Vaccine-derived Polioviruses: Managing the risks
What are vaccine-derived polioviruses and vaccine-associated paralytic poliomyelitis?
The oral poliovirus vaccine (OPV) contains a live, attenuated (weakened) vaccine-virus. When this vaccine is administered, the weakened vaccine-virus replicates in the intestine and enters into the bloodstream, triggering a protective immune response. However, during this replication process, some of the vaccine-virus may genetically mutate from the original attenuated strain and become neurovirulent (able to cause paralysis and circulate in communities). The neurovirulent virus is referred to as vaccine-derived poliovirus (VDPV). This is a very rare event. In the 10 years leading up to 2015 there were around 750 cases of paralysis caused by VDPVs reported worldwide.
There are three recognised categories of VDPV:
- Circulating VDPVs (cVDPVs)
- Immunodeficiency-associated VDPVs (iVDPVs)
- Ambiguous VDPVs (aVDPVs)
Vaccine-associated paralytic poliomyelitis (VAPP) is a rare event associated with OPV, which is caused by a strain of poliovirus that has genetically changed in the intestine from the original attenuated vaccine strain contained in OPV. It is a one-time case, with no risk of spread to others and occurs at a rate of approximately 2 to 4 events per 1 million births. 
VDPV and VAPP Research:
This area of research focuses on understanding and managing the long-term risks of polio relating to VDPVs and VAPP in the post-eradication era, including through the eventual cessation of OPV use in routine immunization programmes as soon as possible after certification of wild poliovirus eradication.
In the first instance, this involves more clearly characterizing the risks of all three types of VDPVs and VAPP.
cVDPVs – Evaluating cVDPV emergence
Environmental surveillance, whereby sewage is sampled for polioviruses in order to track the virus’ transmission through human populations, is being conducted in a number of countries. This area of research aims to further quantify the risk of circulating vaccine-derived poliovirus (cVDPV) emergence in countries that have recently switched from using OPV to IPV or from trivalent to bivalent OPV. Furthermore, these studies provide more evidence on the important role of environmental surveillance which enables us to address polio outbreaks quickly, and target campaigns accurately.
iVDPVs – Defining the risk of iVDPV
A series of studies are measuring the rate of long-term excretion of vaccine-derived poliovirus from immunodeficient people. The purpose of this research is to better define the risk of immunodeficiency-related vaccine-derived polioviruses (iVDPVs).
 It is anticipated that this rate will significantly decline, following the removal of the type 2 component in trivalent OPV in April 2016, which was responsible for approximately 40% of all VAPP cases.
Strategy for response to cVDPV2 2020 – 2021
The Global Polio Eradication Initiative (GPEI) has developed a comprehensive new strategy to stop the spread of type 2 circulating vaccine-derived poliovirus (cVDPV2) outbreaks. This strategy is an addendum to the Polio Endgame Strategy 2019-2023, released in April 2019.