A MOVE CLOSER TO TOTAL DISAPPEARANCE OF POLIO

[And cases are declining sharply in both countries as the Pakistani military has expanded its power, fighting its way into Taliban-controlled areas where most of the vaccine resistance has been. When families are displaced, children are vaccinated at highway checkpoints and border crossings.]

By Donald G. Mcneil Jr.

Health workers gave polio vaccine to a child in Afghanistan in 2014. Credit Diego Ibarra Sanchez for The New York Times

Three years have passed since a case of Type 3 wild polio virus has been detected in the world, which means that particular viral subtype has most likely disappeared forever, the World Health Organization announced this month.

Its demise could speed up the drive to eliminate polio, which has gone on for 27 years and now costs more than $1 billion a year.

The last known Type 3 polio case was an 11-month-old boy in northern Nigeria who became paralyzed on Nov. 10, 2012.

When vaccines were first invented in the 1950s, there were three polio strains, which had nicknames. Type 1, by far the most common, was named Brunhilde, after a chimpanzee in the lab of the scientist leading the work.

Type 2 was Lansing, after the Michigan city where it was isolated from a dying patient. Type 3 was Leon, after a Los Angeles boy who died of it. The names later fell out of favor.

Type 2 has not been seen since 1999, so it is now likely that only Type 1 remains. If so, the fight against the disease may become more efficient.

In 2009, after experts waited a decade to be sure that Type 2 was gone forever, they began removing that strain from the trivalent oral vaccine, which works against all three types.

The three strains of weakened live virus in the oral vaccine compete with one another to attach to receptors in the gut. Removing one type of polio virus meant children developed full immunity after fewer doses.

Once experts are sure Type 3 is gone, they may decide to switch to a monovalent vaccine containing only Type 1.

But all types of polio may be eradicated even before that happens. Wild-type polio, caused by circulating viruses, is now found in only two countries, Pakistan and across the border in Afghanistan.

And cases are declining sharply in both countries as the Pakistani military has expanded its power, fighting its way into Taliban-controlled areas where most of the vaccine resistance has been. When families are displaced, children are vaccinated at highway checkpoints and border crossings.

As of Nov. 17, only 56 cases had been detected, far fewer than the 290 that had been found by the same date in 2014.

Wild-type cases may soon be outnumbered by vaccine-derived polio. Those cases occur when a weakened vaccine strain mutates enough to cause paralysis.

Seventeen cases in five other countries have been detected this year. Those outbreaks are usually stopped by giving all children in the region injections of killed vaccine and follow-up doses of the oral one.

@ The New York Times