Circulating vaccine-derived poliovirus type 2 (VDPV2) Key messages

April 2017

  • Following confirmation in March of a circulating vaccine-derived poliovirus type 2 (VDPV2) in Somalia, the virus has been detected in an environmental sample in Nairobi, Kenya.   
  • No cases of polio have been discovered; no children have been reported paralyzed. Poliovirus has been isolated from environmental samples (sewage) in Mogadishu, Somalia, and Nairobi, Kenya, which indicates circulation and a risk to children. 
  • Response activities are already underway. Neighbouring countries have been alerted to strengthen their surveillance for poliovirus and assess their immunity.


  • In Somalia, three cVDPV2s were isolated from environmental samples collected 4 and 11 January 2018, from Hamarweyn district in Banadir province (Mogadishu).  These latest isolates are genetically linked to previously isolated VDPV2s from 2017, collected on 22 October and 2 November 2017 from environmental samples collected from Waberi district, Banadir province. 
  • In Kenya, one cVDPV2 was isolated from an environmental sample collected on 21 March 2018 from Nairobi, linked to the cVDPV2 previously confirmed in Somalia.
  • Since detection of the initial isolates in 2017, outbreak response campaigns in line with internationally-agreed guidelines have been implemented, consisting of three large-scale immunization activities (SIAs) in Banadir, Lower Shabelle and Middle Shabelle provinces in Somalia, with further campaigns planned for April and May.
  • With detection of the virus in Nairobi, indicating a regional event, the possibility of a region-wide response is now being evaluated.  The exact scale and extent of the response is being finalized.
  • WHO and its partners are continuing to support local public health authorities in conducting field investigations and risk assessments to more clearly assess any potential risk of circulation of the identified cVDPV2, and to continue to support the outbreak response and strengthening of disease surveillance.
  • Health personnel at all levels are undertaking efforts to strengthen surveillance for acute flaccid paralysis (AFP) cases, including by conducting active case searches. The frequency of sampling from the environment is being increased.
  • Somalia and Kenya conducted their last immunization campaigns with trivalent OPV in early 2016, in advance of the trivalent to bivalent OPV switch in April 2016. Searches for residual trivalent OPV stocks are ongoing including public, private and NGOs. No tOPV stocks have been found.
  • Neighbouring countries across the Horn of Africa, including in Yemen, have been alerted, and public health authorities are assessing overall immunity levels and strengthen disease surveillance in those countries.

About vaccine-derived poliovirus (VDPV)

  • A VDPV is a rare strain of poliovirus, genetically changed from its original strain contained in oral polio vaccine (OPV). VDPVs can only occur when population immunity is very low and a population is left susceptible to poliovirus, whether from vaccine-derived or wild poliovirus. In a population with low immunity, the vaccine virus shed by a child who has been given OPV can pass between children who have not received the vaccine, and can mutate along the way. In seriously under-immunized populations, the virus can continue to mutate for a long period of time, and regain its ability to paralyse.
  • Hence, the problem is not with the vaccine itself, but low vaccination coverage. If a population is fully immunized, they will be protected against both vaccine-derived and wild polioviruses.
  • The benefits of OPV far outweigh the extremely low risk of a VDPV. OPV has reduced the global incidence of polio by >99% since 1988, from 350,000 cases every year in more than 125 endemic countries, to 8 cases in 2018.

For further information, please contact:

Mr Oliver Rosenbauer
Communications Officer, Global Polio Eradication Initiative
World Health Organization (WHO) Geneva
Tel: +41 (0)79 500 6536
Email:  rosenbauero [at]