On 17 September 2020, South Sudan received laboratory confirmation of an outbreak of circulating vaccine-derived poliovirus type 2 (cVDPV2). The index case of this outbreak had a date of onset of paralysis on the 16 June 2020, with the last reported case having the onset of paralysis given as the 8 April 2021.[1] This outbreak is documented to have had 91 poliovirus isolates from 59 paralyzed children, 24 AFP contacts, and 8 environmental samples from all ten states of the Republic of South Sudan. Among paralytic-infected children, the majority, 93 percent of cases, were children under 5, and 56 percent were male. The risk of enduring disability and mortality was notably heightened due to the country's sub-optimal immunization coverage, less than 50 percent.[2] To interrupt the outbreak, the Ministry of Health constituted an emergency task force, encompassing the World Health Organization (WHO), the United Nations Children's Fund (UNICEF), and additional partners. At the core of the intervention strategy were two rounds of monovalent Oral Polio Vaccine type 2 (mOPV2) campaigns, implemented across all 80 counties in all 10 states. To enhance the effectiveness of these efforts, supplementary mop-up campaigns were initiated in counties demonstrating less than optimal immunization performance. This rigorous approach successfully interrupted the transmission of cVDPV2 within 29 weeks, which is aligned with the prescriptions given in Polio Outbreak response SOPs.

How did South Sudan, with the support of WHO, achieve this?

WHO secretariat has established a nationwide implementation and service delivery structure for polio eradication. The structure, financed by GPEI, has been in place since South Sudan became an independent state. The WHO structure includes a National Eradication Team, which consists of a team lead, a surveillance officer, and a data/analytics team. Additionally, 10 Expanded Programme on Immunization Officers and 13 Stoppers are deployed at the state level. At the county and sub-county levels, there are 80 Field Supervisors supported by 230 Field Assistants. This polio eradication network of staff supervises, monitors, and supports reporting routine immunization, ensuring comprehensive coverage nationwide. They actively visit all health facilities searching for suspected vaccine-preventable diseases and work closely with Ministry of Health Officers to plan, prepare, and supervise supplemental immunization activities and community surveillance for suspected acute flaccid paralysis (AFP) cases.

^{A child received an anti-polio drop during the campaign launch at Jebel Dinka.}
Photo credit: WHO South Sudan

With this structure, South Sudan made substantial strides in its battle against polio, interrupting the indigenous circulation of the Wild Polio Virus (WPV) in 2001. However, the country still grapples with recurrent outbreaks, often imported, demonstrating the vulnerabilities of a population with low immunity. Less than 50 percent of the nation's children receive vital vaccinations, including those against polio, underscoring the critical need for robust surveillance to ensure early detection and control.[3]

“In the past, many of us believed that the polio virus disease was caused by Witchcraft. Now we know that it is caused by a virus and can be prevented by taking the polio vaccines our children are receiving today.”

- Nyangap Machar Chol, 34-year-old mother, South Sudan

WHO has provided the Ministry of Health with electronic information transmission and data warehousing servers using the Open Data Kit[4] for instantaneous transmission of surveillance data, contact sampling of healthy children, environmental surveillance (ES), and the AVADAR community-based surveillance for paralyzed children. AVADAR, a mobile technology enabling community informants to detect, and report suspected acute flaccid paralysis cases, was piloted in three counties to reinforce the surveillance network. AVADAR was responsible for reporting between 86% and 100% of the total cases in these counties.[5] Additionally, in three states disrupted by civil conflict, the community surveillance network operated by trained local informants is supplemented by technical consultants from the Bill and Melinda Gates Foundation (BMGF).

The Ministry of Health rapidly assembled an emergency task force in response to the outbreak, incorporating all partners from the Global Polio Eradication Initiative (GPEI), including WHO and UNICEF. This task force led the outbreak investigations and conducted risk assessments, concurrently devising a comprehensive response plan. By mobilizing surge staff and essential resources, including a no-regret fund, the WHO coordinated outbreak response activities. 

In this response, WHO supported Polio eradication network detected a cVDPV2 outbreak and provided technical leadership in outbreak investigation, national risk assessment, and the design of a comprehensive response plan. The response plan included measures such as intensified surveillance for poliovirus, three rounds of supplemental immunization activities utilizing type 2 monovalent Oral Polio Vaccines (mOPV2), and changes to national immunization policies and practices, including the introduction of a second dose of Inactivated Polio Vaccines.

In turn, reported cases of acute flaccid paralysis increased from 430 in 2020 to 567 in 2021. In addition, 2.5 million (91%), 2.7 million (99%), and 847,400 (91%) children received vaccinations in rounds 1, 2, and mop-up rounds, respectively. The WHO Lot quality assurance surveys showed improvements from 50% to 90% pass rates between the first and mop-up rounds of supplemental immunization activities, demonstrating WHO-supported polio network's capacity to learn and improve public health service delivery in South Sudan.

My two siblings were crippled by polio when I was 12 years old. To take care of the two disabled persons from childhood to date has been the greatest burden to the family. I made up my mind that my children and others must this time receive the polio drops and I have no doubt that the vaccines will protect them. I thank the government of South Sudan and WHO for making sure that vaccination campaign occurs in Maper and its now our job to ensure that our children are vaccinated.”

- Arok Juong Lueth, 32-year-old mother, South Sudan

To ensure the highest quality implementation, the GPEI dispatched experienced consultants to the states most severely affected by the outbreak. These professionals concentrated on enhancing the skills of health workers, developing meticulous micro-plans, supervising on-site activities, and conducting active surveillance visits to the priority surveillance sites. High-priority locations received regular surveillance visits, and the importance of maintaining high levels of polio vaccination coverage was emphasized to both health workers and community leaders.

The proactive approaches expedited the detection of acute flaccid paralysis cases and enabled timely interventions against the polio outbreak. Over a span of 29 weeks, the cVDPV2 outbreak was successful controlled, with the final case reported with onset of paralysis on 8 April 2021, in Panyijar, Unity state. The successful response underscores the crucial importance of the GPEI/WHO supported polio surveillance network, coordinated efforts/synergies of the polio eradication initiative partners, and rationale for the sustained investments in this network for disease prevention and control.

The intervention resulted in more than two years with no confirmed poliovirus cases. Only two cases of vaccine-derived poliovirus were identified in December 2023, two and a half years later.