The situation at a glance
In the past few months, there have been several detections of genetically linked Sabin-like poliovirus type 2 (SL2) from environmental samples in the United Kingdom of Great Britain and Northern Ireland (UK) and the United States of America.
In the UK, since February 2022, the WHO Global Polio Laboratory Network (GPLN) based at the National Institute for Biological Standards and Control (NIBSC) in London has consistently detected Sabin type 2 poliovirus isolates in sewage samples collected from London. Samples collected on May 24 and 31 had enough mutations to be considered vaccine-derived poliovirus type 2 (VDPV2). Subsequently, due to the new detection of the virus more than two months later, these samples were classified as “circulating” VDPV2 on August 8. As of 5 September, no human case associated with VDPV2 has been reported in the UK.
In the United States, Sabin type 2 poliovirus was consistently detected in environmental samples collected between April 21 and August 26, 2022, from Rockland County, New York, and surrounding counties. In late July, a case of VDPV2 was reported in an unvaccinated individual in Rockland County, who had paralysis. The case had no recent history of international travel. This is the first case of poliomyelitis reported in the country since 2013. Due to the detection of environmental viral sequences (collected on August 3 and August 11) containing more than five nucleotide changes and both linked to the case reported in Rockland County – these viruses are now classified as “circulating” VDPV2.
A virus detected in environmental samples in New York State, United States of America, is genetically related to viruses detected in sewage samples from London, United Kingdom, and in sewage samples collected between January and June 2022 from Jerusalem District, Israel1.
Figure-1 Detection of genetically linked cVDPV2 isolates in the United Kingdom and 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 and causes permanent paralysis (approximately 1 in 200 infections) or death (5-10% of those paralyzed).
The virus is transmitted from person to person, mainly by the fecal-oral route or less often 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, neck stiffness, and limb pain. In a small number 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 have only mild symptoms and the disease is often not recognized.
Vaccine-derived poliovirus is a well-documented type of poliovirus that has mutated from the strain originally contained in the oral polio vaccine (OPV). OPV contains a live, weakened form of poliovirus. In rare cases where they replicate in the gastrointestinal tract, OPV strains change genetically and can spread in communities that are not fully vaccinated against polio, especially in areas with poor sanitation, poor hygiene or overcrowding. Other changes occur when these viruses are spread from person to person. The lower the immunity of the population, the longer this virus survives and the more genetic changes it undergoes. In very rare cases, the vaccine-derived virus can genetically change to a form that can paralyze—this is known as vaccine-derived poliovirus (VDPV).
Detection of VDPV in at least two different sources and at least two months apart that are genetically linked and show evidence of community transmission should be classified as “circulating” vaccine-derived poliovirus type 2 (cVDPV2). cVDPV2 continues to affect different areas of the world.
WHO, in collaboration with national authorities, will continue to evaluate the genetic and epidemiological situation to determine the possible spread of the virus and the potential risk associated with these isolates detected from different locations around the world.
- Public health measures in the United Kingdom
The UK Health Safety Agency (UKHSA) is carrying out further investigations, including assessing risks to public health and implementing response measures. These measures include:
- Enhancing environmental, clinical and laboratory surveillance of polio.
- A catch-up campaign targeting children under 5 years of age in London was implemented in June 2022 and an additional booster inactivated polio vaccine (IPV) targeting children aged 1 to 9 years in London was launched in August 2022.
- The detection of VDPV2 in London was alerted by public health officials, health workers and laboratory staff.
- Health professionals recalled the importance of checking that newly registered routine vaccinations of children and adults are up-to-date, with an emphasis on under-immunized populations (new migrants, asylum seekers and refugees).
- Local and regional laboratories have requested that all stool samples positive for enterovirus be forwarded to the UKHSA.
- Stepping up environmental sampling to assess the extent of the spread of the virus in London. In addition, a number of additional wastewater sampling sites are being constructed across the country.
- Public health measures in the United States of America
- Strengthening supervision of wastewater in polio.
- Ongoing activities to promote polio vaccination and increase vaccination coverage in Rockland and Orange County, New York. Planning is underway to launch an immunization campaign that would provide IPV to Rockland County residents who have been potentially exposed to polio.
- Conducting poliovirus testing in sewage samples in New York and neighboring states, as well as providing confirmatory tests for clinical samples.
- Coordinating acute flaccid myelitis (AFM) surveillance throughout the United States and strengthening paralytic and non-paralytic polio surveillance in areas where poliovirus has been detected in wastewater.
- Health advisories including information about polio, the situation in New York State, and polio immunization were issued to health care providers and hospitals throughout the state and in the county of immediate residence of the case.
- On September 9, a polio emergency was declared in New York State. The declaration allows other healthcare professionals, such as pharmacists, to administer the polio vaccine and enables health professionals to issue standing orders for polio vaccination.
The occurrence of cVDPV2 in the United Kingdom and the United States is a reminder that until polio is eradicated, polio-free countries will continue to be at risk of re-infection or re-emergence of polio. The detection of this VDPV2 strain underscores the importance;
• maintaining high levels of routine polio vaccination at all levels and in all communities to minimize the risk and consequences of any poliovirus circulation.
• have sensitive surveillance systems for early detection of import of VDPV or emergence of VDPV.
Based on WHO UNICEF estimates, coverage of three routine doses of polio vaccine evaluated in children aged 12 months in the United Kingdom and the United States was 93% and 92% in 2021, respectively.
WHO will continue to support the ongoing investigation, risk assessment and response to the outbreak by national authorities.
WHO reiterates to all Member States the importance of achieving and maintaining more than 95% polio vaccination coverage in each district or municipality; maintaining high quality for the three main follow-up indicators: acute flaccid paralysis (AFP) rate, percentage of cases screened within 48 hours, and percentage of cases with a matched sample; optimizing supplementary (environmental and enteroviral) poliovirus surveillance and updating national poliovirus outbreak response plans to rapidly detect and respond to the importation of new viruses or the emergence of VDPV to minimize the consequences of poliovirus transmission and facilitate rapid response.
The thirty-second meeting of the IHR Polio Crisis Committee, held in June 2022 and 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 the interim Recommendation for the next three months.
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