Polio Surveillance in 28 Priority Countries, 2022–2023: Progress and Gaps

Summary

• Poliovirus is found mainly through surveillance for acute flaccid paralysis (AFP), supplemented by testing sewage.
• In 2022–2023, 20 (71.4%) of 28 priority countries — those at high risk for poliovirus transmission — met national AFP surveillance targets, and the number of sewage sampling sites grew. But large national and subnational gaps remain.
• High-quality surveillance is essential to eradicating polio, and tracking its indicators shows where to strengthen it.

Background

Since the Global Polio Eradication Initiative began in 1988, wild poliovirus (WPV) cases have fallen more than 99.9%, and types 2 and 3 have been declared eradicated. By the end of 2023, type 1 (WPV1) was endemic only in Afghanistan and Pakistan — though in 2021–2022, Malawi and Mozambique reported nine WPV1 cases linked to Pakistan (the last paralysis began August 10, 2022). Where vaccination coverage is low, vaccine-derived polioviruses can circulate long enough to regain the ability to paralyze; outbreaks of these circulating vaccine-derived polioviruses (cVDPVs) have been found in 42 countries.

How surveillance is judged

• AFP surveillance, at the national level and in 511 states or provinces, against two targets:
• • a non-polio AFP rate of at least 2 per 100,000 children under 15 — sensitive enough to catch poliovirus;
  • adequate stool specimens from at least 80% of AFP patients (two samples within 14 days of paralysis, at least 24 hours apart, arriving cold and in good condition at an accredited lab).
• Environmental (sewage) surveillance: a site should detect enteroviruses in at least 50% of samples.
• The Global Polio Laboratory Network: 144 WHO-accredited labs in all six WHO regions; 134 can tell wild, vaccine-derived and vaccine (Sabin) strains apart, and 28 do genetic sequencing.

The 28 priority countries: 21 in WHO's African Region, five in the Eastern Mediterranean, and one each in South-East Asia (Indonesia) and the Western Pacific (Papua New Guinea).

Results by region

Combined AFP surveillance indicators by subnational area. Figure from the CDC report.

Sewage surveillance

• 27 of 28 priority countries had at least one site reporting in 2023.
• Africa: sites fell slightly (386 → 378), but the share meeting the sensitivity target rose from 41.7% to 58.8%.
• Eastern Mediterranean: sites jumped from 244 to 571, driven by Pakistan's 308 new sites — yet only 133 (26.7%) of Pakistan's sites reported five or more collections. Somalia (100% → 35.3%) and Sudan (85.7% → 60%) saw the share meeting the target fall.
• Indonesia: sites fell from 16 to 12, but the share meeting the target rose from 25% to 45.5%.

The labs

In 2023, the network tested 233,437 stool specimens. Every WHO region except the Americas met the target for timely virus isolation, and all met the target for timely reporting.

• WPV1: only the South Asia genotype was found — in 42 AFP patients (30 in 2022, 12 in 2023) in Afghanistan and Pakistan, plus one in Mozambique. Pakistan's isolates were in genetic cluster YB3A, except three in 2023 in YB3C; Afghanistan's were all YB3A. Mozambique's 2022 cases were cluster YC2. Five sewage detections were "orphan" viruses — genetically distant from known isolates — a sign of prolonged undetected circulation.
• cVDPVs: 37 separate emergence groups (7 type 1, 30 type 2) were found in 1,320 AFP patients and 607 sewage samples. Type 1 groups fell from seven to four; type 2 groups rose from 18 to 22.

What it means

• The non-polio AFP rate fell in 14 of the 28 countries in 2023, and stool adequacy fell in 15; Botswana, Indonesia, Niger, Papua New Guinea and Sudan missed targets.
• The Malawi–Mozambique outbreak shows how outbreak response can strengthen surveillance: all five responding countries (Malawi, Mozambique, Tanzania, Zambia, Zimbabwe) met the non-polio AFP rate target in 2023; Zambia's stool adequacy rose from 65.9% to 79.2%, just short of target, and in Mozambique the share of subnational areas meeting stool adequacy climbed from 18.2% to 72.7%.

Limits: data can take weeks or months to reach the system, so apparent declines in cases may partly reflect reporting lags; national and regional figures can hide local gaps, especially in hard-to-reach areas; and meeting indicators doesn't guarantee strong surveillance without supervision and training.

The bottom line: good surveillance catches poliovirus early and triggers outbreak vaccination to stop it. Countries must watch their indicators, close gaps, and support field staff to keep eradication on track.

Sources

Based on Nishant Kishore, Elizabeth Krow-Lucal, Ousmane M. Diop and others, "Surveillance To Track Progress Toward Polio Eradication — Worldwide, 2022–2023," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain.