https://qz.com/504830/new-outbreaks-...-caused-polio/
How do vaccine-derived outbreaks happen?
When kids are vaccinated with OPV, the weakened virus stays in their stomach for about two weeks, while the body develops antibodies for it. The virus is excreted through feces, and it can end up contaminating water bodies, especially in areas with poor sanitation, and get in contact with other children, particularly in crowded areas.
When immunization levels are high, that doesn’t matter, because the chances of the virus entering in contact with an unvaccinated kid are extremely low. When, on the other hand, the vaccination levels haven’t been high, the virus might be contracted by an unvaccinated child.
This doesn’t mean that the child will develop polio. However, if the virus spreads from an unvaccinated child to another, or to family members, and the transmission chain continues for long periods (usually at least 12 months, though it can take less), there is a very small chance that the attenuated virus could end up mutating back into an active form.
This is technically known as circulating vaccine-derived polio. It only happens in areas where the vaccination levels are very low to begin with.
How do we handle vaccine-derived outbreaks?
The good news is, vaccine-derived polio outbreaks are much easier to control than wild polio outbreaks, and represent a less serious threat. The immediate response should be to increase vaccination coverage, not scare people away from the vaccine.
In the longer term, once wild polio virus disappears from every corner of the world, OPV immunization can be replaced by IPV, which minimizes the risk of mutations.
Are there any other scenarios of polio infection?
Well, yes. In 0.0001% cases, a child given a first dose of OPV vaccine can develop paralysis. The risk is minimal, however, compared to the risk of not vaccinating children and allowing this highly contagious disease to spread unchecked.
Thanks to global vaccination efforts, since 2000, according to the WHO, over 10 billion doses of OPV have been administered to about 3 billion children. We are now close to a polio-free world. Once that goal is accomplished, there will be no more live virus (in the wild or weakened in the vaccine) and every possible chance of contracting polio will be eliminated.
Kinda close to what I was talking about...
