Nigeria, Democratic Republic of Congo, Central African Republic, and Angola have experienced nine new cases of polio caused by the live virus in oral polio vaccines that has mutated into an infectious form, according to statistics released last week (November 20) by the World Health Organization. That brings the global total of these types of infections to 157 for the year, and it means that more children are paralyzed as a result of such vaccine-derived infections than illnesses caused by the wildtype virus, which has affected 107 people this year.

Other countries in Africa and Asia have also reported such vaccine-derived infections, which have the potential to spark new outbreaks. In Africa alone, there are currently a dozen vaccine-derived polio outbreaks, and another was declared in the Philippines last month—the country’s first cases of the disease in more than 25 years, NPR reports.

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To finally eliminate the world of polio, global leaders convened last week (November 19) at the Reaching the Last Mile (RLM) Forum in Abu Dhabi, pledging $2.6 billion to the effort. The main impediment is vaccination coverage in certain regions, particularly Afghanistan and Pakistan, the last two countries where polio remains to be eradicated.

While Western countries use an injectable solution of inactivated virus, an oral polio vaccine containing the live, attenuated virus is used for vaccination campaigns in Africa and Asia because it is relatively cheap to produce and easy to administer, requiring just two drops of medicine in the mouth. However, the risk is that the attenuated live virus—in particular, type 2, which is at the root of all current vaccine-derived polio cases, the Associated Press reports—can mutate and become pathogenic. Fortunately, vaccination can protect against such vaccine-derived strains. “The solution is the same for all polio outbreaks: immunize every child several times with the oral vaccine to stop polio transmission, regardless of the origin of the virus,” the WHO states. 

“It’s actually crazy because we’re vaccinating now against the vaccine in most parts of the world,” Vincent Racaniello, a virologist at Columbia University, tells NPR, “not against wild polio, which is confined to Pakistan and Afghanistan.”

Starting in April 2016, public health care workers around the world have made the transition from a trivalent vaccine with types 1, 2, and 3 to a bivalent version without type 2 to prevent such vaccine-derived cases.

Jef Akst is managing editor of The Scientist. Email her at jakst@the-scientist.com

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