Surveillance Systems to Track Progress Toward Polio Eradication — Worldwide, 2015–2016

Global measures to eradicate polio began in 1988; as of 2014, four of six World Health Organization (WHO) regions have been certified polio-free. Within the two endemic regions (African and Eastern Mediterranean), Nigeria, Afghanistan, and Pakistan have never interrupted transmission of wild poliovirus (WPV) (1). The primary means of detecting poliovirus transmission is surveillance for acute flaccid paralysis (AFP) among children aged <15 years, combined with collection and testing of stool specimens from persons with AFP for detection of WPV and vaccine-derived polioviruses (VDPVs) (viruses that differ genetically from vaccine viruses and can emerge in areas with low vaccination coverage and cause paralysis) in WHO-accredited laboratories within the Global Polio Laboratory Network (2,3). AFP surveillance is supplemented by environmental surveillance for polioviruses in sewage from selected locations (4). Genomic sequencing of the VP1-coding region of isolated polioviruses enables mapping transmission by time and place, assessment of potential gaps in surveillance, and identification of the emergence of VDPVs. This report presents poliovirus surveillance data from 2015 and 2016, with particular focus on 20 countries in the African Region and six in the Eastern Mediterranean Region that reported WPV or circulating VDPVs (cVDPVs) during 2011-2016, as well as the three countries most affected by the 2014-2015 Ebola virus disease (Ebola) outbreak (Guinea, Liberia, and Sierra Leone). During 2016, 12 (60%) of the 20 African Region countries and all six of the Eastern Mediterranean Region countries met both surveillance quality indicators (nonpolio AFP rates of ≥2 per 100,000 persons aged <15 years per year and ≥80% of AFP cases with adequate stool specimens [stool adequacy]) at the national level; however, provincial-level variation was seen. To complete and certify polio eradication, surveillance gaps must be identified and surveillance activities, including supervision, monitoring, and specimen collection and handling, further strengthened.


Acute Flaccid Paralysis Surveillance
The quality of AFP surveillance is measured by two principal indicators. The first is the nonpolio AFP (NPAFP) rate (i.e., the number of NPAFP cases per 100,000 children aged <15 years per year of observation). An NPAFP rate ≥2 is considered sufficiently sensitive to detect WPV or VDPV cases if poliovirus is circulating. The second indicator is the collection of adequate stool specimens from ≥80% of AFP cases, indicating that surveillance can effectively identify WPV and VDPV among persons with AFP (3). Stool adequacy refers to collection of two stool specimens ≥24 hours apart, within 14 days of paralysis onset, and the arrival of these specimens in good condition* at a WHO-accredited laboratory.
Among 47 African Region countries, 32,250 AFP cases were reported in 2016 and 26,052 in 2015. Although no WPV type 1 (WPV1) cases were reported in the African Region in 2015, all four WPV1 cases that occurred in the African Region in 2016 were reported from Nigeria (5). Eighteen cVDPV cases were reported in the African Region during 2015, including eight cVDPV type 2 (cVDPV2) cases (one from Nigeria and seven from Guinea) and 10 cVDPV type 1 (cVDPV1) cases (all from Madagascar). During 2016, only one cVDPV case was reported in the African Region, a cVDPV2 case from Nigeria (Table 1). Among the 20 countries evaluated in the African Region, 12 met both of the national surveillance indicators in 2016 compared with 10 in 2015. Among the three countries most affected by Ebola (Guinea, Liberia, and Sierra Leone), only Guinea met the NPAFP indicator and only Liberia met the stool adequacy indicator in 2015; however, because of insufficient clinical knowledge about how to exclude Ebola virus from clinical specimens, nearly all stool specimens from 2015 were untested and destroyed. In 2016, all three of the Ebola-affected countries had NPAFP rates ≥2, but only Guinea also achieved ≥80% stool adequacy. Among

Environmental Surveillance
Testing of sewage samples supplements AFP surveillance by identifying poliovirus transmission that might occur in the absence of detected AFP cases (4). In April 2016, all OPVusing countries switched from using trivalent OPV (tOPV) to bivalent OPV (bOPV), containing vaccine virus types 1 and 3, to reduce circulation of type 2 vaccine virus, which is responsible for most cVDPVs (6). Testing sewage is useful for monitoring the decline of oral poliovirus vaccine (OPV) type 2-related poliovirus (OPV2) in the environment after the global switch. The number of environmental surveillance collection sites increased within Afghanistan, Nigeria, and   Stool adequacy dropped to <80% when stool condition was included with timeliness. Timeliness was defined as two specimens collected ≥24 hours apart (≥1 calendar day in this data set), and both within 14 days of paralysis onset. Condition was defined as specimens arriving in good condition (reverse cold chain maintained and received without leakage or desiccation) in a WHO-accredited laboratory. *** Afghanistan, Bahrain, Djibouti, Egypt, Iran, Iraq, Jordan, Kuwait, Lebanon, Libya, Morocco, Oman, Pakistan, Qatar, Saudi Arabia, Somalia, Sudan, Syria, Tunisia, United Arab Emirates, and Yemen. † † † The NPAFP rate for Syria is artificially low because of displaced populations and the lack of official data from areas not under government control. Abbreviations: AFP = acute flaccid paralysis; NPAFP = nonpolio AFP. * The Global Polio Eradication Initiative has set the following targets for countries with current or recent wild poliovirus transmission and their states/provinces: 1) NPAFP detection rate of ≥2 cases per 100,000 persons aged <15 years per year, and 2) adequate stool specimen collection from ≥80% of AFP cases, with specimen adequacy assessed by timeliness and condition. Timeliness was defined as two specimens collected ≥24 hours apart (≥1 calendar day) and both within 14 days of paralysis onset. Good condition was defined as specimens arriving without leakage or desiccation in a maintained reverse cold chain at a World Health Organizationaccredited laboratory.

Global Polio Laboratory Network
The Global Polio Laboratory Network consists of 146 WHO-accredited poliovirus laboratories in all WHO regions. Global Polio Laboratory Network member laboratories follow standardized protocols to 1) isolate and identify poliovirus, 2) conduct intratypic differentiation to identify WPV or screen for Sabin (vaccine) poliovirus and VDPVs (7), and 3) conduct genomic sequencing. Sequencing results help monitor pathways of poliovirus transmission by comparing the nucleotide sequence of the VP1-coding region of poliovirus isolates. To meet standard laboratory timeliness indicators for stool specimen processing, laboratories should report ≥80% of poliovirus isolation results within 14 days of specimen receipt, ≥80% of intratypic differentiation results within 7 days of isolate receipt, and ≥80% of sequencing results within 7 days of intratypic differentiation results. The standard programmatic indicator combining field and laboratory performance is to report intratypic differentiation results for ≥80% of isolates from AFP cases within 60 days of paralysis onset. This indicator considers the entire interval from paralysis onset to specimen testing. The accuracy and quality of testing at Global Polio Laboratory Network member laboratories is monitored through an annual accreditation program that includes onsite reviews and proficiency testing.
Global Polio Laboratory Network laboratories met timeliness indicators for poliovirus isolation for both years in all regions except the European Region in 2015 ( Table 2). The overall timeliness indicator for onset to intratypic differentiation results was met in both years in all regions except the European Region in 2015. The Global Polio Laboratory Network tested 192,250 stool specimens in 2015 and 220,920 in 2016. WPV1 was isolated from 74 AFP case specimens in 2015 and from 37 AFP case specimens in 2016. In addition, cVDPV was detected in 33 AFP case specimens in 2015 and 11 AFP case specimens in 2016.
In 2016, the West Africa B1 (WEAF-B1) genotype was isolated in Nigeria, where it had last been detected in 2014. In Afghanistan and Pakistan, the only genotype isolated in 2016 was South Asia (SOAS); this was the only genotype isolated worldwide in 2015. Overall genetic diversity declined among WPV1 isolates in 2016.
A poliovirus isolate with ≥1.5% nucleotide divergence in genomic sequencing of the VP1-coding region compared with previous isolates is called an "orphan" virus; orphan viruses indicate prolonged undetected virus circulation and gaps in AFP surveillance. In 2016, as in 2015, genomic sequencing indicated that WPV1 and cVDPV cases were likely missed by AFP surveillance. Orphan WPV1 isolates were associated with one of 20 WPV1 cases reported from Pakistan and three of four WPV1 cases reported in Nigeria in 2016. Orphan cVDPVs were isolated from stool specimens of AFP patients in four countries (Pakistan, Afghanistan, Nigeria, and Cameroon) in 2015; in 2016, only Nigeria reported an orphan cVDPV virus from a stool specimen of an AFP case contact in Borno State.
Three countries outside the African and Eastern Mediterranean Regions reported cVDPVs in 2015: Ukraine (cVDPV1), Laos (cVDPV1), and Myanmar (cVDPV2). No additional VDPV cases were detected in Ukraine or Myanmar in 2016; the last case in Laos had onset in January 2016.

Discussion
The number of reported WPV cases declined to the lowest point ever in 2016. Although the majority of national-level surveillance quality indicators improved in 2016, considerable variation was seen at subnational levels. Despite meeting surveillance indicator standards for several years at the state level in Nigeria, the discovery of previously undetected circulation of individual WPV lineages for several years as well as continued inaccessibility of certain geographical areas with underimmunized persons has raised concerns (5), prompting detailed reviews of surveillance and geographic accessibility.

Summary
What is already known about this topic?
Surveillance is a cornerstone of polio eradication programs. Acute flaccid paralysis (AFP) surveillance is the primary means of poliovirus detection, supplemented by environmental surveillance (i.e., the collection of sewage samples for poliovirus testing) to identify poliovirus circulation in the absence of detected AFP cases.
What is added by this report?
Although surveillance performance indicators are improving, gaps remain, including substantial variation at subnational levels (i.e., in 2016, of 20 African Region countries, 19 met the NPAFP target at the national level versus 11 at all subnational levels). The number of environmental surveillance locations has increased substantially (from 21 at the end of 2011 in Afghanistan, Nigeria, and Pakistan to 138 as of February 2017) and has enhanced the ability to detect poliovirus circulation and possible AFP surveillance gaps. In countries previously affected by Ebola, surveillance quality is improving, although further measures are needed to reach preoutbreak levels.
What are the implications for public health practice?
Rapid improvements in AFP surveillance are needed in several African Region countries to ensure timely certification of polio-free status. Gaps in surveillance quality, especially at the subnational level, need to be identified and resolved through well-supervised active and monitored passive surveillance, and supplemental environmental and virologic surveillance. As long as polioviruses continue to circulate in any country, all countries remain at risk.
Although conflict has limited access in several areas (including Somalia, South Sudan, and Syria), effective community-based surveillance provides some assurance of the absence of poliovirus circulation in many of those areas.
Certification of polio-free status requires at least 3 years of timely and sensitive polio surveillance (8), including timely stool collection, and timely and appropriate transport of specimens to the laboratory. Specimen condition was a particular concern in the Democratic Republic of the Congo, Ethiopia, Gabon, Madagascar, and Niger in 2016. With the end of the Ebola outbreak, polio surveillance performance is improving in West Africa, although more work remains to return to preoutbreak surveillance quality indicators.
The findings in this report are subject to at least two limitations. First, the surveillance indicators do not fully reflect security-related issues, issues associated with mobile and difficult-to-access populations, or other factors that affect surveillance performance. For example, in Iraq and the Syria, population movements related to conflict make interpretation of AFP surveillance indicators difficult. Second, high NPAFP rates do not necessarily imply sensitive surveillance, because