Vaccine-Derived Poliovirus Outbreaks Worldwide, January 2023–June 2024

Oral poliovirus vaccine (OPV) gives long-lasting protection against paralysis. But in communities where very few children are immunized, the weakened vaccine virus can keep passing from person to person for a long time, change genetically, and regain the ability to paralyze. The result is a circulating vaccine-derived poliovirus (cVDPV), which can cause outbreaks of paralytic polio as well as silent spread. CDC and the World Health Organization (WHO) track these outbreaks; this report covers January 2023 to June 2024.

Background: the vaccine switch

After wild poliovirus type 2 was declared eradicated in 2015, countries using OPV stopped giving trivalent OPV, which contains types 1, 2 and 3, and switched to bivalent OPV (types 1 and 3) for routine and campaign vaccination. Bivalent OPV is also used against type 1 outbreaks. Monovalent type 2 OPV was held back for responding to type 2 outbreaks. Since 2021 the recommended vaccine for those outbreaks has been novel OPV type 2 (nOPV2), which is more genetically stable and less likely to revert than the older Sabin-strain vaccine. Manufacturing delays have limited its supply at times, including in early 2024.

How outbreaks are counted

Data came from WHO's Polio Information System and the Global Polio Laboratory Network, as of September 18, 2024. An outbreak is confirmed when two or more independent detections of genetically linked vaccine-derived virus are found, through cases of acute flaccid paralysis, sampling of sewage and other environmental sources, or testing of healthy community members. An outbreak counts as interrupted after 13 months or more with no new detection, and as prolonged when transmission lasts 12 months or more.

CDC map: countries and areas reporting cVDPV outbreaks, January 2023–June 2024.

The overall picture

• 74 outbreaks in 39 countries or areas, mostly in Africa. Of these, 47 were new, detected in 30 countries.
• 672 confirmed paralysis cases in 27 of the countries, down from 881 in 2022 — though the number of countries reporting cases stayed about the same.
• 12 countries detected the virus only through environmental or community sampling.
• 15 countries had transmission lasting more than a year into 2024: Algeria, Benin, Cameroon, Central African Republic, Chad, Côte d'Ivoire, Democratic Republic of the Congo, Indonesia, Madagascar, Mali, Mozambique, Niger, Nigeria, Somalia and Yemen.
• No new type 3 emergences were detected.

Type 1: a sharp decline

Type 1 outbreaks were found in only three countries — the Democratic Republic of the Congo (DRC), Madagascar and Mozambique — from four emergences, with no new countries or emergences since 2022. There were 140 confirmed cases, 111 of them in DRC. The latest detections were in April 2024 in DRC and May 2024 in Mozambique; Madagascar's most recent was in September 2023, after years of transmission. The authors see this as evidence that response efforts can work, and as a sign that all type 1 outbreaks could be controlled in 2024. Two countries, DRC and Mozambique, had type 1 and type 2 circulating at the same time.

Type 2: still widespread

Type 2 caused 70 outbreaks from 34 emergences in 38 countries, with 532 confirmed cases in 26 of them. Five countries reported their first type 2 detection since type 2 vaccine was withdrawn from routine immunization in April 2016.

Ten of the 34 emergences spread beyond the country where they were first found. Nigeria and Somalia, both with security-compromised areas, were repeatedly the starting points:

• NIE-ZAS-1, first detected in Nigeria in July 2020, kept circulating there and was found in 17 other countries of WHO's African Region, especially in West Africa.
• SOM-BAN-1, first found in Somalia in October 2017, was reported in Kenya and, for the first time, in Uganda.

Elsewhere, Indonesia detected two more type 2 emergences with seven cases, and an emergence found in 11 environmental samples in Egypt turned up in six samples from the Palestinian Territories in June 2024.

Outbreaks linked to the newer vaccine

Twenty-nine type 2 outbreaks, arising from 19 emergences in 19 countries, were linked to use of nOPV2. They caused 113 paralysis cases in 14 of those countries, 70 of them in DRC; five countries detected them only in environmental samples. One emergence first found in DRC in January 2023 spread to Angola, Mozambique and the Republic of the Congo; another spread from DRC to Angola, Burundi, Côte d'Ivoire, Tanzania and Zambia, with a case in Angola as recently as May 11, 2024.

nOPV2 is more stable than the older vaccine in circulation, but these outbreaks show that vaccine-derived virus can still emerge from it when vaccination responses are late or of poor quality. The longer the vaccine virus circulates in a community, the more chance it has to revert and seed new emergences.

Why outbreaks persist

• Slow or weak responses. Vaccination campaigns were run in 32 of the 39 countries, but delayed and low-quality campaigns allowed further international spread.
• Vaccine supply. Shortages of nOPV2, along with logistical problems and poor access, held up responses.
• Missed children. Routine childhood immunization fell in the early years of the COVID-19 pandemic, leaving many undervaccinated children in African countries with weak health services and raising the risk of new emergences.
• Surveillance gaps. These delay responses and give the virus more time to spread. Work is under way to accredit more laboratories for genomic sequencing.

What it will take

The Global Polio Eradication Initiative's 2022–2026 Strategic Plan aimed to interrupt all cVDPV transmission in 2024; the current target is the end of 2026. Meeting it, the authors write, requires prompt, high-quality vaccination campaigns as soon as a new outbreak is detected, well-resourced responses with cooperation across borders, and stronger efforts — including more work with humanitarian nongovernmental organizations — to reach children in insecure and hard-to-reach communities.

Limitations

Gaps in surveillance may mean cases and spread are underestimated or mislocated, and delays in shipping and testing samples may mean cases, outbreaks and emergences from January to June 2024 are underreported.

Sources

• Apophia Namageyo-Funa, Sharon A. Greene, Elizabeth Henderson, Mohamed A. Traoré, Shahzad Shaukat, John Paul Bigouette, Jaume Jorba, Eric Wiesen, Omotayo Bolu, Ousmane M. Diop, Cara C. Burns and Steven G.F. Wassilak, "Update on Vaccine-Derived Poliovirus Outbreaks — Worldwide, January 2023–June 2024," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/73/wr/mm7341a1.htm
• The source's breakdown of type 1 cases by year does not add up consistently, and it gives two different counts of interrupted outbreaks; this page states neither.