Why are most cases of polio vaccine-derived variants?
Most cases of paralytic polio are now vaccine-derived due to the success of the Global Polio Eradication Initiative. OPV has been instrumental in the near-eradication of wild polioviruses around the world. However, in areas where vaccination rates drop and enough people are susceptible to infection, the weakened virus can replicate. Unfortunately, each round of replication increases the potential for the virus to revert to a form that causes illness and paralysis.
Why was the old oral polio vaccine shelved in 2016?
Following the eradication of type 2 poliovirus in 1999, the only cases of type 2 paralytic polio were vaccine derived. Therefore, to stop these cases, there was a decision to shift from the original trivalent OPV, which contained all three poliovirus serotypes, to a bivalent OPV vaccine, which only contained type 1 and type 3 poliovirus strains. With an additional type 2 specific monovalent vaccine available to contain any vaccine-derived type 2 should any cases arise.
Was that a mistake, as some experts have suggested?
Although this decision was well intentioned, hindsight suggests that the level of vaccine-derived type 2 poliovirus was underestimated. The Global Polio Eradication Initiative commissioned a report into this decision and the draft report, which is open for public comment, has described the switch to bivalent OPV as an “unqualified failure”.
What type of poliovirus vaccine is being used in the current campaign in Gaza?
More than 1.6 million doses of the novel oral poliovirus vaccine type 2 (nOPV2) – a new poliovirus vaccine—will be delivered to the Gaza Strip to provide two doses to more than 640,000 children under the age of 10.
Will it have the same risks as the old poliovirus vaccine? That is, might it get in the wastewater and cause more polio cases?
No, the nOPV2 is a next-generation version of the traditional type 2 monovalent oral polio vaccine that is used to respond to vaccine-derived type 2 poliovirus outbreaks. The key difference is that the new vaccine contains a weakened virus that has been modified to make it more genetically stable and significantly less likely to revert to a more virulent form capable of causing paralysis, thereby increasing the chances of stopping these outbreaks for good.
What other diseases are likely to emerge in Gaza, given the interrupted vaccination campaigns?
Other vaccine-preventable diseases, such as measles and pneumonia, as well as diarrheal diseases, such as rotavirus, all have the potential to emerge, each with its own dangers and complications. Therefore it is really important that as many vaccines as possible are delivered into Gaza.