As we celebrate the near elimination of the polio virus throughout the world, the reports reflect another, often overlooked, number called the cVDPV which stands for “circulating Vaccine‐Derived Polio Virus”. In 2021, the year that only 4 new Type 1 cases were identified, 637 new cVDPV cases were found.
So what are these cases and why are they not considered to be included when the World Health Organization certifies a country to be polio free? To understand this class of polio cases, we have to look at how polio immunization mechanism works. The original Salk vaccine, which is injected, uses killed virus cells to train the child’s immune system to recognize and destroy future polio virus cells. The Sabin oral vaccine, in contrast, uses live polio virus cells to achieve the same result through a much different mechanism.
Once the drops are given to the child they grow and multiply in the gut for about 6 weeks (or longer if the
child is malnourished) until the immune system learns how to overcome the invaders and proceeds to kill them. During this learning period, the child in effect becomes a walking polio factory and can spread polio via fecal eliminations.
In areas where workers can immunize a large percentage of the kids, this is not a problem. Siblings and other kids coming in contact each other are also vaccinated and can ward off the virus. In areas where immunizations are spotty and some kids are getting their drops while others are not that the unwanted spread happens and cVDPV cases proliferate.
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