Outbreaks of Circulating Vaccine-Derived Poliovirus type 2 are popping up in a number of countries since the switch to a vaccine that only attacks type two polio in April of 2016. Some of these outbreaks are spreading over more than one country. As a comparison, in the three years before the switch there were 8 cVDPV2 outbreaks in five countries. Since the switch, there have been 47 cVDPV2 outbreaks in 20 countries. While outbreaks were anticipated, what was not anticipated was the number and scale of these outbreaks, some of which have proven very difficult to stop.
Circulating Vaccine Derived cases are exactly what the label indicates, that in countries, particularly in Africa where there is a growing cadre of children without immunity to polio from infrequent and inadequate vaccination programs, it is rare but possible, to find cases of Circulating Vaccine Derived polio.
This happens because the oral vaccine used in immunization programs is made from the live poliovirus. It is the same vaccine discovered by Albert Sabin in 1964. In very rare cases, where campaign quality is poor and not enough children are reached with the vaccine, we run a risk of the live virus occasionally morphing back into a form virulent enough to cause the acute flaccid paralysis that marks the normal polio patient who caught it in the wild.
The problem is that the only vaccine we have today is the Sabin live-virus vaccine called monovalent Oral Poliovirus Vaccine2 or mOPV2.
Understand that many cVDPV2 outbreaks in the past have been stopped using the mOPV2. Because the occurrence of cVDPV2 outbreaks, while serious, is comparatively rare, it is currently the only tool available.
To better address the evolving risk of cVDPV2, the GPEI partners are working to deploy an additional innovative tool – novel Oral Polio Vaccine type 2 (nOPV2). The vaccine is a modified version of the existing mOPV2, which clinical trials have shown provides comparable protection against poliovirus while being more genetically stable and less likely to revert into a form which can cause paralysis in low immunity settings. The novel vaccine’s increased genetic stability means there is also a reduced risk of seeding new cVDPV2 outbreaks, as is currently experienced with the mOPV2. nOPV2 is being considered for deployment under WHO’s Emergency Use Listing procedure (EUL) to enable rapid field availability. The mOPV2 vaccine will continue to be used until a new and more genetically stable oral polio vaccine, known as nOPV2, which is currently under clinical development, is available.
Accelerating the development and roll-out of a new vaccine to be less likely to seed outbreaks is the current strategy to eradicating the circulating Virus threat in under-immunized populations.
Clinical trials for the nOPV2 are underway. COVID-19 has had an effect on the development program for the novel vaccine, but development, approval and roll-out of this vaccine remain a top priority. Works remain on track to have 200 million doses available by the end of 2020.
