Why do most polio vaccine-derived variants arise?
Most cases of paralytic polio are now vaccine-derived, thanks to the success of the Global Polio Eradication Initiative. OPV has played a key role in nearly eradicating wild poliovirus worldwide. However, in areas where vaccination coverage is low and enough people are susceptible to infection, weakened viruses can replicate. Unfortunately, each round of replication increases the likelihood that the virus will revert to the form that causes disease and paralysis.
Why was the old oral polio vaccine put on hold in 2016?
Since the eradication of poliovirus type 2 in 1999, the only cases of paralytic poliotype 2 have been vaccine-derived. Therefore, to prevent such cases, it was decided to switch from the original trivalent OPV containing all three poliovirus serotypes to a bivalent OPV vaccine containing only poliovirus strains types 1 and 3. If cases occur, an additional type 2-specific monovalent vaccine containing vaccine-derived type 2 will be provided.
Could it have been a mistake, as some experts have claimed?
The decision was made in good faith, but in retrospect it appears to have underestimated the level of vaccine-derived poliovirus type 2. The Global Polio Eradication Initiative commissioned a report on the decision, and a draft report released for public comment described the switch to bivalent OPV as a “complete failure.”
What type of polio vaccine is being used in the current campaign in the Gaza Strip?
More than 1.6 million doses of a new polio vaccine, novel oral polio vaccine type 2 (nOPV2), will be delivered to Gaza, providing two doses to more than 640,000 children under 10 years of age.
Will this vaccine have the same risks as the original poliovirus vaccine, meaning it could end up in wastewater and cause more polio cases?
No, nOPV2 is a next-generation version of the original single-valent oral polio vaccine type 2 that is used to combat vaccine-derived type 2 poliovirus outbreaks. The key difference is that the new vaccine contains a weakened virus that has been modified to be more genetically stable and significantly less likely to revert to the more virulent form that can cause paralysis, increasing the chances of stopping these outbreaks permanently.
What other diseases are likely to emerge in Gaza if the vaccination campaign is halted?
Other vaccine-preventable diseases, such as measles and pneumonia, and diarrheal diseases, such as rotavirus, are all possible, and each has its own risks and complications, so it is really important that as many vaccines as possible are available in Gaza.
