As the world moves "closer than ever to ending polio," more than 150 countries are taking part in what the Global Polio Eradication Initiative calls the "largest and fastest" global rollout of a routine vaccine, NPR reports. The endeavor, which entails swapping out an older oral polio vaccine, began on Sunday, reports the New York Times, and officials hope it wraps up by May 1. Why the switch? The old vaccine protected against three strains of polio (types 1 through 3), which, per the BBC, can cause paralysis (as was the case with FDR). The new vaccine covers two strains (types 1 and 3). Type 2 polio was eradicated in 1999, per the Times. And, as oral polio vaccines are made with live, weakened strains of the virus, the weakened strain can mutate and become dangerous. That has happened most often with type 2 polio.
"So removing type 2 from the vaccine takes away that risk," a CDC doctor tells the BBC, "and ensures we have a vaccine which will work better dose by dose." And for the switch to accomplish that, global buy-in is needed. The changeover will be monitored by thousands of people in the 155 participating countries, which include Afghanistan and Pakistan (the only two countries that are still polio-endemic) and more developed countries, such as Russia and Mexico. One potential hiccup, per the Times, is the fact that the new boxes and vials of vaccine are almost identical. According to reports, millions of vials of old vaccine will need to be destroyed by incineration, boiling, or being entombed in concrete and buried, among other methods. However, a WHO official put the number at a "few vials, not trucks full of vaccine." (Refusal to vaccinate is behind the resurgence of measles, study says.)