Figure-1 Detection of genetically linked cVDPV2 isolates in the United Kingdom and the United States of America from February to August 2022 
Epidemiology of Poliomyelitis
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 (5-10% of those paralyzed).
The virus is transmitted from 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 and death. Initial symptoms of polio include fever, fatigue, headache, vomiting, stiffness of the neck and pain in the limbs. In a small proportion of cases, the disease causes paralysis, which is often permanent. There is no cure for polio but it can be prevented by immunization.
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 only mild symptoms and the disease often goes unrecognized.
Vaccine-derived poliovirus is a well-documented type of poliovirus that has mutated from the strain originally contained in the oral polio vaccine (OPV). The OPV contains a live, weakened form of poliovirus. 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. Further changes occur as these viruses spread from person to person. 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 paralyze – 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). cVDPV2 continues to affect different areas of the world.
The United Kingdom Health Security Agency (UKHSA) is conducting further investigations including assessing the public health risk and implementing response measures. These measures include:
The emergence of cVDPV2 in the United Kingdom and in the United States of America is a reminder that until polio is eradicated, polio-free countries will remain at risk of polio re-infection or re-emergence. The detection of this VDPV2 strain underscores the importance of;
Based on the WHO UNICEF estimates, the vaccine coverage for three routine doses of polio vaccine evaluated in children aged 12 months in the United Kingdom and United States of America were 93% and 92% respectively in 2021.
WHO will continue to support the ongoing investigation, risk assessment and outbreak response by national authorities.
WHO reiterates to all Member States the importance of reaching and maintaining polio vaccination coverage of more than 95% in each district or municipality; maintaining high quality for three main surveillance indicators: acute flaccid paralysis (AFP) rate, percentage of cases investigated within 48 hours, and percentage of cases with an adequate sample; optimizing supplementary (environmental and enterovirus) poliovirus surveillance and updating national poliovirus outbreak response plans in order to rapidly detect and respond to new virus importations or VDPV emergence to minimize the consequences of poliovirus transmission and facilitate a rapid response.
The thirty-second polio IHR Emergency Committee meeting held in June 2022, convened under the International Health Regulations (2005), agreed that the risk of international spread of poliovirus remains a Public Health Emergency of International Concern (PHEIC) and recommended the extension of Temporary Recommendations for a further three months.
Citable reference: World Health Organization 14 September 22 2022). Disease Outbreak News; Detection of circulating vaccine derived polio virus 2 (cVDPV2) in environmental samples– the United Kingdom of Great Britain and Northern Ireland and the United States of America . Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2022-DON408
[1] No further information is available on VDPV2 detections in Israel at this stage as investigations are ongoing
