Health experts around the world are expressing cautious optimism about news out of Africa of the first-ever full year without a reported case of polio.

They point to rigorous approaches to surveillance and vaccination in countries such as Nigeria, where the disease had long been endemic, for the encouraging news of the first polio-free year in the continent's history.

August 11 marked the end of a full year since the last confirmed case of polio in Somalia, believed to be the final evidence of an outbreak that began in Nigeria.

However, the United Nations says despite the progress, the announcement cannot be considered "official" until laboratory results confirm no new cases in Nigeria, and surveillance by the World Health Organization African Region finds no further cases for a full two years.

If that happens, then the Africa Regional Certification Commission could declare the continent polio-free.

"Globally, we are on the verge of totally eradicating a disease for only the second time in history," Peter Crowley, the head of the UN Children Fund's (UNICEF) Polio unit, wrote in a blog.

Africa was also the last stronghold of the other disease he was referring to — smallpox — with the world's last case detected in Somalia in 1977.

"With Africa now on track, we are left with only two countries where polio transmission has never been interrupted: Pakistan and Afghanistan," Crowley continued wrote. "Here too, despite enormous challenges, communities, governments and partners are working with courage and determination to end polio once and for all."

A viral disease, polio can be transmitted from person to person, multiplying in the intestines and spreading from there to the nervous system.

 Symptoms can include fever, headache, fatigue, vomiting, a stiff neck and pain in the arms and legs. In a small proportion of cases, permanent paralysis can result.

Once diagnosed, there is no cure, which is why immunization is so important, experts say.

Despite the encouraging news out of Africa, vigilance is still required, says Dr. Hamid Jafari, director of the UN's Global Polio Eradication Initiative.

"This is a big success, but it's still fragile," he says. "There's always a worry that there could be an undetected case in a population you're not reaching."

In 1988, when the eradication initiative was begun, the virus led to paralysis of more than 350,000 children every year. At a cost of around $1 billion a year, that figure has been below 2,000 since 2001 and just 359 last year.

Just 34 cases have been confirmed this year, all in Afghanistan and Pakistan.