Surveillance To Track Progress Toward Poliomyelitis Eradication — Worldwide, 2021–2022

Acute Flaccid Paralysis Surveillance

Performance Indicators. AFP surveillance quality is assessed using two performance indicators: 1) the nonpolio AFP (NPAFP) rate, by which an NPAFP rate of at least two per 100,000 persons aged <15 years is deemed sufficiently sensitive to detect circulating poliovirus, and 2) stool adequacy,^† with the target of ≥80% adequate stool specimens collected from AFP patients indicating effective identification of poliovirus. Surveillance indicators were assessed in 34 priority countries during 2021–2022 and summarized by region (Table 1).

African Region. Among 24 priority countries in the World Health Organization (WHO) African Region (AFR), 19 (79.2%) met both national AFP surveillance indicator targets in 2022 compared with 17 (70.8%) in 2021 (Table 1). In 2022, all 24 countries met the NPAFP rate target nationally, and 19 (79.2%) met the stool adequacy target. Indicators at the first subnational administrative level (i.e., province or state) were similar both years, with 73.2% and 72.4% of subnational areas meeting both targets in 2021 and 2022, respectively (Figure). In 2022, ≥80% of subnational areas in 21 (87.5%) countries met the NPAFP rate target compared with 20 (83.3%) countries in 2021. The stool adequacy target was met by more than 80% of subnational areas in 10 (41.7%) countries in 2022 compared with 11 (45.8%) countries in 2021. Subnational area stool adequacy performance varied; nine countries (37.5%) reported more subnational areas meeting the target in 2022, and six (25.0%) reported fewer. In 2022, eight WPV1 cases were detected, and one WPV1 case had onset in 2021. The number of VDPV cases increased from 512 (13 cVDPV type 1 [cVDPV1] cases and 499 cVDPV type 2 [cVDPV2] cases) in 2021 to 653 (173 cVDPV1 and 480 cVDPV2) in 2022.

Eastern Mediterranean Region. All seven priority countries in the WHO Eastern Mediterranean Region (EMR) met both national surveillance indicator targets in 2021 and 2022. In 2022, 88.2% of subnational areas met both indicator targets compared with 89.0% in 2021. EMR reported 22 WPV1 and 168 cVDPV2 cases in 2022 compared with five WPV1, three cVDPV1, and 118 cVDPV2 in 2021.

South-East Asia Region. In the WHO South-East Asia Region (SEAR) priority country of Burma (Myanmar)^§, the NPAFP rate improved from 0.2 to 1.1 cases per 100,000 but remained below the target. In 2021 and 2022, Burma met stool adequacy targets (84.8% and 88.0%, respectively). Three (16.7%) of 18 subnational areas met both NPAFP rates and stool adequacy targets in 2022 compared with none in 2021. No poliovirus cases were detected in Burma during 2021–2022.

Western Pacific Region. Of the two priority countries in the WHO Western Pacific Region (WPR), neither Papua New Guinea nor the Philippines met both national surveillance indicator targets during the assessment period. In Papua New Guinea, the NPAFP rate (1.3 in 2021 and 1.4 in 2022) was below target; stool adequacy increased from 50% in 2021 to 57.8% in 2022 but remained below target. In 2021 and 2022, the Philippines met NPAFP rate targets (2.6 and 2.2, respectively) but missed stool adequacy targets (78.4% and 79.3%, respectively). Across both countries, the number of subnational areas meeting both indicators declined from 20.5% in 2021 to 10.3% in 2022. No poliovirus was detected in the two WPR priority countries in 2021 or 2022.

Environmental Surveillance

Environmental surveillance for poliovirus, the systematic collection and testing of sewage samples for poliovirus, supplements the sensitivity of AFP surveillance by detecting poliovirus circulation in the absence of confirmed paralytic polio cases. In 2022, 32 (94.1%) of the priority countries^¶ reported 725 environmental surveillance sites compared with 553 sites in 2021, a 31.1% increase. Performance (sensitivity to detect poliovirus) is assessed by the annual enterovirus isolation rate, the proportion of environmental samples that have positive test results for any polio or nonpolio enterovirus, with a target of ≥50%.**

In AFR, the number of sites increased 11.9%, from 371 in 2021 to 415 in 2022, and the overall proportion of sites meeting the target increased from 27.8% to 40.5%. Eleven (47.8%) countries reported an increase in the proportion of sites meeting the indicator in 2022, and seven (30.4%) countries reported a decrease.

The number of sites in EMR increased from 162 in 2021 to 294 in 2022, including 120 added in Pakistan. More than 85% of sites met the indicator in each year.

In SEAR and WPR, Burma’s single reporting site met the environmental surveillance indicator. The Philippines had 17 environmental sites in 2021, five (29.4%) of which met the indicator. In 2022, 15 sites reported, and four (26.7%) met the indicator.

Global Polio Laboratory Network

The Global Polio Laboratory Network (GPLN) consists of 144 WHO-accredited laboratories in the six WHO regions, monitored through a standardized quality assurance program of annual on-site audits and proficiency tests (10). All 144 GPLN laboratories are responsible for isolating polioviruses; 134 conduct intratypic differentiation to identify WPV, VDPV, and Sabin (oral poliovirus vaccine)^†† polioviruses; and 28 conduct genomic sequencing. These 28 laboratories participated in global proficiency testing, analyzed the region of the poliovirus genome that codes for the capsid viral protein 1 (VP1), and demonstrated their ability to accurately characterize poliovirus in stool specimens (10).

In 2022, GPLN tested 193,945 stool specimens collected from patients with AFP (Table 2). Three of six regions (the Americas [AMR], EMR, and European [EUR]) did not meet the timeliness indicator for poliovirus isolation (results reported for ≥80% of specimens ≤14 days after receipt); however, all regions met the timeliness indicator for intratypic differentiation (results reported for more than 80% of specimens, both ≤7 days of receipt of isolate and ≤60 days of paralysis onset).

During 2021–2022, the South Asia (SOAS) genotype was the only circulating WPV1 isolated from 36 AFP patients, 27 from the two endemic countries (Afghanistan and Pakistan), and nine from two nonendemic countries (Malawi and Mozambique). In Pakistan, 21 WPV1 cases were related to the YB3C genetic cluster (i.e., groups of polioviruses sharing ≥95% sequence identity in the region coding VP1), two of which were orphan viruses.^§§ The YB3C cluster was also found in the nine cases from Malawi and Mozambique and two cases detected in Afghanistan. Cluster YB3A was detected in three cases in Afghanistan and in environmental samples in Pakistan. Two additional distinct clusters (YB3B and XC2B) were detected in environmental samples collected in Pakistan during 2021.

In priority countries during 2021–2022, 38 cVDPV emergence groups (eight cVDPV1 and 30 cVDPV2) were isolated from 1,454 AFP patients and 750 environmental samples. The number of cVDPV1 emergence groups increased from four isolated from 16 AFP patients and 33 environmental samples in 2021 to seven from 173 patients and 99 samples in 2022. The number of cVDPV2 emergence groups decreased from 24 isolated from 617 AFP patients and 449 environmental samples in 2021 to 17 emergence groups isolated from 648 patients and 169 samples in 2022.

Discussion

After the COVID-19 pandemic weakened poliovirus surveillance performance (7,8), NPAFP rates during 2022 improved in most priority countries in AFR and EMR; improvements in stool adequacy, however, were only marginal. All high-priority countries in AFR and EMR met national NPAFP rate targets, and 26 of 31 countries met stool adequacy targets. Subnational surveillance gaps exist, particularly for stool adequacy, with 14 of 24 AFR countries and one of seven EMR countries reporting <80% of subnational areas meeting the target. SEAR and WPR priority countries and subnational areas showed improvement in 2022 but not enough to meet targets.

The detection of WPV1 in Malawi and Mozambique (4,5) highlights the risk for importation and the importance of monitoring surveillance performance to detect transmission. Substantial surveillance gaps persist in both countries, with <25% of the population living in areas that met both AFP surveillance indicators in 2022. The 653 VDPV cases detected in 42 countries during 2022 also emphasize the importance of sensitive and timely surveillance to help response activities interrupt poliovirus transmission. While polioviruses continue to circulate, all countries remain at risk for importation and must strengthen and maintain surveillance.

The findings in this report are subject to at least four limitations. First, the NPAFP rate depends on the accurate identification of AFP cases; however, the data presented in this study might include cases not meeting the AFP case definition and exclude actual AFP cases that were not reported. Environmental surveillance improves sensitivity without relying on AFP case detection. Second, AFP surveillance measures of timeliness depend on the accurate identification of paralysis onset date during the field investigation. Third, performance measures reported at regional and national levels can obscure variation at lower administrative levels. Finally, populations living in hard-to-access areas might not be adequately identified by the surveillance system and could affect subnational surveillance indicators and limit their interpretation.

High-quality surveillance is critical for the timely detection of circulating poliovirus and the rapid activation of outbreak response vaccination activities to stop transmission. Countries should maintain high-quality surveillance by monitoring surveillance indicators to identify gaps, enhance the sensitivity and timeliness of surveillance activities, and guide program decision-making toward polio eradication.