The switch does not come without risks. There are three strains of polioviruses. The new vaccine will not have a component that protects against Type 2. For the most part, that is not a problem — Type 2 polioviruses haven’t been seen since 1999 and have been declared eradicated.  

But after the switch, infants in some parts of the world won’t have any protection against Type 2 polio. The old vaccine includes live-but-weakened polioviruses; if there are any Type 2 vaccine viruses left in the environment — or unleashed by the unauthorized or unwitting use of stocks of the old vaccine — those children could become infected and crippled. 

Mathematical modeling suggests there probably will be at least one outbreak caused by Type 2 vaccine viruses after the switch, Dr. Stephen Cochi, a polio expert at the Centers for Disease Control and Prevention, told reporters last week. The Global Polio Eradication Initiative, a public-private partnership that includes the CDC and the World Health Organization, has prepared for that possibility, stockpiling vaccines that could be used to control an outbreak if the need arises. 

A little background on polio and the vaccines that protect against it is useful to understand what is at work here. 

Back in 1988, the WHO, CDC, UNICEF, and the service club Rotary International embarked on an ambitious plan to get rid of polio. (The Bill and Melinda Gates Foundation joined the polio eradication partnership in the last decade.) At the time, there were three strains of the virus circulating.

Since then, Type 2 polioviruses have been declared eradicated. Type 3 viruses haven’t been spotted in more than three years and are also thought to be gone. 

The eradication toolbox contained — then and now — only two tools: an injectable vaccine, which is made with killed viruses, and the oral vaccine, developed by Albert Sabin. 

Each has strengths and weaknesses. The injectable vaccine, which is used in the United States and most affluent countries, is safer; it does not paralyze. But it is more expensive and because it is injected, it must be administered by a health professional. The oral vaccine costs pennies a dose, and anyone with a few minutes of training can squeeze the drops into a child’s open mouth. 

Children who get the oral vaccine excrete those live-but-weakened vaccine viruses for a time when they have bowel movements. In places where sanitation is rudimentary, that initially was beneficial. Vaccinate some kids in a community and others will eventually also be protected as the vaccine viruses spread.

But as vaccine viruses move from one child’s gut to the next, they evolve and can regain the ability to paralyze. When polio was crippling hundreds of thousands of children a year, the oral vaccine’s advantages far exceeded its negatives. But as the polio math has shifted, so too have the scales on which the vaccine’s risks and benefits are weighed. 

Last year there were 74 children in the world paralyzed by polio. But 37 children were paralyzed by vaccine-derived polioviruses. Over 90 percent of those cases in recent years have been caused by the Type 2 component of the oral vaccine. But given that there is no longer any risk from Type 2 polioviruses, that vaccine component is now more of a detriment than a benefit. 

Polio eradication campaigners decided that in order to stop Type 2 vaccine virus cases from cropping up, that part of the vaccine had to go. 

When the idea for the switch was proposed, experts who advise the WHO recommended a prerequisite for making the change. Before countries that use oral vaccine move to the version without Type 2, they should give all their children at least one dose of the injectable vaccine, to try to minimize the dangers inherent in the process. 

That hasn’t happened, admitted Michel Zaffran, director of the WHO’s polio eradication team. Vaccine manufacturers weren’t able to scale up production sufficiently to make enough of the injectable vaccine. At this point, about 30 countries haven’t given children doses of the injectable vaccine; 20 won’t get the needed supplies until some time in 2017. “This is one piece that has not worked as effectively as we would have hoped,” Zaffran said. “Nonetheless the progress has been quite remarkable.” 

Orenstein said it would have been “desirable” if all countries had managed to give a round of injectable vaccine. But it was still felt the switch should go ahead. 

But an expert committee, which met last week, expressed concern about the potential impact of the vaccine shortfall. 

The available injectable vaccine supplies have been directed to countries felt to be at the highest risk of having vaccine virus outbreaks. Those that have been told they’ll have to wait are countries where vaccination rates are known to be high — meaning many children are already protected against the Type 2 vaccine viruses. When vaccine coverage is high, vaccine derived viruses are less likely to circulate.

Over the two weeks of the switch, thousands of program monitors will be checking hospitals, clinics, and other venues where oral polio vaccine is given to ensure any remaining supplies of the old vaccine are removed from the supply chain and rounded up for safe disposal. 

In addition to fixing the Type 2 vaccine virus problem, the switch will serve as a dry run for farther down the road, when all oral vaccine use must stop. “We are very close to eradicating polio. We have never been this close,” said Zaffran. “And we believe that we will be interrupting transmission in 2016.” 

That would open the door to a declaration of eradication in 2019, after a three-year monitoring period needed to ensure the virus is actually history.