T

he world is in the process of trying something it has never attempted before. Over the next two weeks, 155 countries must stop using a vaccine that has been protecting children from paralyzing polioviruses for more than a half-century.

Designed in the 1950s, the vaccine has helped take the world to the edge of polio eradication. In the 1980s, polioviruses crippled 350,000 children annually; this year they have maimed 10 in the only two countries where polioviruses still spread, Pakistan and Afghanistan.

It has been a monumental achievement in public health.

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But for a while now, a component of the vaccine has caused more problems than it has solved, and has actually resulted in a relatively small number of cases of paralysis. So between this past Sunday and May 1, all countries that use the oral polio vaccine developed more than 60 years ago must stop administering the current formula and replace it with a new version.

The unprecedented synchronized campaign, over 18 months in the planning, is appropriately known as “the switch.”

Hundreds of thousands of health workers around the globe have been involved. Countries have had to develop implementation plans, train volunteers, and manage vast inventories of vaccines, including by ensuring that stocks of the old one are discarded.

“I’ve been working in immunization since 1974, and nothing like this has ever happened before,” said Dr. Walter Orenstein, a polio expert at Emory University in Atlanta.

If the planners’ assumptions are correct and the switch is executed as intended, the world’s children will be safer.

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’s 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 if they are unleashed by the unauthorized or unwitting use of stocks of the old vaccine — those children could become infected and crippled.

Mathematical modeling suggests there will likely 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 in a briefing 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.

Albert Sabin - polio vaccine
Dr. Albert Sabin, who developed a polio vaccine more than 60 years ago, examines a bottle containing isolated pure strains of polio virus in October 1956. Gene Smith/AP

A little background on polio and the vaccines that protect against it is useful to understand what’s 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’s 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.

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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, who consults on polio eradication for the Gates Foundation, said it would have been “desirable” if all countries had managed to give a round of injectable vaccine. But it was still believed the switch should go ahead.

“I think the feeling is that the risks of continuing to use the [old] vaccine with the potential of generating more outbreaks of circulating vaccine-derived polio [Type] 2 … outweighed the risk of not having every child born in the world getting [the injectable vaccine] for some period,” he said.

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.

Pakistan Polio
Pakistani health workers gives a polio vaccine to child in Islamabad. Muhammed Muheisen/AP

Some countries may take an innovative approach, using a special injection method to stretch supplies. Intradermal injections — injecting vaccine into the skin — is actually a more effective way of stimulating the immune system’s response to vaccine. A fraction of a dose induces as much protection as a full dose injected into the muscle, the way polio vaccine is normally injected.

India, which was one of the last countries to stop polio transmission, will be using this approach, said Orenstein, who noted two normal doses can be stretched to protect five children when administered intradermally.

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.

Zaffran said it is not expected that there will be much to throw away; national programs planned ahead to try to ensure they ordered only as much of the old vaccine as they would use before the switch took place.

In addition to fixing the Type 2 vaccine virus problem, the switch will serve as a dry run for further down the road, when all oral vaccine use must stop. Once it appears that polio transmission has ceased, the oral vaccine must be discontinued to ensure vaccine viruses also disappear.

The original target for eradication was 2000. It was missed, as were several targets that followed. But with cases of polio at never-before seen lows, optimism is high that the end of this very long fight is in sight.

“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.

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