Starting in April 2016, 155 countries around the world have agreed to switch from one polio vaccine to another in what is being called the Global Polio Eradication Initiative (GPEI). Currently these countries use the oral trivalent polio vaccine (OPV), which protects against three types of the virus, but they will be transitioning over to the bivalent version, which blocks two types of polio. Currently the wild form of type 2 of the virus has been eradicated, but there are fears that the live, oral vaccine could mutate and cause reinfection, and switching to the bivalent version could help mitigate this risk.

Although the likelihood of this is exceedingly rare—according to the World Health Organization (WHO), billions of doses of the OPV have been administered between 2000 and 2015 and have resulted in fewer than 760 cases of the vaccine-derived poliovirus—this initiative will help lessen the risk. If successful, the switch will be “one of the most ambitious global synchronized projects in the history of vaccines,” according to the GPEI.

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