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[NEW DELHI] The global fight to eliminate polio has narrowed down to a single strain of the wild polio virus and to two countries, says the World Health Organization (WHO).
 
On 24 October, WHO in Geneva announced that the world was now free of wild polio virus Type 3 and Type 2. That leaves the Type 1 strain now circulating only in Afghanistan and Pakistan, countries which reported 88 cases of polio this year (Pakistan accounted for 72), which is far higher than last year’s total of 33 cases.     

“We must eradicate the remaining wild polio virus Type 1 and also address circulating vaccine-derived poliovirus outbreaks, particularly in Africa”

Oliver Rosenbauer, Global Polio Eradication Initiative

The hike in the number of cases was said to be due to vaccine hesitancy and assaults on polio workers in conflict-hit areas that disrupted campaigns. In September, polio resurfaced in the Philippines after two decades, largely due to the interruption in vaccine delivery as withdrawal of a dengue vaccine set off rumours, resulting in loss of confidence in immunisation.   

All three wild polio virus strains are transmitted most often by under-five children and display the same symptoms — weakness, paralysis and, in severe cases, resulting in death. But because the three strains differ from each other genetically and virologically, it is necessary to eradicate all of them to rid the world of polio.

Wild polio virus Type 2 was certified eradicated in 2015, the last case having been detected in 1999 in India. The certification of Type 3 was made after assessing evidence that the strain was not detected across the globe for seven years, the last case having been detected in 2012 in Nigeria. 

Oliver Rosenbauer, spokesperson, Global Polio Eradication Initiative, WHO, tells SciDev.Net that the entire African region will be eligible to be certified polio-free by June 2020.

Some experts, however, say that the re-emergence of polio from the circulating vaccine-derived poliovirus strains continues to be a worry and that complete eradication may be some way off. The oral polio vaccine – which is used to combat wild polio virus – contains live but attenuated form (unable-to-cause-an-infection type) of wild polio virus strains. In rare occasions though, this vaccine virus undergoes a genetic mutation and again becomes infectious. This is referred to as vaccine-derived polio virus and is transmitted through the faecal-oral route.

Apart from Type 1 cases, incidences of circulating vaccine-derived poliovirus, mostly from the Type 2 polio virus antigen in oral polio vaccine (OPV) have surfaced, thanks to a rare reversion of the vaccine strain in severely immune-compromised individuals, according to Gavi, the Vaccine Alliance.

“As a critical step, the OPV Type 2 antigen was removed in April 2016 in a globally synchronised effort,” the Alliance said.   

“We must achieve final success or face the consequences of renewed global resurgence of this ancient scourge,” says Rosenbauer. “We must eradicate the remaining wild polio virus Type 1 and also address circulating vaccine-derived poliovirus outbreaks, particularly in Africa.”

Oyewale Tomori, a Nigeria-based virologist and former president of the Nigerian Academy of Science, tells SciDev.Net that improving surveillance, ensuring high quality immunisation and rapid response and containment of any outbreak of Type 1 virus is important.

“But eradication of poliomyelitis will be certified when there are no more cases of poliomyelitis caused by circulating vaccine-derived poliovirus,” he adds.    

This piece was produced by SciDev.Net’s Asia & Pacific desk.