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two-week campaign to phase out a polio vaccine that is now considered harmful to the effort to eradicate the disease appears to have been completed, polio program leaders at the World Health Organization said Monday.

There had been worries that a few countries — China and Russia among them — might not be able to get the job done by May 1. But all 155 nations involved in the unprecedented operation known as “the switch” have complied, director Michel Zaffran told STAT in an interview from Geneva.

“I believe we can be quite confident that all countries withdrew the vaccine by yesterday,” he said.

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About three-quarters of the countries involved in the switch had completed the work by the end of last week. Roughly 35 countries made the shift over the weekend, said Diana Chang Blanc, who was coordinating the campaign for the WHO.

The WHO has sent monitors to countries involved in the operation to verify that any remaining stocks of the phased-out vaccine have been withdrawn from circulation and collected for destruction, Zaffran said.

The switch was designed to shift countries that use oral polio vaccine from a version that contains protection against three types of polio to one that protects against only two.

It was done in a coordinated fashion — the largest withdrawal of a vaccine ever — to minimize the risks created by the move, which will leave some children in the world without protection against one strain of polio for a period of time.

The United States was not involved in the switch. It stopped using oral polio vaccine altogether in 1999. US children are immunized with the injectable polio vaccine designed by Jonas Salk in the 1950s.

The switch was needed to address a risk associated with the oral vaccine, still used widely in low- and middle-income countries because it costs pennies a dose and is simple to administer.

The oral vaccine, designed by Albert Sabin, is made with live but weakened polioviruses. Children who swallow the vaccine drops excrete those vaccine viruses in their stools for a period of time after vaccination. Like regular polioviruses, the vaccine viruses can circulate from child to child in places where sanitation is poor.

As they spread, the vaccine viruses can regain the ability to paralyze. That happens rarely — and was a risk the partners in the Global Polio Eradication Initiative felt was acceptable back in the days when tens and even hundreds of thousands of children a year were being paralyzed by so-called wild polioviruses.

But the risk-benefit ratio has shifted for one strain of polio, type 2. That strain of polioviruses has been declared eradicated; no type 2 viruses have been seen since 1999.

Type 2 vaccine viruses are the ones most likely to regain the ability to paralyze. So with no risk of infection from wild type 2 viruses, it was decided the type 2 component had to be removed from the vaccine.

Last year there were 74 children paralyzed by wild polioviruses in the only two remaining countries — Pakistan and Afghanistan — where circulation of the viruses has never been interrupted. So far this year, there have been 12 cases reported — eight in Pakistan and four in Afghanistan.

But last year, 32 children were paralyzed by vaccine-derived polioviruses. And so far this year, three such cases have been reported.

The Global Polio Eradication Initiative originally included the WHO, the Centers for Disease Control and Prevention, UNICEF, and the service club Rotary International; the Bill and Melinda Gates Foundation joined in the last decade.

For polio eradication to be successful, all transmission of both wild viruses and the viruses from the oral vaccine must cease.

Eventually, use of all oral vaccine will have to stop. The polio program has been viewing this campaign as a test run for the final withdrawal of the oral vaccine.

After 28 years and more than $14 billion, the polio finish line appears tantalizingly close — closer than it’s ever been. Experts hope that polio transmission will stop sometime this year and the virus will be declared eradicated in 2019.

 

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An earlier version of this story incorrectly stated that the polio eradication campaign has cost more than $11 billion. It has cost more than $14 billion.

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