Surveillance to Track Progress Toward Polio Eradication — Worldwide, 2018–2019

Acute Flaccid Paralysis Surveillance

Two primary surveillance performance indicators assess AFP surveillance quality. The first is the nonpolio AFP (NPAFP) rate^§ (the number of NPAFP cases per 100,000 children aged <15 years per year); an NPAFP rate ≥2 is considered sufficiently sensitive to detect circulating poliovirus. The second is the collection of adequate stool specimens from AFP patients (i.e., two stool specimens collected ≥24 hours apart and within 14 days of paralysis onset) and arrival of these specimens at a WHO-accredited laboratory by reverse cold chain (storing and transporting samples at recommended temperatures from the point of collection to the laboratory) and in good condition (i.e., without leakage or desiccation) from ≥80% of persons with AFP, which ensures adequate sensitivity and specificity to track poliovirus circulation (3).

Among the 47 countries in the WHO African Region (AFR), the number of priority countries increased from 18 (38%) in 2018 to 30 (64%) in 2019 because of the increase in the number of VDPV outbreaks (2,4). To describe the previous 2 years’ performance for this year’s priority countries, surveillance performance was assessed for 2018 and 2019 for the 30 2019 priority countries in AFR (Table 1). In 2018, cVDPV type 2 (cVDPV2) cases or environmental surveillance isolations were detected in five countries (Democratic Republic of the Congo [DRC], Kenya, Mozambique, Niger, and Nigeria) and, in 2019, in 14 countries (Angola, Benin, Burkina Faso, Cameroon, Central African Republic, Chad, Côte d’Ivoire, DRC, Ethiopia, Ghana, Niger, Nigeria, Togo, and Zambia). In 2018 and 2019, both the NPAFP rate and adequate stool collection AFP surveillance performance indicators were met nationally in 27 (90%) and 20 (67%) of the 30 2019 priority countries, respectively (Table 1). Numerous subnational pockets of low surveillance performance were identified during 2018–2019 (Table 1) (Figure). September 2019 marked 3 years since the last reported WPV1 isolation in AFR (in Borno, Nigeria); during this period, populations living within security-compromised areas in Nigeria decreased and community-based surveillance and specimen collection increased (5).

Among the 21 countries in the WHO Eastern Mediterranean Region (EMR), the number of priority countries decreased from 11 (52%) in 2018 to five (24%) in 2019. Surveillance performance was assessed for the five 2019 priority countries in EMR (Afghanistan, Djibouti, Pakistan, Somalia, and Sudan) for 2018 and 2019 (Table 1). From 2018 to 2019, the number of WPV1 cases increased from 21 to 29 in Afghanistan (38% increase) and from 12 to 147 in Pakistan (1,125% increase). In 2019, 22 cVDPV2 cases were also reported in Pakistan (Table 1). In Somalia, 12 cVDPV cases (type 2 and 3) were reported in 2018 (including one coinfection with types 2 and 3), and three cVDPV2 cases were reported in 2019. Four of the five EMR priority countries met both surveillance indicators in 2018 and 2019; in Djibouti only 16% of the population lived in areas meeting both indicators in 2019 (Figure).

In the Western Pacific Region (WPR), surveillance performance was assessed for three countries (Malaysia, Papua New Guinea, and Philippines) (Table 1). No priority country met both AFP surveillance indicators in 2018 and 2019. Two cVDPV type 1 (cVDPV1) cases reported in Philippines in 2019 were genetically linked to three cVDPV1 cases reported in Malaysia; 13 cVDPV2 cases were also reported in Philippines in 2019. Environmental surveillance also detected genetically linked cVDPV1 and cVDPV2 isolates in both countries. One cVDPV2 case was reported in China in 2019. Subnational NPAFP rate and stool adequacy indicators were suboptimal in Philippines and Malaysia in 2018 and 2019, indicating gaps in AFP case detection or investigation. Although Papua New Guinea met the NPAFP target performance indicator in both years, the stool adequacy target was not met at the national level.

In the South-East Asia Region (SEAR), surveillance performance was assessed for two countries (Indonesia and Burma [Myanmar]^¶) (Table 1). In 2018, both countries met both surveillance indicators and in 2019, one Burma (Myanmar) met both indicators. Six cVDPV1 cases were reported in Burma (Myanmar) in 2019, which had subnational weaknesses in NPAFP surveillance (Figure). No cVDPV1 cases were reported in Indonesia in 2019 after detection of one cVDPV1 case in 2018; however, weaknesses in subnational surveillance performance were identified in 2019.

Environmental Surveillance

Environmental surveillance enhances the sensitivity of poliovirus surveillance by identifying poliovirus circulation that might occur in the absence of detected AFP cases (6), as occurred in 2018 and 2019 in Cameroon, Côte d’Ivoire,** and Kenya; environmental surveillance confirmed cVDPV circulation well before AFP case detection in China, Central African Republic, Ghana, Malaysia, Nigeria, Philippines, and Somalia. In Iran, WPV1 was isolated from sewage in 2019 in the absence of detected AFP cases.

In Nigeria, environmental surveillance resulted in 45 cVDPV2 isolates in 2018 and 60 in 2019. In 2018, six WPV1 genetic clusters (isolates with ≥95% genetic relatedness) were detected in environmental surveillance from seven provinces in Afghanistan. In Pakistan, eight genetic clusters were detected from 28 districts in four provinces and in the Islamabad Capital Territory. In Pakistan, the number of WPV1 environmental surveillance detections increased in 2019, compared with that in 2018, with the largest increase in the Sindh province.

Global Polio Laboratory Network (GPLN)

GPLN comprises 145 poliovirus laboratories in the six WHO regions. GPLN laboratories implement standardized protocols to 1) isolate polioviruses (all laboratories); 2) conduct intratypic differentiation (ITD) to identify WPV, Sabin (oral polio vaccine) poliovirus and VDPV (134 laboratories); and 3) conduct genomic sequencing (28 laboratories). Poliovirus transmission pathways are monitored through sequence analysis of an isolate’s viral capsid protein (VP1) coding region. Standard AFP timeliness indicators specify that laboratories should report ≥80% of poliovirus virus isolation results within 14 days of specimen receipt, ≥80% of ITD results within 7 days of isolate receipt, and ≥80% of sequencing results within 7 days of ITD result. The combined field and laboratory performance indicator is reporting of ITD results for ≥80% of isolates within 60 days of paralysis onset in AFP cases. The accuracy and quality of testing at GPLN laboratories are monitored through an annual accreditation program of onsite reviews and proficiency testing (7). Another accreditation checklist is used for laboratories conducting environmental surveillance, with separate timeliness indicators.

GPLN tested 190,055 stool specimens in 2018 and 219,049 stool specimens in 2019 (Table 2). WPV1 was isolated from 33 stool specimens in 2018 and from 156 stool specimens in 2019. cVDPVs were isolated from 105 AFP patients in 2018 and from 437 in 2019. From 2018 to 2019, the number of stool specimens with cVDPV isolates increased from 65 to 303 (366%) in AFR, from 13 to 50 in EMR (284%), from one to 10 (90%) in SEAR, and from 26 to 74 (185%) in WPR. In 2018 and 2019, all regions met the timeliness indicator for poliovirus isolation.

In 2019, the South Asia genotype (the only WPV1 genotype circulating globally since 2016) was detected in Afghanistan and Pakistan. There were no “orphan” WPV1 isolates (those with ≤98.5% genetic identity in VP1, compared with other isolates) from AFP patients in 2018, and there were five in 2019 (two in Afghanistan and three in Pakistan), indicating possible gaps in AFP surveillance. The genetic diversity of WPV1 isolates in Afghanistan and Pakistan increased during the reporting period because of the high level of WPV1 circulation during the low season from October to May (8). Genomic sequence analysis identified seven cVDPV2 emergences in six countries in 2018 and 39 cVDPV2 emergences in 19 countries in 2019 (4,5).

Discussion

Although many of the 40 priority countries evaluated met national-level AFP surveillance performance indicators, the percentage of 2019 priority countries meeting both indicators declined overall from 83% in 2018 to 63% in 2019. Critical subnational gaps were also reported in almost all countries assessed, and the decline in the number of countries meeting the stool adequacy target from 2018 to 2019 indicates challenges in timely detection and investigation of suspected AFP cases or in specimen transport and handling. GPEI has outlined activities to enhance polio surveillance in high-priority countries (9), and the surveillance status report (10) details efforts to address current challenges; despite efforts, however, shortcomings remain in detection, investigation, reporting, and monitoring. Competing priorities, limited logistical support, and heavy workloads could all contribute to suboptimal surveillance performance. The coronavirus disease 2019 (COVID-19) pandemic might exacerbate these existing challenges and present new ones in polio immunization, surveillance, and laboratory testing activities^†† as a result of diminished access to health care and immunization and concerns about exposure to COVID-19 cases.

The findings in this report are subject to at least three limitations. First, matters related to security, hard-to-reach subpopulations, and other factors could affect subnational AFP surveillance indicators and limit their interpretation. Second, high NPAFP rates do not necessarily indicate highly sensitive surveillance because not all reported AFP cases meet the case definition, and some AFP cases might not be detected. Finally, the accuracy of stool specimen collection timeliness depends on whether the field investigator can elicit the actual date of paralysis onset.

High-quality AFP surveillance is critical to detecting poliovirus transmission. Important activities to enhance and maintain sensitive surveillance include effective case detection, investigation, reporting, monitoring, and supervision. Where the effects of COVID-19 are particularly devastating, efforts at the national and subnational levels should be made to restore curative health systems, preventive services, and overall infectious disease surveillance and control activities and, in the process, ensure availability of resources to enhance poliovirus surveillance and safeguard progress toward polio eradication. Continuous assessment of surveillance performance at the national and subnational levels must be undertaken to identify and promptly address gaps to achieve eradication of poliovirus worldwide.