For the first time, the number of children paralyzed by mutant strains of the polio vaccine are greater than the number of children paralyzed by polio itself. So far in 2017, there have been only six cases of wild" polio reported anywhere in the world. By "wild," public health officials mean the disease caused by polio virus found naturally in the environment. By contrast, there have been 21 cases of vaccine-derived polio this year. These cases look remarkably similar to regular polio. But laboratory tests show they're caused by remnants of the oral polio vaccine that have gotten loose in the environment, mutated and regained their ability to paralyze unvaccinated children. The oral polio vaccine used throughout most of the developing world contains a form of the virus that has been weakened in the laboratory. But it's still a live virus. (This is a different vaccine than the injectable one used in the U.S. and most developed countries. The injectable vaccine is far more expensive and does not contain live forms of the virus.) WHO is attempting to phase out the use of live oral polio vaccine to eliminate the risk that the active virus in the vaccine could mutate into a form that can harm unvaccinated children. But for now, the live vaccine continues to be the workhorse of the global polio eradication campaign. The killed vaccine doesn't fully block the virus from spreading because a person who is immunized can still carry and spread the polio virus.