In the next two weeks, 155 countries are switching to a new polio vaccine and destroying the old vaccine version that’s been protecting children from the poliovirus for over 60 years. The vaccine change has never been attempted before, but is deemed a critical step in succeeding at total eradication of the poliovirus.
"We're closer than ever to ending polio worldwide, which is why we are able to move forward with the largest and fastest globally synchronized switch ever," said Michel Zaffran, Director of Polio Eradication at the World Health Organization (WHO).
But why is the switch necessary? The old oral polio vaccine (tOPV) was used as part of the World Health Organization’s polio eradication program beginning in 1988, and polio cases significantly diminished from the hundreds of thousands to 74 cases in 2015. Given this successful streak, why not continue vaccination with the old vaccine? Why is it necessary to mess with a complicated and confusing vaccine switch that could result in error?
The reasoning for the switch lies in the components that made the vaccine so successful in the first place: live viruses. The tOPV contains attenuated, or weakened, forms of three polioviruses – Type 1, 2, and 3. The attenuated vaccines work by mobilizing the immune system to resist the poliovirus, but in some rare cases the vaccines can become virulent and cause harm.
While the benefits of vaccinations have always significantly outweighed its risks, the polio vaccine is at a delicate crossroad, as it contains the weakened form of Type 2 poliovirus, which hasn’t been seen in the wild since 1999. Thus, to reduce the instance of vaccine-derived Type 2 polio outbreaks, a new type of vaccine must be used to exclude the Type 2 poliovirus. And that’s exactly the difference between the old vaccine (tOPV) and the new vaccine (bOPV).
"We have to stop using OPV [oral polio vaccine] as soon as possible. As long as we're using it, we're introducing reverted strains into the environment," said Vincent Racaniello, a Columbia professor of microbiology and immunology. The ultimate goal is to eradicate polio so completely that vaccines will no longer be necessary. But we still have many steps before that point.
As outlined by the WHO, the first step is the current switch from the old polio vaccine to new ones that don’t contain Type 2 poliovirus. The WHO is also closely assessing whether the oral vaccine, which contains live attenuated virus, can be replaced with a single vaccine injection, which would contain an inactivated form of the virus. Their goal is to have oral vaccines completely phased out by 2020, to be replaced by vaccine injections only. The idea is to reduce polio incidence so dramatically that the safer, but more costly and difficult to administer injections will be sufficient.
Currently, 155 countries are engaged in an unprecedented undertaking to switch out the polio vaccines. They have until May 1 to complete the transition, and experts are monitoring the situation intensely, as improper procedures could lead to the renewed spread of Type 2 poliovirus. But in the unlikely case of such event, the WHO is holding a global stockpile of type 2 vaccines for deployment.