line with those found in Mauritania (90%) [5] and in Guinea (94%) [19]. However, Nigeria( 59%) [1] and Spain( 20%) [17] reported lower AFP cases with fever at onset. Besides, our study found lower rate asymmetrical paralysis (31.78%) than a study conducted in Nigeria (63.7%) [1]. During the study six years, the majority of AFP cases (54%) were reported in the last trimester of 2017 to the second trimester of 2019, reaching the highest pick in this trimester (Fig. 3). Trimesters 3 and 4 correspond to the rainy season in Niger, which are periods of high polio transmission, with potential outbreaks associated to AFP. This could explain high AFP cases in Zinder and Maradi provinces. Our findings were in the line with studies that found out high AFP cases during the rainy seasons[20–22]. Besides, our result showed that NPAFP raised from 2017 with 6.39 per 100,000, reaching the highest incidence rates in 2018 and 2019 with 8.77 and 7.84 per 100,000, respectively. Then, the NPAFP felt down in 2020 and 2021 with 5.00 and 5.20, respectively. This could be explained by the impact of COVID-19 pandemic on AFP surveillance. The COVID-19 pandemic has also had an impact on the average arrival time of samples at the central level and at the level of the reference laboratory. A study undertaken in East and Southern African countries found a decline in the core AFP surveillance (non-polio) NP-AFP rate, and percentage of stool adequacy in these countries during the COVID-19 pandemic [23]. Besides, the very high non-polio AFP rate observed between 2018 and 2019 may be explained by the deployment of polio consultants to support the health districts of Diffa and Zinder as part of the Lake Chad Basin response. Our study revealed an average of non-polio AFP rate of 5.93 per 100,000 children under age 15. Our finding was not far way to Uganda 3.0[24], Indonesia 3.85 [16] and Mauritania 4.61 per 100,000 children under 15[14]. On the other hand, non-polio AFP rate was higher than our findings in Guinea with 7.09 [25] and Nigeria 9.1 per 100,000 children under 15 [25]. Of the AFP cases reported during our study period, 79.48% of cases received more than 3 doses of OPV, compared to Guinea with 70% [25] and Uganda with 82.16 [24].
Our study also found that 33 cases of cVDPV2 were recorded, predominantly in Zinder region. In contrast, we did not record any case of cVDPV1 and cVDPV3. However, Kennedy et al. reported an unexpectedly low seroprevalence against type 2 poliovirus of 38·3% in Liberia after two nationwide campaigns with nOPV2 [26]. These findings suggest that this novel vaccine might not be the answer that is needed to fight against cVDPV2. An increasing number of new cVDPV2 outbreaks are attributable to mOPV2 use and the geographical spread of established cVDPV2 emergences is rapidly increasing with declining population immunity [27]. This study revealed more than 80% of AFP cases received at least 3 doses of OPV vaccine from 2016 to 2021. This is similar to study findings in Ethiopia [28], Kenya [29], Uganda [24] and Iran [30] which also found that most AFP cases had received three or more doses of oral polio vaccine. In contrast, during the six years of our study, the polio vaccination coverage did not reach the target of 95% as the recommended vaccination coverage to prevent the reintroduction of the virus [31]. By the way, recent outbreaks reported in Niger of OPV2 may be attributable to low routine immunization and population immunity during the COVID-19 periodThe WHO Strategic Advisory Group of Experts (SAGE) recommended that at least one dose of IPV should be used in routine immunization in all countries to protect against paralysis from all poliovirus, including serotype 2 [32]. By the way the cost-effectiveness of switching from OPV to IPV is needful in Niger. Furthermore, particular emphasize should be placed on Agadez, Diffa, Maradi and Zinder provinces where most of the cVDPV2 cases were found.
Reviewing the surveillance of AFP performance indicators for Niger from 2016 to 2021, Annualized non-polio AFP rate, AFP with one faecal specimen within 14 days of paralysis onset, reported AFP cases with follow-up exam at least 60 days after paralysis onset, NPEV, and AFP cases with laboratory results < 28 days of receiving samples at laboratory performed well during the six years. In contrast, the proportion of AFP cases with stool specimens received in good condition poorly performed in 2016 with 69% and the proportion of AFP cases with two adequate stool specimens was only above 80% in 2018. In the same line, the average delivery time to transport stools samples to the laboratory was beyond 7 days during the six years. Even though, the proportion of stool adequacy was above 80% in all the eight provinces of Niger during the six years, our study has shown that five districts namely Guidan Roumdji, Tahoua, Arlit, Tera, and Bosso reported less than 80% of stool adequacy. Comparatively to findings reported to other districts including Ethiopia[28], some eastern and southern African countries [23], and Liberia [33], our findings showed that it is even more critical that the AFP surveillance system should be strengthened in Niger. Niger should analyse routine AFP index to identify any subnational gaps to guide AFP activities and thereby minimize the consequences of any new virus introduction. Priority should be given to areas at high-risk of importations and where OPV3 coverage is < 80%[12]. Besides, environmental surveillance (ES) is a supplementary polio surveillance system, and has become an essential component of the overall polio surveillance program. ES also allows the programme to assess the quality of outbreak response, as it can detect the vaccine-virus used in immunization activities [34]. In Niger, two additional polio environmental surveillance sites were opened in the Zinder region in addition to the eight (8) already existing in the Diffa (2), Maradi (2) and Niamey (4) regions, bringing thus at 10 the total number of functional sites in the country [10].
The Polio Eradication Strategy 2022–2026 lays out the roadmap to securing a lasting and sustained world, free of all polioviruses, and transition and polio post-certification efforts are ongoing to assure that the infrastructure built up to eradicate polio will continue to benefit broader public health efforts, long after the disease is gone [2]. As part of strengthening AFP surveillance in Niger, the WHO Office supported the MSP for the following achievements [10]: the response to the cVPDV2 epidemic in the regions of Agadez, Diffa, Maradi and Zinder through vaccination of monovalent OPV type 2, strengthening of epidemiological surveillance and routine EPI, provision to the MSP of technical assistance (international consultants seniors, international STOP Team consultants, national epidemiologist consultants and data managers), preventive vaccination campaigns, extension of community-based AFP surveillance in districts, Support for Centre for Medical and Health Research (CERMES), and Support for the implementation of the activities of the national poliomyelitis committees. This support has made it possible, on the one hand, to strengthen poliomyelitis surveillance in Niger, which has led to the rapid notification of cVPDV2 cases and the implementation of appropriate vaccination responses in the districts, on the other hand [10]. The capacities of the teams have been strengthened with the close support of staff deployed in the field by WHO and other partners [10].
The strengths of study reside in the fact that we included a large sample AFP sample size highlighting AFP epidemiological profile, its performance indicators and gaps in Niger from 2016 to 2021. This period, the country went through concomitant outbreaks including AFP and COVID-19 and armed conflicts. This study highlighted the importance of increasing polio vaccination coverage, improving the average delivery time of stools samples transport, and regional and sub-regional stool adequacy in Niger. Other study strengths included the fact that the age, hospital admission and stool condition were associated with AFP cases findings. The major study limitations are those in observational studies such as recall bias and missing data on OPV doses uptake.