Questions and answers: Circulating vaccine-derived poliovirus type 2 (VDPV2)

Did this virus spread from Somalia into Kenya?

Investigations are currently ongoing to determine the source and origin of this virus, where it initially emerged and where it moved to.  However, it is clear that two countries in the Horn of Africa are currently affected, and the overriding priority is to stop this virus in the known affected areas and prevent it from spreading further.

How can an outbreak response be warranted even if there are no detected cases?

Poliovirus has been isolated from environmental samples only; no associated cases of paralysis have been detected at this time. Genetic evidence confirms that this virus is circulating, and even if no cases have been detected, represents a public health threat requiring a response.

Is this the beginning of a new outbreak similar in scale as the Syria outbreak?

The cVDPV2 outbreak in Syria and the cVDPV2 event in the Horn of Africa are different by nature. In Syria, virus has been detected in human cases showing symptoms of paralysis, whereas in the Horn of Africa, virus has been detected in the environment alone. However, similarities in conditions exist such as the inaccessibility of children for vaccination over extended periods of time, high levels of population movement in and out of the infected region/s, and protracted conflict and insecurity. It is precisely to prevent an outbreak on a scale like in Syria that a response is being implemented in Somalia, Kenya and potentially other areas of the Horn of Africa, even though virus has been detected in the environment only at this time.

How much of a factor will insecurity be in the response?

Insecurity always complicates both immunization response and the ability to conduct disease surveillance.  However, much has been learned from the 2013-14 outbreak of wild poliovirus in Somalia and the successfully implemented outbreak response, and the same measures will now again be implemented. 

Direct conflict has disrupted a number of campaigns in Somalia over the past few years, however, successful campaigns have been carried out despite these conflicts.

We work with any group whose aim is to provide for the health of the community.

Will the virus spread to other parts of the Horn of Africa, including Yemen which is already affected by a humanitarian crisis?

There is a high risk of spread of the virus in Somalia and Kenya and into neighbouring countries, especially those where access to immunization has been compromised. Given this risk, a swift and targeted immunization response is needed. Unhindered access to affected populations will be essential..

Do the vaccine viruses detected predate the Switch?

Genetic sequencing of the isolated virus of type-2 vaccine-derived poliovirus indicates 38 nucleotides divergence from the Sabin 2 vaccine virus. The viruses are not linked to any previously-detected VDPV2s, suggesting this virus has been circulating in the area undetected for a period of at least two years. As the global trivalent-bivalent oral polio vaccine switch took place in April 2016, it is likely that the viruses detected predate the switch.

Is GPEI concerned that the use of mOPV2 in response to this situation risks seeding future cVDPV in the Horn of Africa?

The goal of any immunization response using mOPV2 is to achieve high levels of vaccination coverage to stop the outbreak. As cVDPVs can only emerge in under-immunised populations, achieving sufficient vaccination coverage to stop an outbreak should also be sufficient coverage to prevent new cVDPV from emerging.

The decision to use mOPV2 in any outbreak response is strictly guided by an advisory committee, which carefully evaluates the risks and benefits of using mOPV2. In addition, mOPV2 can only be released for use under the authority of the WHO Director-General, operating on the recommendation of the advisory committee.

There are risks of using mOPV2 in challenging areas such as Banadir region. But the risks posed by the current situation far outweigh the risk of potentially seeding cVDPV in the future.

Why have you not detected cases of polio? Does this mean you have missed cases?

Key surveillance indicators are currently meeting international standards at national and subnational levels in Somalia and Kenya, reflecting a functioning surveillance system. Public health authorities are carrying out a thorough investigation of surveillance activities to determine any gaps. Undetected cases however, cannot be ruled out. The overriding priority now is to rapidly implement a response, to protect children from this virus. The secondary priority is to determine more clearly how this virus circulated for an extended period of time without detection.

At the same time, health personnel at all levels (Ministry of Health, WHO and UNICEF) 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, and other supplementary surveillance activities are being carried out. 

Polio is considered a Public Health Emergency of International Concern (PHEIC), as it is a disease targeted for eradication...

Polio is considered a Public Health Emergency of International Concern (PHEIC), as it is a disease targeted for eradication. Type 2 poliovirus, it could be argued, is a pathogen that has already been eradicated (particularly after the switch from trivalent OPV to bivalent OPV). What are the global implications of the re-emergence of a strain which has already been eradicated?

The emergence of cVDPV2s following the switch is to be anticipated, particularly in the 12-18 months after the switch. Global response mechanisms are in place to rapidly respond to such events, including through use of a global stockpile of mOPV2 and new outbreak response SOPs which came into effect on 1 May 2016 (after the switch), and which clearly outline the mechanisms for responding to detected VDPV2s after the switch.

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]