A crippling strain of polio virus is no more. Officials confirmed Thursday that global health efforts have wiped it out, moving humanity one step closer to completely eradicating the highly infectious virus from the planet.

The obliterated strain—wild poliovirus type 3 (WPV3)—is one of only three wild strains of polio. It is the second to be globally eradicated. Health officials declared WPV2 eradicated in 2015. That leaves only one wild strain remaining: WPV1.

This “historic” announcement falls on World Polio Day and is based on the recent conclusion of the independent Global Commission for the Certification of Poliomyelitis Eradication, set up in part by the World Health Organization. The announcement comes after years of careful and painstaking global surveillance to certify that WPV3 no longer exists anywhere in the world, apart from specimens preserved in secure containment. The last known case of WPV3 occurred in northern Nigeria in 2012.

“Wild poliovirus type 3 is globally eradicated,” commission chair and vaccine expert David Salisbury declared at a celebration at WHO headquarters in Geneva, Switzerland. “This is a significant achievement that should reinvigorate the eradication process and provides motivation for the final step—the eradication of wild poliovirus type 1.”

That final step is not expected to be an easy one. Currently, WPV1 only circulates in two countries: Afghanistan and Pakistan. But vaccination efforts to stop it are plagued by vaccine misinformation and harmful rumors, armed conflicts and insecurity, and fears and misinformation about vaccine-derived polio infections, which are rare but a considerable threat.

Endgame

Polio viruses in general spread via the fecal-oral route, therefore thriving in areas with poor sanitation and low, to no, vaccination. The virus typically affects young children and can cause permanent paralysis and even death in a small number of those infected. Some people infected will have no symptoms, though, making surveillance particularly difficult.

Oral polio vaccines use weakened versions of the virus to train the immune system to fight the devastating disease. Thus, those vaccinated can harbor these weakened viruses in their guts for a short period of time, excreting them in waste. In under-immunized populations with poor sanitation, the weakened viruses can circulate and spread just like the three wild strains.

In rare instances when the weakened strains are able to circulate for extended periods of time (at least 12 months), they can mutate and collect dangerous traits from other circulating viruses, creating hybrid viruses that can, once again, cause paralysis. These cases are called circulating vaccine-derived poliovirus (cVDPV). In well-immunized populations, cVDPVs are not an issue.

In past years, there have been over two-dozen outbreaks of cVDPV in at least 21 countries. So far in 2019, there have been 94 cases of WPV1 (in Afghanistan and Pakistan) and 102 cases of cVDPV worldwide. Most cases of cVDPV occurred in Angola, the Central African Republic, the Democratic Republic of the Congo, and Nigeria.

The endgame for polio eradication is to continue supporting immunization programs worldwide and to switch to inactivated polio vaccine (IPV), which is given by an injection.

In a statement Thursday, WHO Director-General Tedros Adhanom Ghebreyesus said, “The achievement of polio eradication will be a milestone for global health... We remain fully committed to ensuring that all necessary resources are made available to eradicate all poliovirus strains. We urge all our other stakeholders and partners to also stay the course until final success is achieved.”

So far, only two infectious diseases have been globally eradicated: smallpox and rinderpest, a viral disease that affects cattle.