Progress Toward Polio Eradication Worldwide, 2022–2023

From Morbidity and Mortality Weekly Report (MMWR), Vol. 73, No. 19 — Keri Geiger, Tasha Stehling-Ariza and colleagues, CDC Global Immunization Division and the World Health Organization.

Summary

• Known before: Afghanistan and Pakistan are the last countries where wild poliovirus (WPV) is endemic, while many countries face outbreaks of circulating vaccine-derived poliovirus (cVDPV).
• New here: WPV cases fell in 2023, but sewage testing found WPV beyond known reservoirs, and eight new countries reported cVDPV outbreaks — wider spread than in 2022.
• Implication: raising routine immunization in endemic areas and running better vaccination campaigns are needed to stop transmission.

Background

• Eradication since 1988: the Global Polio Eradication Initiative (GPEI) has eradicated WPV types 2 and 3; type 1 (WPV1) persists only in Afghanistan and Pakistan. A WPV1 outbreak linked to Pakistan reached Malawi in November 2021 and Mozambique in 2022.
• cVDPV: when weakened oral polio vaccine (OPV) strains pass person to person for a long time in under-immunized populations, they can mutate back into a virus that paralyzes.
• Vaccines: since May 2016, OPV-using countries give bivalent OPV (types 1 and 3) plus at least one dose of inactivated polio vaccine (IPV, all three types). mOPV2 and the more genetically stable novel OPV2 (nOPV2) are reserved for type 2 outbreaks; nOPV2, used under emergency listing since March 2021, won WHO prequalification in December 2023.
• The GPEI goal of stopping all poliovirus transmission by December 2023 was missed.

Vaccination

• Routine coverage with 3 doses (Pol3) by age 12 months: 84% in 2022, up from 81% in 2021, but still below the 85–87% of 2014–2019, before COVID-19 disruptions. First-dose IPV coverage reached 84%, above the pre-pandemic high of 83%.
• Afghanistan: Pol3 76%, IPV1 71%; Pakistan: 85% and 90% — with much lower coverage in the reservoir areas.
• Campaigns, 2023: 119 in 30 countries — about 675 million bOPV doses in 10 countries (114 million in Afghanistan, 264 million in Pakistan), 524 million nOPV doses in 24 countries, 10 million mOPV2 in Sudan, and 8 million tOPV in Somalia and Yemen. A single nOPV2 manufacturer has meant shortages.

Surveillance

• Acute flaccid paralysis (AFP) surveillance in children under 15 is the main tool, confirmed at 144 WHO-accredited labs; environmental surveillance (ES) of sewage can find virus without cases.
• 8 of 28 high-risk countries (29%) missed national AFP surveillance targets in 2023.
• ES tested 15,886 samples from 1,462 sites in 68 countries in 2023, up from 14,498 samples from 1,117 sites in 2022.

Wild poliovirus

Afghanistan's cases came from provinces along the Pakistan border with security problems, and sewage positivity quadrupled. In both countries, sewage detected WPV1 in provinces with no cases — evidence of wider transmission. The Africa outbreak ended with one case in Malawi and eight in Mozambique; no WPV1 case has occurred outside the two endemic countries since August 10, 2022.

Vaccine-derived poliovirus

• Cases fell 41%, from 881 in 2022 (193 type 1, 687 type 2, one type 3 in Israel) to 524 in 2023 (133 type 1, 391 type 2).
• Outbreaks: 32 countries in 2023. Eight countries with 2022 outbreaks stopped them; eight new countries had outbreaks; and 24 countries failed to stop outbreaks despite campaigns — a sign that too many children are missed.

Wild and vaccine-derived poliovirus cases and environmental isolates worldwide. CDC.

Discussion

WPV1 persists where insecurity limits health services; efforts continue to improve campaign quality, reach more children, synchronize campaigns across the Pakistan–Afghanistan border, and strengthen AFP surveillance. For cVDPV, responses must improve — yet nOPV2 supply has fallen short, and interruptions in 2024 will delay campaigns and risk further spread. Where outbreaks persist amid insecurity, new strategies are needed to reach missed children.

Limitations: administrative coverage relies on population estimates that may be wrong where census data are old or migration is heavy; and AFP counts may include cases that don't fit the definition and miss unreported ones.

Implications

Eradication needs effective, innovative campaigns in insecure areas, community engagement, accountability down to frontline supervision, enough vaccine for outbreak response, and limits on outbreaks spreading into polio-free areas.

Sources

Based on "Progress Toward Poliomyelitis Eradication — Worldwide, January 2022–December 2023," MMWR 73(19), Centers for Disease Control and Prevention (PDF), data as of April 10, 2024. MMWR material is in the public domain. Country lists and full tables are in the original.