Surveillance Systems to Track Progress Toward Polio Eradication — Worldwide, 2014–2015

Acute Flaccid Paralysis Surveillance

In the African Region, the number of reported AFP cases increased from 22,447 in 2014 to 26,238 in 2015. The number of WPV type 1 (WPV1) cases identified decreased from 17 in four countries in 2014 to zero in 2015. Date of onset of the last WPV1 case was July 24, 2014, in Nigeria. During 2014, a total of 34 cVDPV cases (33 cVDPV type 2 [cVDPV2] and one cVDPV type 1 [cVDPV1]) were identified in four countries. During 2015, a total of 18 cVDPV cases (eight cVDPV2 and 10 cVDPV1) were identified in three countries (Table 1).

In the Eastern Mediterranean Region, 12,546 AFP cases were reported in 2014 and 13,171 in 2015. The number of WPV1 cases identified declined from 342 in five countries in 2014 to 74 in the two countries with endemic polio in 2015 (20 in Afghanistan and 54 in Pakistan). During both 2014 and 2015, most of the WPV1 cases (89% and 73%, respectively) and all cVDPV2 cases in the region were identified in Pakistan (Table 1).

The quality of AFP surveillance is measured by two principal indicators. The first is the nonpolio AFP rate, (i.e., the number of nonpolio AFP cases per 100,000 children aged <15 years per year). A nonpolio AFP rate ≥2 is considered sufficiently sensitive to identify WPV or cVDPV cases if poliovirus is circulating (5). The second indicator is the collection of adequate stool specimens from ≥80% of AFP cases, indicating surveillance can effectively identify WPV and VDPV among individuals with AFP (5). Adequacy is defined as the collection of two stool specimens ≥24 hours apart, within 14 days of paralysis onset, and arrival at a WHO–accredited laboratory in “good” condition.^†

Among the 20 countries evaluated in the African region, 13 (65%) and 10 (50%) met both national indicators in 2014 and 2015, respectively. Among the three countries affected by the Ebola outbreak, Guinea and Liberia did not meet one of the national indicators and Sierra Leone did not meet either indicator in 2015. All six Eastern Mediterranean Region countries examined met both indicators in 2014 and 2015 (Table 1). However, national-level surveillance indicators masked differences in surveillance performance at subnational levels (Figure).

Environmental Surveillance

Testing of sewage samples supplements AFP surveillance by identifying poliovirus transmission that might occur in the absence of detected AFP cases (6). Environmental surveillance collection sites increased within Afghanistan, Nigeria, and Pakistan from 21 at the end of 2011 to 83 as of March 2015. Overall, environmental surveillance is conducted in 34 countries without recent active WPV transmission, including nine on the African continent.

In Nigeria, sampling is currently conducted at 43 sites in 10 states and the Federal Capital Territory. No WPV has been isolated since May 2014 when WPV1 was isolated from one sample in Kaduna. Continued transmission of cVDPV2 that emerged in Nigeria in 2005 and of cVDPV2 imported from Chad in 2013 was documented from samples collected in six states in 2014 and only Kaduna in 2015. Environmental sampling in Afghanistan is conducted at 14 sites in five WPV high risk provinces. WPV1 was detected in samples collected in Helmand, Kandahar, and Nangarhar in 2014 and in all five provinces in 2015. In Pakistan, sampling is conducted at 40 sites in five provinces/regions. The proportion of samples positive for WPV1 decreased from 35% in 2014 to 20% in 2015. WPV1 was detected in all five provinces/regions in both years.

Global Polio Laboratory Network

The GPLN comprises 146 WHO–accredited poliovirus laboratories in all WHO regions. GPLN member laboratories follow standardized protocols to 1) isolate and identify poliovirus, 2) conduct intratypic differentiation to identify WPV or screen for Sabin-like poliovirus (isolates that display ≤1% nucleotide sequence difference from the parental vaccine strain [≤0.6% for type 2]) and VDPV (7), and 3) conduct genomic sequencing. Sequencing results are used to monitor pathways of poliovirus transmission by comparing the nucleotide sequence of the coding region for one of the viral capsid proteins (VP1) of poliovirus isolates. Genomic sequencing of an isolate with ≥1.5% nucleotide divergence in the VP1-coding region from previously identified poliovirus isolates (i.e., an “orphan” virus), indicates prolonged undetected circulation and gaps in AFP surveillance.

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 identifying isolate intratype. The standard programmatic indicator combining field and laboratory performance is to report intratypic differentiation results for ≥80% of isolates within 60 days of paralysis onset for AFP cases. This indicator takes into account the interval from paralysis onset to specimen testing (the Eastern Mediterranean Region uses a 45-day time frame). The accuracy and quality of testing at GPLN laboratories is monitored through an annual accreditation program of onsite reviews and proficiency testing.

GPLN laboratories met timeliness indicators for poliovirus isolation in all regions for both years except the Western Pacific Region in 2014 and the European Region in 2015 (Table 2). The overall timeliness indicator for onset to intratypic differentiation results was met in all regions in both years except the European Region in 2015. As of March 5, 2016, the GPLN had tested 203,698 stool specimens in 2014 and 192,250 in 2015. WPV1 was isolated from 412 AFP case samples in 2014 and from 74 in 2015. In addition, cVDPV was detected from 80 AFP case samples in 2014 and 32 in 2015.

For the first time since 2005, the majority of cVDPV cases detected globally in 2015 were caused by Type 1. Among the 31 cVDPV cases identified, 19 (61%) occurred as part of type 1 outbreaks in Laos (7), Madagascar (10) and Ukraine (2); the remaining cVDPV cases were type 2 (Guinea [7], Myanmar [2], Nigeria [1], and Pakistan [2]).

Genetic diversity declined among WPV1 isolates in 2015. In 2014, West Africa B1 (WEAF-B1) and South Asia (SOAS) were the only WPV1 genotypes circulating globally. Although WEAF-B1 genotype was detected in five countries in 2014, the only genotype detected in 2015 was SOAS from Afghanistan and Pakistan.

Sequence analysis continues to indicate that, as in 2014, WPV1 and cVDPV cases were likely missed by AFP surveillance in 2015. Orphan WPV1 isolates were associated with six of 54 WPV1 cases reported from Pakistan and two of 20 WPV1 cases reported in Afghanistan. Orphan cVDPV viruses were also isolated from stool specimens of AFP cases in Guinea, Laos, Madagascar, and Ukraine.

Discussion

No WPV transmission was identified on the African continent in 2015. Certification of polio-free status requires at least 3 years of timely and sensitive surveillance (8). However, AFP surveillance indicators were not met in half of the 20 African Region countries examined. Rapid strengthening of AFP surveillance is critical to ensure timely polio-free certification of the region. This includes improving timeliness for stool collection and ensuring appropriate transport of specimens. Specimen condition was a particular concern for the Democratic Republic of the Congo, Equatorial Guinea, Ethiopia, Gabon, Niger, and the Republic of the Congo and was a factor in six countries that did not attain ≥80% specimen adequacy in 2015. Urgent efforts are also needed to improve AFP surveillance in Guinea, Liberia, and Sierra Leone, where health systems were severely disrupted by the Ebola outbreak (9). During 2013–2015, nonpolio AFP rates and stool specimen adequacy (i.e., timeliness and condition) declined in all three countries (10). Furthermore, specimen shipping and testing was suspended during portions of 2014–2015. Efforts are also needed to improve immunization services in these three countries. Since 2014, eight cVDPV2 cases have been detected in Guinea; cVDPV emergence is the result of low population immunity to poliovirus. The WHO African Regional Office is collaborating with eight high-risk countries on an initiative to strengthen AFP surveillance.

The national AFP surveillance quality indicators continued to be met in all six Eastern Mediterranean Region countries, including Afghanistan and Pakistan in 2015, where 94% and 99% of the population aged <15 years, respectively, lived in areas meeting both indicators. Nonetheless, detection of orphan viruses and environmental surveillance findings indicate continued gaps in AFP surveillance in both countries.

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 Syrian Arab Republic, population movements related to conflict make interpretation of AFP surveillance indicators difficult. Second, high nonpolio AFP rates do not necessarily imply sensitive surveillance, because a proportion of reported AFP cases might not be true AFP cases and not all true AFP cases might be reported. Because AFP cases in this report are considered true AFP cases, findings are presumed to accurately depict surveillance activities in each country. Supervision and monitoring of AFP surveillance can help ensure that all true AFP cases are identified, reported, and investigated appropriately.

As the number of reported polio cases declines, sensitive AFP surveillance becomes increasingly critical, and environmental surveillance will continue to be an important supplement to AFP surveillance. The risk for WPV and cVDPV importation, and cVDPV emergence exists even in countries within polio-free regions. To promptly identify and respond to all cases of polio, surveillance performance must be continuously assessed and quality must be maintained globally.