Will Polio Make a Comeback ? The world has spent billions of dollars in vaccination efforts to eradicate this virus over the past 15 years, with encouraging results. Incidence has plummeted from 350,000 cases in 1988 (estimated) to a few dozen cases in 2016. But in recent years, the incidence of polio has begun to rebound. The reason has to do with the types of vaccines used in some parts of the world, mainly low- and middle-income countries. While the U.S. and other Western countries have vaccinated against an inactivated virus that poses no risk of transmission and have eliminated polio, some countries rely on oral vaccines. Oral vaccines are inexpensive and easy to administer, and two or more doses of the vaccine can provide lifelong immunity. But it’s made with a live, weakened virus. This brings up a problem.
Those who have been vaccinated with live virus shed the virus in their feces, and in places with poor sanitation, the virus can be spread through sewage. If the virus remains weakened, it could even expose people who have not been vaccinated against polio to the virus and confer immunity. But if it mutates and regains its virulence, unvaccinated people could become infected with vaccine-induced polio after exposure to contaminated wastewater. Now in parts of Europe, the Middle East and Asia, new, vaccine-caused polio outbreaks are emerging in countries that had previously eradicated polio.
Polio experts say one reason for the rise in cases is that gaps in immunization in recent years have created more chances for the unvaccinated to become infected. “The vaccination movement has definitely been affected by the outbreak,” said Raul Andino, a virologist at the University of California, San Francisco. The composition of the oral vaccine is also a factor. In 2016, global health officials changed the composition of the oral vaccine after seeing an increase in polio from the vaccine. The vaccine previously protected against all three types of wild polio – viruses that spread naturally in the environment. They then removed one of the types—the type that caused most vaccine-derived cases but whose wild type had been successfully eradicated. But the development of the situation was unexpected. This type of vaccine-derived poliovirus is still spread through earlier versions of oral vaccines, and now with reconfigured vaccines, more and more people are no longer immune to this type of virus. This further intensified the spread.
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