Wild Polio Returns to Southeastern Africa: The Malawi and Mozambique Outbreak, 2021–2022

This page reports the outbreak response as of April 7, 2023.

Since the Global Polio Eradication Initiative (GPEI) began in 1988, wild poliovirus cases have fallen by more than 99.99%. Five of the six World Health Organization regions have been certified free of indigenous wild poliovirus, and types 2 and 3 have been declared eradicated worldwide. Type 1 (WPV1) remains endemic only in Afghanistan and Pakistan. It had not been seen in southeastern Africa since the 1990s, and on August 25, 2020, the WHO African Region was certified free of indigenous wild poliovirus.

Then, on February 16, 2022, WPV1 was confirmed in a child in Malawi who had become paralyzed on November 19, 2021. Genetic analysis showed the virus came from Pakistan. More cases followed in Mozambique. A report by CDC, WHO, UNICEF, the Gates Foundation, the Task Force for Global Health, South Africa's polio reference laboratory and the five countries' health ministries describes the response.

The cases

From November 2021 to December 2022, nine children and adolescents were paralyzed by WPV1: one in Lilongwe, Malawi, and eight in Mozambique's Tete Province, in the country's northwest, the last with paralysis beginning on August 10, 2022. Several of the Mozambique cases lived near the borders with Malawi, Zambia and Zimbabwe.

• Patients were aged 5 months to 14 years (median 59 months); five were 5 or older.
• Only two had received at least 3 doses of oral polio vaccine, the minimum for adequate protection against type 1.
• Every case was confirmed late: a median of 53 days (range 36–96) passed between paralysis and genetic sequencing results, mostly because shipping stool samples across borders was slow, but also because of slow detection and testing.

The virus's closest relative was a WPV1 lineage found in Sindh, Pakistan, in October 2019. The many genetic changes among the outbreak samples suggest it had been circulating in southeastern Africa for about 2 years before the Malawi case was found.

Gaps in routine immunization

Routine vaccination of every child is the foundation of polio prevention. In 2021, none of the five countries in the response reached the WHO target of 90% coverage with 3 doses of bivalent oral polio vaccine (bOPV, which protects against types 1 and 3):

All five worked to strengthen routine immunization through 2022.

Finding the virus

Polio is tracked mainly through surveillance for acute flaccid paralysis (AFP) in children under 15, with stool samples tested at WHO-accredited laboratories. In 2022 GPEI sent staff to high-risk districts in the two affected countries and the three at-risk neighbors (Tanzania, Zambia and Zimbabwe) to visit health facilities, train clinicians to recognize and report AFP, search communities for paralyzed children, investigate cases and speed samples to laboratories.

• Sensitivity. A rate of at least 2 non-polio AFP cases per 100,000 children under 15 shows surveillance is sensitive enough to catch polio (3 during outbreaks). In 2021, 370 of 554 districts (67%) met that benchmark; in 2022, 512 (92%) did.
• Stool adequacy. At least 80% of AFP patients should have stool collected within 14 days of paralysis and delivered in good condition. Tanzania and Zimbabwe met this nationally in both years, but district-level gaps remained: in 2022 the benchmark was met in 96% of Tanzania's districts and 87% of Zimbabwe's, but only 43% in Malawi, 52% in Mozambique and 34% in Zambia. Without adequate samples, cases go undetected and the full extent of the outbreak cannot be known.
• Sewage testing. Before the outbreak only Tanzania and Zambia had reliably working environmental surveillance. In 2022 it began in Malawi and expanded elsewhere. Sites count as reliable when at least half their samples yield non-polio enteroviruses; that was achieved at 6 of 12 sites in Malawi, 3 of 14 in Mozambique, 10 of 11 in Tanzania and 11 of 11 in Zambia. No sewage sample from 2021–2022 tested positive for WPV1.

Vaccination campaigns

Within 33 days of the Malawi notification, the five countries began mass vaccination campaigns with bOPV for children under 5, known as supplementary immunization activities (SIAs). Malawi, Mozambique, Tanzania and Zambia synchronized their first round in March 2022, but logistics kept later rounds apart.

Because five of Mozambique's eight cases were aged 5 or older, its sixth round was widened to everyone under 15.

Campaign quality was checked within a week by lot quality assurance sampling: 10 children in each of six random settlements per district, with the district passing if at least 57 of the 60 had been vaccinated, evidence that coverage was approaching 90%. After the first rounds in Malawi, Mozambique and Zambia, fewer than 35% of districts passed. Quality improved markedly, and by the third round in Mozambique and Tanzania and the fourth in Malawi and Zambia, more than 70% passed; Zimbabwe also improved between its two rounds. But three of the six districts where cases occurred failed in half of their country's rounds, including Moatize in Mozambique, where four of the nine cases were found.

What helped, and what did not

The authors credit strong collaboration from international coordinators to frontline health workers, high government engagement despite competing priorities, national emergency operations centers, and surge staff from GPEI who helped reach remote areas and strengthen surveillance.

The challenges were serious. Circulating vaccine-derived poliovirus type 1, found in Malawi and Mozambique in 2022, showed that many children remained susceptible to type 1, and vaccine-derived type 2 outbreaks in Mozambique and Zambia competed for resources. Global shortages of bOPV delayed some campaigns or made them smaller, and campaign quality in the affected districts may have been too low to stop transmission. Surveillance gaps and testing delays could hide ongoing spread, coverage figures based on administrative data may overstate immunity, and the quality surveys may not reflect how many children were really reached.

WHO's Africa Regional Certification Commission judged that the outbreak did not yet threaten the region's polio-free certification, because it began with an importation from Pakistan. But if transmission continued 12 months or more after the outbreak was confirmed, certification would be at risk. The authors call for more sensitive and timely surveillance, better-quality campaigns, stronger routine immunization and continued cooperation across borders.

Sources

• Davlantes E, Greene SA, Tobolowsky FA, et al. "Update on Wild Poliovirus Type 1 Outbreak — Southeastern Africa, 2021–2022." MMWR 72(15). https://www.cdc.gov/mmwr/volumes/72/wr/mm7215a3.htm
• The report's maps and charts are not reproduced here because it was prepared jointly with international organizations and national health ministries.