Why are most polio cases variants from the polio vaccine?
Most cases of paralytic polio are now vaccine-related due to the success of the Global Polio Eradication Initiative. OPV has been instrumental in nearly eliminating wild polioviruses worldwide. However, in areas where vaccination rates are degenerating and enough people are susceptible to infection, the weakened virus can replicate. Unfortunately, each round of replication increases the likelihood that the virus will revert to a form that causes disease and paralysis.
Why was the senior oral polio vaccine withdrawn in 2016?
After the eradication of poliovirus type 2 in 1999, the only cases of paralytic polio type 2 were vaccine-related. Therefore, to contain these cases, the decision was made to switch from the original trivalent OPV vaccine, which contained all three serotypes of poliovirus, to a bivalent OPV vaccine, which contained only type 1 and type 3 poliovirus strains. With an additional monovalent type 2-specific vaccine that could contain any type 2 from the vaccine, should any cases occur.
Was this a mistake, as some experts suggest?
Although this decision was well-intended, in retrospect it appears that the level of vaccine-derived poliovirus type 2 was underestimated. The Global Polio Eradication Initiative commissioned a report on the decision, and the draft report, which is open for public comment, describes the switch to bivalent OPV as “failure is possible”.
What type of polio vaccine is currently being used in the Gaza campaign?
More than 1.6 million doses of the modern oral polio vaccine type 2 (nOPV2) – the modern polio 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 senior polio vaccine? That is, could it get into sewage and cause more cases of polio?
No, nOPV2 is a next-generation version of the established monovalent oral polio vaccine type 2 that is used to respond to vaccine-induced outbreaks of polio virus type 2. The key difference is that the modern vaccine contains a weakened virus that has been modified to make it more genetically stable and much less likely to revert to a more virulent form capable of causing paralysis, thereby increasing the chances of permanently stopping these outbreaks.
What other diseases could 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, have the potential to emerge, each with its own risks and complications. That is why it is really essential that as many vaccines as possible are delivered to Gaza.