VAPP or vaccine-associated paralytic polio are cases of polio that are actually caused by the polio vaccine.
Very rarely, the attenuated (weakened) virus in the oral polio vaccine can revert to a form that can cause the person who was vaccinated or their close contact to actually develop polio.
How rarely? VAPP only occurs in about 1 in every 1.27 million children receiving their first dose of OPV.
The vaccine’s benefit outweighs the risk of VAPP while polio is endemic (lots of cases) in a region, after all, without the vaccine, hundreds of thousands of children would get polio and would be paralyzed.
In polio-free countries, the risk of VAPP becomes greater than the risk of polio and they move to the inactivated polio vaccine. That helps prevent a situation in which the polio vaccines actually causes more cases of polio than wild type polio viruses.
Eventually, all countries will move to the IPV vaccine as we move closer to polio eradication.
For more information:
- Oral polio vaccine (OPV) cessation
- Replacing trivalent OPV with bivalent OPV
- Polio Eradication and Endgame Strategic Plan 2013–2018