Beginning in late April, 155 countries will switch to a new version of the polio vaccine. The switch, which will take place over the course of two weeks, will go from the trivalent (tOPV) version of the vaccine to the bivalent (bOPV) version, which protects against the two remaining wild polio strains. The maneuver will involve moving millions of doses of the new drug into nations across the world while also sequestering what remains of the old vaccine. If all goes to plan, the bOPV will help protect children against the wild strains of the disease while lessening their chances of contracting vaccine-induced polio, which happened on rare occasions with the tOPV.

“This is the largest, the fastest, and [the most] unique event that is taking place,” Dr. Michel Zaffran, the director of polio eradication at the World Health Organization (WHO), said. “This is an unprecedented event that has never been done before in the world.”

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