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nd then there was one.

Three years ago Tuesday, an 11-month-old child in Yobe, Nigeria, was paralyzed by a polio virus. It was a type 3, one of a trio of strains of the virus that have been the targets of a 27-year long eradication struggle.

The day after the child in Yobe got sick, a type 3 virus was found in a sewage sample collected in Lagos. (Sewage testing is a cornerstone of polio surveillance.)

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Since then, there has not been a single detection of a type 3 virus anywhere. In the world of polio, that means type 3 polio is now probably eradicated — although the World Health Organization hasn’t yet officially said so.

“I think we are increasingly confident that it’s gone,” said Dr. Hamid Jafari, director of the WHO’s Global Polio Eradication Initiative.

The polio eradication partners — the WHO, the US Centers for Disease Control and Prevention, UNICEF, Rotary International, and the Bill and Melinda Gates Foundation — aren’t planning to seek a formal declaration at this point. A rigorous review of surveillance data from all of Africa has to be completed in order to decree that type 3 viruses are extinct, and that hasn’t yet been done.

But unofficially, the working assumption is that there is now only one remaining family of polioviruses in the eradication program’s crosshairs, type 1.

A missed goal

Type 2 viruses were last seen in 1999. However it wasn’t until September of this year that the Global Commission for the Certification of Poliomyelitis Eradication declared type 2 eradicated.

For years, the thinking had been there wouldn’t be a piecemeal process, said Dr. Walter Orenstein, associate director of the Emory Vaccine Centre at Emory University in Atlanta and a WHO polio vaccine adviser. The world would wait until transmission of all three types had halted before adding polio to the list of eradicated human diseases — which presently comprises only smallpox.

The original target date for eradicating polio was 2000, and in 1999 the world seemed to be closing in on that goal. But the target was missed, and after just 483 cases in 2001, polio roared back to nearly 2,000 cases the next year. It took another decade to get the annual case total down below 500 again.

Vaccine switch

As the battle dragged on, an unforeseen problem added a new wrinkle: The viruses in the oral polio vaccine, which can spread, were on rare occasions causing disease.

The United States uses an injectable polio vaccine that contains killed viruses. But the oral vaccine, used in many parts of the world, contains weakened, live polioviruses.

The vaccine viruses can spread, and if they mutate, can reacquire the ability to paralyze. Type 2 vaccine viruses are the best at doing that. So the polio program, on the advice of its vaccine experts, decided in October that the world should stop using oral vaccine containing type 2 protection and should switch to a vaccine containing only types 1 and 3.

That unprecedented event, dubbed “the switch,” will take place over a two-week period next April.

Declaring type 2 viruses eradicated was part of the preparation for the switch, explained David Salisbury, the European representative on the Global Commission for the Certification of Poliomyelitis Eradication.

Salisbury said it’s not clear at this point whether a similar process will be undertaken for type 3 viruses.

Toward full eradication

The current target date for full eradication of polio is 2019, which would require transmission of polio to stop by the end of next year. If that goal appears to be on track, there would be no point in going to the trouble of declaring type 3 viruses eradicated. Type 1 and type 3 could be handled in a single declaration.

But if the process drags on — and that has been the history of the polio eradication effort — then a type 3 declaration may be considered. “So it’s all in the hands of Pakistan and Afghanistan,” said Salisbury.

Those two countries are the only places on the globe where wild polioviruses (as opposed to the vaccine viruses) continue to circulate. So far this year there have been 38 children in Pakistan and 13 in Afghanistan paralyzed by polio.

Those figures are promising; last year at this point there had been 277 cases. But Pakistan in particular has been a tough nut to crack, with people in parts of the country actively resisting vaccination efforts. Since 2012, more than 70 polio vaccinators or police officers hired to protect them have been killed on duty.

So the job isn’t over. But this week, people involved in the extraordinary effort to rid the world of polio are quietly celebrating. “It’s another step. It’s another … piece that we can tick off,” Salisbury said.

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