Circulating vaccine-derived poliovirus type 2 – Sudan

On 18 December 2022, the IHR National Focal Point for Sudan notified WHO of the detection of a circulating vaccine-derived poliovirus type 2 (cVDPV2) in a 48-month-old male with acute flaccid paralysis (AFP), from West Darfur in Western Sudan. The case had onset of paralysis on 31 October. The isolated virus is most closely related to a strain that circulated in Borno State, Nigeria, in 2021, and is unrelated to a cVDPV2 strain that caused an outbreak in Sudan in 2020. On 28 November, the Federal Ministry of Health (FMOH) launched an immunization campaign for children under the age of 13 years in the affected areas. Field investigations were immediately launched by local and national public health authorities, with support from partners of the Global Polio Eradication Initiative (GPEI).

On 18 December 2022, the IHR National Focal Point for Sudan notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in a 48-month-old male with acute flaccid paralysis (AFP), from West Darfur, Western Sudan. The case had the onset of paralysis on 31 October. Two stool specimens were collected on 10 and 12 November 2022 and were confirmed to be cVDPV2 on 16 December. Sequencing results showed that the isolate has undergone 38 nucleotide changes.  The isolated virus is most closely related to the strain that circulated in Borno state, Nigeria, in 2021. The current case is a new importation and is unrelated to the cVDPV2 strain that caused an outbreak in Sudan in 2020. 

In 2020, the cVDPV2 outbreak affected 15 of 18 states in Sudan. The outbreak was caused by a cVDPV2 imported from Chad, which caused paralysis in 58 children. The end of the outbreak was declared in August 2022, after the Outbreak Response Assessment (OBRA) was conducted from 2 July to 1 August 2022.

According to the WHO-UNICEF 2021 estimates of national polio immunization coverage in Sudan, the oral poliovirus vaccine third dose (OPV3) and inactivated poliovirus vaccine first dose (IPV 1) coverage were 85% and 94% respectively.

Polio is a highly infectious disease that largely affects children under five years of age, causing permanent paralysis (approximately 1 in 200 infections) or death (2-10% of those paralyzed).

The virus is transmitted by person-to-person, mainly through the fecal-oral route or, less frequently, by a common vehicle (e.g., contaminated water or food) and multiplies in the intestine, from where it can invade the nervous system and cause paralysis. The incubation period is usually 7–10 days but can range from 4–35 days. Up to 90% of those infected are either asymptomatic or experience mild symptoms and the disease usually goes unrecognized.

Vaccine-derived poliovirus is a well-documented strain of poliovirus mutated from the strain originally contained in OPV. OPV contains a live, weakened form of poliovirus that replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. On rare occasions, when replicating in the gastrointestinal tract, OPV strains genetically change and may spread in communities that are not fully vaccinated against polio, especially in areas where there is poor hygiene, poor sanitation, or overcrowding. The lower the population immunity, the longer this virus survives and the more genetic changes it undergoes.

In very rare instances, the vaccine-derived virus can genetically change into a form that can cause paralysis as does the wild poliovirus – this is what is known as a vaccine-derived poliovirus (VDPV). The detection of VDPV in at least two different sources and at least two months apart, that are genetically linked, showing evidence of transmission in the community, should be classified as ‘circulating’ vaccine-derived poliovirus type 2 (cVDPV2). 

The FMOH declared the outbreak of polio in Sudan within 24 hours after the laboratory confirmation of the case. Additionally, the FMOH, with support from WHO, UNICEF and partners of the GPEI, have implemented the following public health measures:

• Completed detailed case investigation.
• An internal consultation of experts was convened to discuss the public health actions to be taken.
• On 28 November, the Federal Ministry of Health (FMOH) launched an immunization campaign for children under the age of 13 years in the affected areas.
• A risk assessment was conducted on 22 December 2022 and based on the outcome of the assessment, two rounds of National Immunization Days were proposed to the Outbreak Response and Preparedness Group, with a target population of 8 964 477 children under the age of five for each round.

The emergence of cVDPV2 indicates gaps in routine immunization.  It also highlights the importance of maintaining high levels of routine vaccination coverage everywhere to minimize the risk and consequences of the circulation of any poliovirus, as well as the need to ensure quality surveillance for early detection of any poliovirus.

Furthermore, extensive population movement within country and across the borders coupled with sub optimal population immunity due to low vaccination coverage increase the risk of disease spreading across the country.

It is important that all countries, in particular those with frequent travel and contacts with polio-affected countries and areas, strengthen surveillance for AFP cases and commence planned expansion of environmental surveillance in order to rapidly detect any new virus importation and to facilitate a rapid response. Countries, territories and areas should also maintain uniformly high routine immunization coverage at the district level to minimize the consequences of any new virus introduction.

WHO’s International International Travel and Health and Health recommends that all travelers to polio-affected areas be fully vaccinated against polio. Residents (and visitors for more than four weeks) from infected areas should receive an additional dose of oral polio vaccine (OPV) or inactivated polio vaccine (IPV) within four weeks to 12 months of travel. 

Based on the advice of an Emergency Committee convened under the International Health Regulations (2005), the risk of the international spread of poliovirus remains a Public Health Emergency of International Concern (PHEIC). Countries affected by poliovirus transmission are subject to Temporary Recommendations. To comply with the Temporary Recommendations issued under the PHEIC, any country infected by poliovirus should declare the outbreak as a national public health emergency, ensure the vaccination of residents and long-term visitors and restrict at the point of departure travel of individuals, who have not been vaccinated or cannot prove their vaccination status.

WHO does not recommend any travel and/or trade restrictions to Sudan based on the information available for this current event.

• Global Polio Eradication Initiative: https://polioeradication.org/
  
• Poliomyelitis Fact sheets: https://www.who.int/health-topics/poliomyelitis#tab=tab_1
• WHO immunization dashboard: https://immunizationdata.who.int/
• GPEI Public health emergency status: http://polioeradication.org/polio-today/polio-now/public-health-emergency-status/
• International travel and health: https://www.who.int/travel-advice
• Vaccine-derived polioviruses: http://polioeradication.org/polio-today/polio-prevention/the-virus/vaccine-derived-polio-viruses/
• Polio outbreak in Sudan successfully stopped and declared closed: https://www.emro.who.int/polio-eradication/news/polio-outbreak-in-sudan-successfully-stopped-and-declared-closed.html

Citable reference: World Health Organization (12 January 2023). Disease Outbreak News;Circulating vaccine-derived poliovirus type 2 (cVDPV2) – Sudan . Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2023-DON362