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

Abstract

Circulating vaccine-derived polioviruses (cVDPVs) can emerge and lead to outbreaks of paralytic polio as well as asymptomatic transmission in communities with a high percentage of undervaccinated children. Using data from the World Health Organization Polio Information System and Global Polio Laboratory Network, this report describes global polio outbreaks due to cVDPVs during January 2023–June 2024 and updates previous reports. During the reporting period, 74 cVDPV outbreaks were detected in 39 countries or areas (countries), predominantly in Africa. Among these 74 cVDPV outbreaks, 47 (64%) were new outbreaks, detected in 30 (77%) of the 39 countries. Three countries reported cVDPV type 1 (cVDPV1) outbreaks and 38 countries reported cVDPV type 2 (cVDPV2) outbreaks; two of these countries reported cocirculating cVDPV1 and cVDPV2. In the 38 countries with cVDPV2 transmission, 70 distinct outbreaks were reported. In 15 countries, cVDPV transmission has lasted >1 year into 2024. In Nigeria and Somalia, both countries with security-compromised areas, persistent cVDPV2 transmission has spread to neighboring countries. Delayed implementation of outbreak response campaigns and low-quality campaigns have resulted in further international spread. Countries can control cVDPV outbreaks with timely allocation of resources to implement prompt, high-quality responses after outbreak confirmation. Stopping all cVDPV transmission requires effectively increasing population immunity by overcoming barriers to reaching children.

Introduction

Live, attenuated oral poliovirus vaccine (OPV) induces long-term protection against paralytic disease, and limits virus shedding in vaccinated persons with infection (1). Circulating vaccine-derived poliovirus (cVDPVs)* outbreaks occur when OPV-related strains undergo prolonged circulation in communities with very low immunity against polioviruses, and the genetically reverted virus has regained neurovirulence (vaccine-derived poliovirus [VDPV] emergence) (2,3). After declaration of wild poliovirus type 2 eradication in 2015, and in an effort to lower the risk for cVDPV type 2 (cVDPV2) outbreaks, immunization programs in countries using OPV switched from using trivalent OPV (tOPV) (containing types 1, 2, and 3 Sabin strains) in routine and supplementary immunization activities (SIAs) to bivalent OPV (bOPV) (containing types 1 and 3 Sabin strains) (4); bOPV is used for cVDPV type 1 (cVDPV1) outbreak response. After the switch from tOPV to bOPV, monovalent type 2 OPV (mOPV2) (Sabin-strain type 2) was reserved for cVDPV2 outbreak responses. Since 2021, novel oral poliovirus vaccine type 2 (nOPV2), a more genetically stable vaccine with reduced risk for reversion to neurovirulence than Sabin-strain OPV2, has been the recommended vaccine for cVDPV2 outbreak response (5). However, nOPV2 supply has been periodically restricted because of manufacturing delays, including during a period in early 2024. Despite the goal of permanent cessation of cVDPV2 transmission by switching from tOPV to bOPV, new cVDPV2 polio outbreaks continue to be reported in multiple countries (5,6). This report describes global polio outbreaks due to cVDPVs during January 2023–June 2024 and updates previous reports.

Methods

Data Sources

The surveillance and virologic data on cVDPV outbreaks in this report (as of September 18, 2024)^† were gathered from the World Health Organization (WHO) Polio Information System^§ and the Global Polio Laboratory Network (GPLN).^¶ Genomic sequencing and analyses were conducted by WHO-accredited sequencing laboratories within GPLN. A cVDPV outbreak is confirmed when two or more independent detections of genetically linked VDPV emergences are identified through acute flaccid paralysis (AFP) surveillance, environmental surveillance (ES),** or from sampling of healthy community members^†† (2,3). Each unique VDPV emergence group is labeled by the country or area (country) and geographic subnational region of the emergence and the number of emergences in each subnational region.

Data on outbreak control were also reviewed. Based on WHO’s Emergency Committee under the International Health Regulations on the international spread of poliovirus, outbreaks were considered to have been interrupted when no detections were identified ≥13 months since the onset of paralysis or collection of the most recent positive environmental or other sample. Outbreaks were considered to be prolonged when transmission persisted for ≥12 months.

Analysis

VDPV outbreaks were tabulated and mapped by country, serotype, source of detection, emergence group, and other characteristics. cVDPV emergences with ongoing transmission detected outside of the country of first isolation during 2024, either through AFP cases or ES, were also plotted. Descriptive analyses were conducted using R software (version 4.4.1; R Foundation). These activities were reviewed by CDC, deemed not research, and were conducted consistent with applicable federal law and CDC policy.^§§

Results

cVDPV Outbreaks

During January 2023–June 2024, a total of 74 cVDPV outbreaks were detected in 39 countries, with 672 confirmed AFP cases identified in 27 of the 39 countries (Figure 1) (Table). Twelve countries reported cVDPV circulation detected only through ES or sampling of healthy community members. Cocirculation of cVDPV1 and cVDPV2 was detected in two countries in the WHO African Region (Democratic Republic of the Congo [DRC] and Mozambique). During the reporting period, no new cVDPV3 emergences were detected. Overall, the number of cVDPV AFP cases declined from 881 in 2022 to 672 during January 2023–June 2024 (Supplementary Figure, https://stacks.cdc.gov/view/cdc/164302). Despite the decline in AFP case counts, the number of countries reporting AFP cases remained approximately the same. The number of cVDPV1 AFP cases declined substantially over the reporting period.

cVDPV1 Outbreaks

During January 2023–June 2024, cVDPV1 circulation was detected in three countries (DRC, Madagascar, and Mozambique) from four cVDPV1 emergences (Figure 1) (Table). No new countries or emergences were reported since 2022. A total of 140 AFP cases were confirmed, with 111 reported from DRC, 106 (75%) in 2023, and five (5%) in the first half of 2024. Recent cVDPV1 detections occurred in April 2024 in DRC (RDC-TAN-1) and May 2024 in Mozambique (MOZ-NPL-2). The latest cVDPV1 detection in Madagascar occurred in September 2023.

cVDPV2 Outbreaks

During January 2023–June 2024, a total of 70 cVDPV2 outbreaks from 34 emergences were reported in 38 countries (Figure 1) (Table); 532 AFP cases were confirmed in 26 countries. During the reporting period, five countries reported their first cVDPV2 detection since type 2-containing OPV was removed from OPV-using countries’ routine immunization programs in April 2016. Ten (29%) of the 34 emergences spread outside the country of first detection. In Nigeria and Somalia, countries with security-compromised areas, persistent cVDPV2 transmission has spread frequently to neighboring countries. The transmission from Nigeria to its neighbors led to further international spread. The NIE-ZAS-1 emergence, first detected in Nigeria in July 2020, continued to circulate within Nigeria during the reporting period and was detected in 17 other countries in the African Region, particularly in West Africa (Figure 2). SOM-BAN-1, detected in Somalia in October 2017 (7), was reported in Kenya, and for the first time, was detected in Uganda during the reporting period. Since the previous reporting period (January 2021–December 2022) (7), Indonesia has detected two additional cVDPV2 emergences, with seven AFP cases. The EGY-NOR-1 emergence detected in 11 ES samples collected in Egypt was detected in six ES samples collected in the Palestinian Territories in June 2024 (Table).

Among the 70 cVDPV2 outbreaks, 29 (comprising 19 VDPV2 emergences) were linked to nOPV2 use in 19 countries. Two of the 19 emergences (RDC-SKV-1 and CAF-KEM-1) were first detected in September 2022 and December 2022, respectively. A total of 113 AFP cases were reported in 14 of the 19 countries with emergences; the highest number (70; 62%) was reported in DRC. Five countries detected outbreaks linked to nOPV2 use through ES only, with no confirmed AFP cases. The RDC-KOR-1 emergence was first detected in DRC in January 2023 and spread to Angola, Mozambique, and Republic of the Congo. The RDC-SKV-1 emergence was first detected in DRC and spread to Angola, Burundi, Côte d’Ivoire, Tanzania, and Zambia, with the most recent detection of an AFP case in Angola on May 11, 2024.

Outbreak Control

Of the 74 cVDPV outbreaks, 47 were new outbreaks detected during the reporting period in 30 of the 39 countries reporting outbreaks (Table). The remaining 27 outbreaks had been detected before the reporting period began and are ongoing (<13 months since most recent case) in 20 of the 39 countries. During January 2023–June 2024, SIAs were conducted to control cVDPV outbreaks in 32 of the 39 outbreak countries. Among the 74 cVDPV outbreaks, 11 (in seven countries) were documented to have been interrupted. Prolonged transmission of a cVDPV outbreak (≥12 months from first to most recent detection) was observed in 15 countries: Algeria, Benin, Cameroon, Central African Republic, Chad, Côte d’Ivoire, Democratic Republic of the Congo, Indonesia, Madagascar, Mali, Mozambique, Niger, Nigeria, Somalia, and Yemen. In seven of the 74 outbreaks across six countries, no cVDPV was detected ≥13 months since the most recent positive sample.

Discussion

Continuing cVDPV outbreaks impede attainment of the Global Polio Eradication Initiative (GPEI) 2022–2026 Strategic Plan goal of eradicating polio, particularly that of interrupting all cVDPV transmission in 2024 (2). During January 2023–June 2024, cVDPV2 outbreaks remained as prevalent as those during previous reporting periods (7,8). Although the number of countries reporting outbreaks is approximately the same during each of the most recent years, as some countries interrupt transmission, newly infected or reinfected countries are reporting confirmed outbreaks.

Since 2022, no new countries have reported cVDPV1 emergences or outbreaks. Although cVDPV1 detections were reported in Mozambique and DRC in early 2024, no detections have been reported in Madagascar since September 2023, following multiyear transmission in each of these countries. This development reflects the ultimate success of outbreak response efforts and highlights the possibility of controlling all cVDPV1 outbreaks in 2024. The decline in routine childhood immunization coverage during the early years of the COVID-19 pandemic (9) has resulted in an accumulation of undervaccinated, susceptible children in many African countries with weak essential health services, increasing the risk for new cVDPV1 emergences.

The spread of cVDPV2 emergence groups such as NIE-ZAS-1 and SOM-BAN-1 outside the country of first detection, often with further international spread, reveals gaps in the effectiveness and timeliness of outbreak responses. In light of the many social, economic, and political challenges, promptly interrupting transmission of cVDPV2 requires sufficient resources, including those mobilized within countries, to implement intensive response efforts with cross-border collaborations.

Low supplies of nOPV2, compounded with logistical challenges and insufficient access, have led to delays in implementation of outbreak responses, impeding efforts to achieve the high population immunity required to stop cVDPV2 transmission. During January 2023–June 2024, cVDPV2 outbreaks were linked to nOPV2 use in 19 countries. Whereas nOPV2 is genetically more stable than Sabin strain OPV2 in community circulation, these findings highlight that cVDPVs can develop with nOPV2 use when the timing and quality of vaccination responses are suboptimal (10). Prolonged community circulation of the vaccine strain leads to reversion to neurovirulence, seeding new emergences.

Gaps in poliovirus surveillance can delay outbreak response activities and provide a longer opportunity for virus to spread. Efforts are underway to strengthen surveillance systems and improve the capacity to confirm cVDPV outbreaks by increasing the number of laboratories accredited by GPLN to perform genomic sequencing.

The current GPEI target is to stop all cVDPV transmission by the end of 2026. Continued circulation of cVDPVs highlights the need for 1) increased urgency to implement prompt, high-quality SIAs upon detection of new cVDPV outbreaks and 2) enhanced efforts, such as more engagement with humanitarian nongovernmental organizations, to vaccinate children in security-compromised areas and in hard-to-reach communities.

Limitations

The findings in this report are subject to at least two limitations. First, existing gaps in polio surveillance systems might lead to the underestimation of cases and transmission levels and inaccuracies in the geographic spread of cVDPVs. Second, delays in the transportation of polio samples and testing by reference laboratories might result in underreporting of cases, outbreaks, and emergences during January–June 2024.

Implications for Public Health Practice

GPEI currently aims to eradicate polio by 2026; the key challenges are ending transmission in security-compromised areas and hard-to-reach communities and preventing any further international spread. Ending transmission by 2026 will require a focus on implementing intensive efforts to vaccinate children in security-compromised and hard-to-reach communities to achieve the goal of sustained cVDPV2 interruption. Countries can control cVDPV outbreaks with timely allocation of resources to implement prompt, high-quality responses after outbreak confirmation. Stopping all cVDPV transmission requires effectively increasing population immunity by overcoming the barriers to reaching children.