Background
Poliomyelitis, commonly known as Polio, is an infectious disease caused by the Poliovirus [
1,
2]. There are three serotypes of the Wild Poliovirus (WPV); type 1, 2 and 3 – types 2 and 3 have been eradicated. The Poliovirus is transmitted from person to person primarily through contaminated fecal matter entering the oral route [
1,
3]. It can also be transmitted in rare cases through saliva [
3,
4]. Human beings are the only known reservoir for the poliovirus [
5]. The Poliovirus replicates in the intestine of its human host and consequently spreads to the central nervous system [
3,
6].
The burden of polio disease is higher among children under 5 years of age [
3,
7,
8]. Poliovirus infection causes irreversible paralytic disease, presenting as Acute Flaccid Paralysis (AFP), in 1 per 200 to 1 per 1000 cases [
9] with a Case Fatality Rate (CFR) of 5 to 10% [
6]. Poliomyelitis has no cure [
10]. However, it is possible to prevent the disease through vaccination [
10,
11].
The forty-first World Health Assembly (WHA) in Geneva Switzerland in May 1988 was a breakthrough for the commencement of The Global Polio Eradication Initiative (GPEI) led by national governments in partnership with the World Health Organization (WHO), Rotary Foundation, Bill and Melinda Gates Foundation (BMGF), United States Centers for Disease Control (US CDC), and United Nations Children’s Fund (UNICEF) [
5,
8,
12]. The initiative targeted to eradicate the poliovirus worldwide by the year 2000 using four proven strategies [
13]. These are: 1) maintaining high population immunity using OPV and IPV through the Expanded Programme on Immunization (EPI), 2) detect and interrupt circulation of all suspected cases of Poliomyelitis through sensitive AFP surveillance, 3) Supplemental Immunization Activities (SIAs), and 4) mop-up campaigns [
14,
15]. A sensitive AFP surveillance system is central to the overall polio eradication initiative [
4,
16]. The GPEI has a set of performance indicators to monitor progress and evaluate performance of countries [
1]. These global efforts have reduced the number of WPV cases by more than 99%: from an estimated 350,000 of WPV cases from 125 countries in 1988 to only 33 cases by the end of 2018 [
17,
18]. Since August 2016, WPV cases have not been detected outside Afghanistan and Pakistan [
17‐
19].
In Kenya, under the Ministry of Health (MOH), the department of Disease Surveillance and Response (DDSR) in collaboration with the National Vaccine and Immunization Program (NVIP) collaborate on the Polio eradication activities. The activities aim to ensure the certification of the country as free of indigenous wild poliovirus in the shortest possible time and maintain polio free status. The last case of WPV (non-indigenous) in Kenya was detected in 2013, with a paralysis onset date of 14 July 2013 [
20]. This was an imported AFP case from neighboring Somalia [
20].
Describing characteristics of AFP cases and assessing performance towards set goals are of great importance in providing quantitative evidence to support policymakers in making informed decisions. For this reason, this study aims to describe Kenya’s AFP surveillance performance from 2016 to 2018 based on the WHO recommended epidemiological performance indicators [
1] for AFP surveillance.
Discussion
The present study aimed to describe AFP surveillance performance in Kenya from 2016 to 2018 based on the WHO recommended performance indicators for AFP surveillance. It demonstrated the mean national performance disaggregated by counties and provided evidence on performance of key AFP surveillance indicators improved markedly throughout the study periods.
The main goal of any AFP surveillance system is to detect, investigate, report, disseminate and promptly implement control measures in a timely manner for indigenous and imported cases of WPV or cVDPV [
29]. This goal is also expandable to certify polio free status [
7,
19]. No case of poliomyelitis was reported from 2016 to 2018. However, a cVDPV type 2 was isolated from an environmental sample in Nairobi County in March 2018. The Poliovirus isolated had similar nucleotide sequence structure to poliovirus isolated in Somalia. This highlights that there is a potential for Poliovirus importation in Kenya especially from polio outbreak neighboring countries thereby justifying the need for strengthening AFP surveillance [
7,
30]. As part of the outbreak response, five successive rounds of polio SIAs were conducted in 12 high-risk counties representing about 25% of all counties in Kenya. The SIAs were conducted between May to September 2018. Active surveillance for AFP cases was conducted during the SIAs.
Our finding showed that majority of the AFP cases were below the age of 5 years (59 months). This is in agreement with findings from previous studies in Ghana, Iran, Nigeria and India [
8,
24,
31]. On the contrary, a study in Italy [
32] highlighted a smaller percentage of AFP cases occurring among under five children.
Our findings also give credence to the differential of reported AFP cases by gender. Our results revealed that the mean proportion of AFP cases was higher in boys (55.1%) as compared to girls (44.9%), although the difference was not statistically significant. The result from this study is in line with previous studies conducted in Ghana, Iran, Ethiopia and Italy [
8,
24,
32,
33].
The current study also evidenced that higher percentage of children had developed fever at onset of paralysis, which progressed within 3 days and was asymmetric. Authors believe that this could be accredited to appropriate implementation of AFP case definition during case investigation to determine if the reported case meets the case definition for AFP or not. Different studies across the globe have upheld the same finding [
24,
33,
34].
Two types of polio vaccine are used in polio eradication efforts [
8,
35]; the Inactivated Polio Vaccine (IPV) which is administered through an injection and the Oral Polio Vaccine (OPV) which is administered orally; it is live but attenuated (weakened) virus developed by Sabin [
36]. The Oral Polio Vaccine is the main vaccine towards polio eradication. When OPV is administered, the weakened vaccine-virus replicates in the intestine and enters into the bloodstream, triggering a protective immune response. During replication process, some of the vaccine-virus may genetically mutate from the original attenuated strain and become neurovirulent. The neurovirulent virus from the polio vaccine is referred to as Vaccine-Derived Poliovirus (VDPV), which is able to cause paralysis [
20]. The VDPV is able to circulate in the community, known as circulating Vaccine-Derived Poliovirus (cVDPV) [
20], most likely due to weak AFP surveillance, suboptimal routine immunization coverage, and constant movement of populations [
20] in cross-border high-risk areas. In the case of the IPV vaccine, the virus is inactivated. Therefore, it cannot regain virulence after it has been administered to a child. The Inactivated Polio Vaccine is not a replacement for OPV but rather used in addition to OPV to protect against polio serotype 2 which is not in the current OPV and also strengthen immunity against the other two serotypes. A vast majority of the AFP cases had received the recommended three and above doses of OPV. It is possible that the presented proportion of AFP cases that is said to have received at least three doses of OPV might have been under- or overestimated by recall bias from parents or caregivers of the children whose immunization cards could not be traced [
24,
37]. In addition to the routine immunization program, the high polio vaccination coverage in reported AFP cases is attributable to high quality polio SIAs conducted sixth times with greater emphasis on high risk and underserved population sub-groups. High immunization coverage through routine immunization and SIAs boost herd immunity and, disrupts chain of infections and silent circulation of poliovirus [
38‐
40]. The findings of our study highlight that Kenya has sensitive and quality AFP surveillance system capable of detecting and reporting Poliovirus cases. The health workers’ industrial action in 2017 might have greatly affected the performance of AFP surveillance and routine immunization activities as depicted by lower performance in 2017 in comparison with 2016 but better performance in 2018 that in 2016. It is worth noting that the minimum NP-AFP rate and stool adequacy targets set by the WHO were surpassed at the national level in all the 3 years analyzed. Furthermore, drill-down analysis of the mean NP-AFP rate by counties showed heterogeneous performances. Some counties such as Tana-River, Wajir, Isiolo and Garissa outperformed while others for instance Lamu, West-Pokot, Migori and Trans-Nzoia were least performers.
The findings of the study showed that the mean proportion of AFP cases with adequate specimens surpassed the WHO recommended target of 80% and above from 2016 through 2018. Like, NP-AFP rate, there were disparities in stool adequacy performance among counties. Lamu, West-Pokot, Marsabit, Turkana, Siaya, Garissa and Busia counties were unable to meet WHO recommended target for collection of two adequate specimens within 14 days of onset of paralysis. As important as case detection, laboratory analysis is crucial to the confirmation of the poliovirus [
28]. Thus, the stool specimens collected from AFP cases need to be of a quality that meets minimum laboratory requirements. Subsequently, with high stool adequacy rates coupled with no poliovirus isolation during the study period, it is reasonable to believe that there was no wild poliovirus circulating in the country during the study period, although the risk of poliovirus importation from neighboring countries with relatively weaker health systems remains high due to continued influx of refugees and general movement of people.
In our study, we noted that the NPEC classified 23 AFP cases as polio compatible. The 23 polio compatible cases lent an unprecedented opportunity to cluster investigation of this subgroup of AFP cases by geographical area and time [
1], which was very important towards polio eradication activities [
31]. Cluster investigation and additional passive and active case searches conducted verified that there were no missed polio cases and dismissed the presence of circulation with confidence.
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