(CNN)Polio has been declared eradicated from Africa, the World Health Organization said Tuesday.
"Today we come together to rejoice over a historic public health success, the certification of wild poliovirus eradication in the African region," Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said during a livestreamed event.
"The end of wild polio in Africa is a great day," said Tedros, who is also the chair of the polio oversight board. "Your success is the success of the world. None of us could have done this alone."
Polio once was a common virus. In some young children it can affect the nerves and cause muscle weakness or paralysis. There is no treatment and no cure but getting vaccinated can prevent infection.
Governments and nonprofits have been working since 1996 to try to eradicate the virus from the African continent with sustained vaccination campaigns. Almost 9 billion polio vaccines have been delivered, Tedros said.
A large part of the eradication effort has been through the Global Polio Eradication Initiative (GPEI), which was created in 1988 and is led by national governments and five partners -- Rotary International, WHO, UNICEF, US Centers for Disease Control and Prevention and the Bill and Melinda Gates Foundation.
US Health and Human Services Secretary Alex Azar, US Centers for Disease Control and Prevention Director Dr. Robert Redfield and Bill and Melinda Gates Foundation co-chair Bill Gates congratulated Africa on the eradication of polio during the certification event for this milestone on Tuesday.
"Today is a historic day for global health, and it's a cause for celebration for everyone who works to improve Africans health," Azar said in a prerecorded message. "Congratulations to everyone who helped make it possible."
Officials wanted to make sure polio was really gone and have waited four years since the last case of wild virus was diagnosed.
Rose Gana Fomban Leke, who chairs African Regional Certification Commission for Polio eradication, told CNN that the decision to declare the region free of the virus came after many years of polio surveillance, immunization, and laboratory analysis of the region's 47 member states on the continent.
"The work has gone on for years and now we can see the results. It's such a huge milestone. I'm confident to say that in the region, for the last four years we haven't seen one poliovirus," she said.
According to a WHO statement, "The last case of wild poliovirus in the region was detected in 2016 in Nigeria. Since 1996, polio eradication efforts have prevented up to 1.8 million children from crippling life-long paralysis and saved approximately 180,000 lives."
This doesn't mean no child will develop polio symptoms. The weakened virus used to make the oral vaccine can sometimes survive in populations that are under-immunized, and if it circulates long enough, it can morph back into a dangerous form. It's called a circulating vaccine-derived poliovirus. WHO says 16 African countries are currently fighting outbreaks of this vaccine-derived strain.
"The small risk of circulating vaccine-derived poliovirus pales in significance to the tremendous public health benefits associated with the oral polio vaccine," WHO says on its website.
"Every year, hundreds of thousands of cases due to wild polio virus are prevented. Well over 10 million cases have been averted since large-scale administration of oral polio vaccine began 20 years ago."
That's why kids in developing countries get two doses of vaccine -- the oral vaccine, which is given as easy-to-administer drops, and a shot, which is made using a completely inactivated polio virus that cannot be re-activated. Children in developed countries only get the shot, but they need four doses to be completely protected.
"Circulating vaccine-derived polioviruses in the past have been rapidly stopped with 2--3 rounds of high-quality immunization campaigns. The solution is the same for all polio outbreaks: immunize every child several times with the oral vaccine to stop polio transmission, regardless of the origin of the virus," WHO said.
The last region to eradicate wild polio was Southeast Asia. Polio has been eradicated in the Americas, Europe, most of Australasia and now in Africa. Wild strains of polio only circulate now in two countries: Afghanistan and Pakistan.
"Wild poliovirus cases have decreased by over 99% since 1988, from an estimated 350,000 cases in more than 125 endemic countries then, to 175 reported cases in 2019," WHO said.
Bill Gates, founder of Microsoft and co-chair of the Bill and Melinda Gates Foundation, called it an "historic milestone, and a public health triumph for Africa."
But he cautioned that now, more than ever, it is important to protect the progress made. Wild polio is still present in Pakistan and Afghanistan, and until it is gone everywhere it can still return, he said.
The CDC's Redfield also highlighted that there is still work to be done. He said each country must continue to vaccinate to sustain high levels of protection against vaccination.
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
"This moment is Africa's to celebrate and to savor, and I want you to know that CDC stands with you today, until the day that polio is finally eradicated," Redfield said in a prerecorded interview.
"You have demonstrated how much can be achieved when determination, partnership and resources come together, no matter the circumstances," he said.
