Vaccine-Derived Polio Outbreaks Worldwide, 2021–2022

This report covers outbreaks from January 2021 through December 2022, with data as of February 14, 2023. The targets it mentions for 2023 and 2024 were the goals at that time.

Oral poliovirus vaccine (OPV) contains weakened Sabin strains of poliovirus types 1, 2 and 3. Where too few children are vaccinated, those strains can circulate for a long time and genetically revert into forms that paralyze. The result is a circulating vaccine-derived poliovirus (cVDPV) outbreak.

Wild poliovirus type 2 was declared eradicated in 2015, and in April 2016 countries switched together from trivalent OPV (types 1, 2 and 3) to bivalent OPV (types 1 and 3 only) for routine immunization. Since then, outbreaks of cVDPV type 2 (cVDPV2) have been reported around the world. From 2016 through 2020 the response relied on Sabin-strain monovalent OPV2, which could itself seed new outbreaks if campaigns missed too many children. A more genetically stable vaccine, novel OPV2 (nOPV2), became available in 2021 to reduce that risk, although supplies have often run short for prompt campaigns.

The World Health Organization (WHO) considers an outbreak interrupted once 13 months or more have passed since the last case or positive sample.

The overall picture

• 172 cVDPV outbreaks have been reported since 2016.
• cVDPV outbreaks affected 46 countries during 2021–2022.
• Paralytic cVDPV cases of all types fell 36%, from 1,117 in 2020 to 715 in 2022.
• The share of those cases caused by type 1 (cVDPV1) rose from 3% in 2020 to 18% in 2022, and two countries had type 1 and type 2 outbreaks at the same time.

Type 1: rising

Since 2016, 14 cVDPV1 outbreaks from 12 separate emergences have been reported in 10 countries. Nine were active in 2021–2022, in the Democratic Republic of the Congo (DRC), Madagascar, Malawi, Mozambique and Yemen, including five new outbreaks in the first four. Surveillance for acute flaccid paralysis found 127 paralytic cVDPV1 cases in 2022, up from 35 in 2020 and 16 in 2021.

• DRC detected two cVDPV1 outbreaks in 2022 with 91 cases by December, 72% of the world's cVDPV1 cases that year. It also had type 2 outbreaks.
• Madagascar has had continuous cVDPV1 transmission since September 2020, with 13 cases in 2021 and 2022 and a new emergence confirmed in February 2022.
• Mozambique's first patient in one outbreak became paralyzed in July 2020, but the outbreak was not confirmed until July 2022, once linked cases turned up. Genetic analysis showed the virus had emerged about four years before it was first detected, a sign of large gaps in surveillance. It spread to Malawi, where it was found in September 2022. Mozambique also has a wild poliovirus type 1 outbreak linked to Malawi.

The rise followed a sharp fall in routine immunization and the suspension of preventive campaigns during the COVID-19 pandemic. In the WHO African Region, the share of children who had their third polio vaccine dose by age 1 was 70% in 2022, down from 74% in 2019; in the Eastern Mediterranean Region it was 83% in both years.

Type 2: fewer cases, still spreading

Since August 2016, 154 cVDPV2 outbreaks from 82 emergences have been reported in 48 countries. During 2021–2022, 76 outbreaks were active across 42 countries, and 49 of them, in 35 countries, were first reported in that period: 33 in 23 countries of the WHO African Region and 10 in six countries of the Eastern Mediterranean Region. Nine new emergences were identified in 2021 and five in 2022. In 34 of the 76 active outbreaks, the last detection was at least 13 months old, so transmission is considered interrupted.

Paralytic cVDPV2 cases have declined from their 2020 peak: 1,082 in 2020, 682 in 2021, and 587 in 2022, a count that could still grow to match or pass 2021's as samples from late 2022 are processed. The DRC and Yemen accounted for 75% of the 2022 cases. The DRC reported 283, up from 28 in 2021 and 48% of the world total; Yemen reported 160, up from 66, or 27%.

Some emergences travel far. Thirteen of the 82 spread beyond the country where they were first found:

• one first detected in Nigeria in January 2018 has reached 18 other African countries, with active transmission in 13 during the period;
• another, first found in Nigeria in July 2020, has been detected in 12 more countries since 2021;
• one first detected in Yemen in 2021 has spread to Egypt and Somalia;
• one first detected in Pakistan in 2019 reached Tajikistan in 2020 and then Ukraine, where two cases were found in October–December 2021.

In 2022, genetically related type 2 viruses turned up in Israel, the United Kingdom and New York in the United States: one polio case and 12 environmental samples in the United States, five non-patient samples in the United Kingdom and one in Israel. In Indonesia, a new outbreak with one case was reported in November 2022; its strain had apparently circulated undetected for about three years.

Type 3

Four cVDPV3 outbreaks have occurred since 2016. Two were active in the period: one in Israel in 2021–2022 with one paralytic case, and one in the Palestinian Territories in 2022 with 16 environmental detections.

How well outbreaks are being stopped

The Global Polio Eradication Initiative (GPEI) measures response by whether transmission stops within 120 days of an outbreak being announced. Of outbreaks confirmed in 2021, 19 of 29 (66%) met that mark, compared with 28 of 45 (62%) in 2019 and 27 of 50 (54%) in 2020, so the rate had not changed much. Campaigns were postponed in March–June 2020, and many after that were late or poorly run, allowing breakthrough viruses in many outbreaks.

The drop in new type 2 emergences is likely linked to nOPV2. Since its first use in an outbreak response in March 2021, under WHO's Emergency Use Listing, more than 590 million doses had been given in 24 countries by March 2023. In a statement on March 16, 2023, GPEI reported type 2 detections in Burundi and the DRC that have been linked with nOPV2 use. International spread, however, has not declined.

GPEI's 2022–2026 strategy aimed to stop all cVDPV outbreaks by the end of 2023 and to have no cVDPV detections in 2024. The authors named two main obstacles: surveillance good enough to find the virus quickly, and outbreak responses strong enough to prevent spread across borders. Countries also face late detection, delayed vaccine, competing outbreaks such as measles, cholera and Ebola, insecurity and limited resources. The authors call for higher routine immunization, especially at subnational levels, stronger surveillance, and fast, high-quality vaccination campaigns. Delays in shipping and testing specimens, and gaps in surveillance, mean the true number of cases may be higher.

Sources

• Bigouette JP, Henderson E, Traoré MA, et al. Update on Vaccine-Derived Poliovirus Outbreaks — Worldwide, January 2021–December 2022. MMWR Morb Mortal Wkly Rep 72(14). https://www.cdc.gov/mmwr/volumes/72/wr/mm7214a3.htm
• GPEI statement on cVDPV2 detections in Burundi and the Democratic Republic of the Congo: https://polioeradication.org/news-post/gpei-statement-on-cvdpv2-detections-in-burundi-and-democratic-republic-of-the-congo/
• The report's own totals do not all agree. It gives 88 active outbreaks in 2021–2022, but its counts by type (76, nine and two) do not add up to that. It gives 17 countries with their first post-switch type 2 outbreak in the period, but names 11. Its counts of interrupted outbreaks and of type 2 outbreaks still active at the end of 2022 also differ by one from the other figures. This page leaves out the figures that conflict.
• The report was written jointly by CDC and WHO; its map is not reproduced here.