Polio could soon become the second human disease after smallpox to be completely wiped out worldwide after two of the three polio viruses were eradicated.

The three strains of immunologically identical but virologically distinct wild polio virus – WPV1, WPV2, WPV3 – cause irreversible paralysis in children.

In order to eradicate the disease, the circulation of the three strains of the virus needs to be completely wiped out.

While the WPV2 was eradicated in 2015, the last case of WPV3 was reported from Nigeria in 2012. After seven years of surveillance, an independent team of researchers from Global Polio Eradication Initiative (GPEI) announced the elimination of WPV3 on Thursday.

"The achievement of polio eradication will be a milestone for global health. Commitment from partners and countries, coupled with innovation, means of the three wild polio serotypes, only type one remains," said Dr. Tedros Adhanom Ghebreyesus, director-general of the World Health Organization.

The global polio elimination campaign that started in 1988 has faced a series of obstacles. The resistance of parents to get their children immunized against the disease became a significant challenge for policymakers. Ensuring a cold chain to maintain the efficacy of the oral polio drops in non-electrified villages came as another logistical issue.

But the most violent cases emerged from Pakistan and Afghanistan, where terror groups attacked polio volunteers. More than 70 polio workers have been killed in both countries, severely hampering polio eradication campaigns.

"This virus remains in circulation in just two countries: Afghanistan and Pakistan," said David Salisbury, chair of the independent Global Commission for the Certification of Poliomyelitis Eradication.

"We do have good news from Africa, no wild polio virus WPV1 has been detected anywhere on the continent since 2016 in the face of ever-improving surveillance," he added.

Eradication of the second wild polio virus comes as a relief to many governments, but the emergence of vaccine-derived polio has become a new challenge.

The virus used in the oral vaccine administered to children mutated after being passed through their stool, infecting unvaccinated children.