On 22 November 2021, the International Health Regulations national focal point (IHR NFP) for Yemen notified WHO of the detection of circulating vaccine-derived poliovirus type 2 (cVDPV2) in stool samples from two children with acute flaccid paralysis (AFP) in Yemen.
The first case, a nine-year-old girl, from Thubab district, Taiz governorate, south-western Yemen, experienced onset of paralysis on 30 August. The child had not been vaccinated against polio. Two stool samples were collected on 31 August and 2 September, and VDPV2 was confirmed on 22 November, with 10 nucleotide differences from the Sabin type 2 poliovirus vaccine strain.
The second case, a 26-month-old girl, from Marib district, Marib governorate, north-east of Sana’a city, experienced onset of paralysis on 1 September. The child had also not been vaccinated against polio. Two stool specimens were collected on 4 and 5 September, and VDPV2 was confirmed on 22 November with 11 nucleotide differences from the Sabin type 2 poliovirus vaccine strain.
The two districts do not share a border and are approximately 430 km apart. The two VDPV isolates are genetically linked, and therefore, classified as cVDPV2. This is a new emergence and the viruses detected are not related to other known cVDPV2s globally.
The confirmation of cVDPV2 comes in the context of an ongoing outbreak of circulating vaccine-derived poliovirus type 1 (cVDPV1), which has to date paralyzed 35 children (three in 2021, 31 in 2020, and one in 2019).
The last VDPV2 case in the country was reported in June 2016 in Aden governorate, southern Yemen, and was classified as ambiguousVDPV2 (aVDPV2). According to the WHO/UNICEF estimates of national immunization coverage, polio vaccine third dose (POL3) coverage was reported to be 66% in 2020.
Public health response
The Ministry of Health, supported by the Global Polio Eradication Initiative (GPEI) partners, conducted a field investigation, and started implementing the polio outbreak preparedness and response plan. In addition, AFP surveillance continues to be enhanced.
WHO risk assessment
The risk of further spread of this strain in Yemen is exacerbated by decreased immunization rates during the ongoing COVID-19 pandemic. In addition, the detected cases are from two governorates with accessibility and security issues, with high risk of population displacement to other governorates due to the recent security situation in Marib and Western Coast of Yemen which has led to significant levels of population movement, including to and from the Horn of Africa. The ongoing humanitarian crisis is a major risk factor to be considered for the spread of the disease.
Based on the above-mentioned gaps and challenges, WHO considers the risk of international spread and/or emergence of cVDPV2 to be high. Additionally, the existing cVDPV1 outbreak as well as the emergence of cVDPV2 highlight the gaps in routine immunization coverage and inadequate outbreak response.
The Emergency Committee under the IHR (2005) convened a meeting in November 2021 on the international spread of poliovirus in multiple countries. As per the report released, the Emergency Committee expressed concern at the continued rapid spread of cVDPV2 to many countries and noted that the risk of international spread of cVDPV2 is currently high.
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 Travel and Health recommends that all travelers to polio-affected areas be fully vaccinated against polio. Residents (and visitors for more than 4 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.
As per the advice of the Emergency Committee, the risk of international spread of poliovirus remains a Public Health Emergency of International Concern (PHEIC) and implementation of advice aimed at reducing this risk is strongly encouraged. 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 travel of individuals at the point of departure if not vaccinated or cannot prove their vaccination status.