Why are most cases variants derived from the polio vaccine?
Most cases of paralytic polio are now vaccine-related, thanks to the success of the Global Polio Eradication Initiative. Oral polio vaccine has been instrumental in nearly eradicating wild polioviruses worldwide. However, in areas where vaccination rates decline and there are enough people susceptible to infection, the weakened virus can replicate. Unfortunately, each round of replication increases the chance that the virus will revert to a form that causes disease and paralysis.
Why was the old oral polio vaccine abandoned 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, it was decided to switch from the original trivalent OPV vaccine, which contained all three poliovirus serotypes, to a bivalent OPV vaccine, which contained only type 1 and type 3 poliovirus strains. In addition, a type 2-specific monovalent vaccine containing any vaccine-derived type 2 was available in case cases arose.
Was it a mistake, as some experts have suggested?
Although this decision was well-intentioned, hindsight suggests that the level of vaccine-derived poliovirus type 2 was underestimated. The Global Polio Eradication Initiative commissioned a report on this decision and the draft report, which is open for public comment, has described the switch to bivalent oral polio vaccine as a “absolute failure”.
What type of polio 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 shipped to the Gaza Strip to provide two doses to more than 640,000 children under 10 years of age.
Will it pose the same risks as the old polio vaccine? That is, could it end up in wastewater and cause more cases of polio?
No, nOPV2 is a next-generation version of the traditional monovalent oral polio vaccine type 2 used to respond to vaccine-derived poliovirus type 2 outbreaks. The key difference is that the new vaccine contains a weakened virus that has been modified to make it genetically more stable and significantly less likely to revert to a more virulent form capable of causing paralysis, 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, can also occur, each with its own dangers and complications. It is therefore very important that as many vaccines as possible are delivered to Gaza.