Polio Surveillance in High-Risk Countries, 2021–2022

This page summarizes a WHO and CDC report on polio surveillance in 2021–2022.

Since the Global Polio Eradication Initiative began in 1988, wild poliovirus cases have fallen by more than 99.9%, and wild types 2 and 3 have been declared eradicated. By the end of 2022, wild type 1 (WPV1) was endemic only in Afghanistan and Pakistan. But in 2021–2022, Malawi and Mozambique reported nine WPV1 cases genetically linked to Pakistan, and outbreaks of circulating vaccine-derived poliovirus (cVDPV) were found in 42 countries. cVDPVs arise from the weakened virus in oral polio vaccine when it circulates for a long time among people with low immunity, regaining the ability to cause paralysis.

How polio is found

• Acute flaccid paralysis (AFP) surveillance looks for sudden paralysis in children under 15, confirming polio by testing stool.
• Environmental surveillance tests sewage for poliovirus, catching circulation even when no paralyzed cases are found.

Both were weakened by the COVID-19 pandemic in 2020 and improved in 2021. This report covers 34 priority countries with surveillance gaps or high vulnerability.

AFP surveillance is judged by two targets: a non-polio AFP rate of at least 2 per 100,000 children under 15, showing the system is sensitive enough to catch polio; and stool adequacy, at least 80% of AFP patients with two good stool samples collected within 14 days of paralysis onset, 24 or more hours apart, and delivered cold to a WHO-accredited laboratory.

In 2022, 26 of the 34 countries (76.5%) met both targets nationally, up from 24 (70.6%) in 2021, but large gaps remained below the national level.

Region by region

Cases. The African Region recorded eight WPV1 cases in 2022 and one with onset in 2021, and its vaccine-derived cases rose from 512 in 2021 (13 type 1, 499 type 2) to 653 in 2022 (173 type 1, 480 type 2). The Eastern Mediterranean Region reported 22 WPV1 and 168 type 2 vaccine-derived cases in 2022, up from 5 WPV1, 3 type 1 and 118 type 2 vaccine-derived cases in 2021. No poliovirus was found in Burma, Papua New Guinea or the Philippines.

Sewage surveillance

In 2022, 32 of the 34 priority countries reported 725 environmental surveillance sites, up 31.1% from 553 in 2021. A site is considered sensitive enough if at least half its samples contain some enterovirus, polio or not.

• African Region: sites rose from 371 to 415, and the share meeting the target from 27.8% to 40.5%.
• Eastern Mediterranean Region: sites rose from 162 to 294, including 120 added in Pakistan, and more than 85% met the target each year.
• Burma's single site met the target; the Philippines had 17 sites in 2021 (5 meeting the target) and 15 in 2022 (4).

The laboratory network

The Global Polio Laboratory Network's 144 WHO-accredited laboratories all isolate poliovirus; 134 can tell wild, vaccine-derived and vaccine (Sabin) viruses apart, and 28 do genetic sequencing. In 2022 they tested 193,945 stool samples from AFP patients. The Americas, Eastern Mediterranean and European regions missed the target of reporting isolation results within 14 days for 80% of samples, but all regions met the targets for identifying virus types.

Sequencing traced the wild viruses: all WPV1 in 2021–2022, from 36 AFP patients, was the South Asia genotype, 27 cases in Afghanistan and Pakistan and 9 in Malawi and Mozambique. The YB3C genetic cluster linked 21 cases in Pakistan, 2 in Afghanistan and all 9 African cases; two of the Pakistani viruses were "orphans," so genetically distant from known viruses that they signal undetected, prolonged circulation. In priority countries, 38 separate vaccine-derived outbreaks (8 type 1 and 30 type 2) were found in 1,454 AFP patients and 750 sewage samples.

What it means

Paralysis surveillance recovered from the pandemic in most African and Eastern Mediterranean priority countries, but stool adequacy improved only slightly, and subnational gaps persisted, especially in 14 of 24 African countries. In Malawi and Mozambique, where WPV1 had been found, less than a quarter of the population lived in areas meeting both targets in 2022. While polioviruses still circulate, every country is at risk of importation and must keep surveillance strong.

Limitations: the paralysis rate depends on accurate case finding; timeliness depends on accurate onset dates; national and regional figures can hide local gaps; and people in hard-to-reach areas may be missed.

Sources

• Stehling-Ariza T, Wilkinson AL, Diop OM, et al. "Surveillance To Track Progress Toward Poliomyelitis Eradication — Worldwide, 2021–2022." MMWR 72(23). https://www.cdc.gov/mmwr/volumes/72/wr/mm7223a1.htm
• WHO, Global Polio Surveillance Action Plan 2022–2024: https://polioeradication.org/wp-content/uploads/2022/02/Global-Polio-Surveillance-Action-Plan-2022-2024.pdf
• The report's discussion refers to "653 VDPV cases detected in 42 countries during 2022," while its results give 653 as the African Region's 2022 total alone; this page gives the regional counts. Its map is not reproduced because the report was prepared jointly with WHO.