Never before in the history of polio have so few children in so few countries contracted the crippling virus, but the international community cannot rest until the number of cases is zero, the UN Children's Fund (UNICEF) said here Friday on the eve of World Polio Day.

"Progress to end polio is real and dramatic, with now just two countries in the world where the wild poliovirus has never been interrupted: Afghanistan and Pakistan," said Peter Crowley, head of the Polio Unit at UNICEF. "But -- and it's a big but -- until all children everywhere are consistently and routinely immunized against polio, the threat remains."

"We cannot let down our guard; we have to keep going until there is not a single child anywhere who remains unvaccinated," Crowley said.

World Polio Day was established for annual observance on Oct. 24 by Rotary International more than a decade ago to commemorate the fight against poliomyelitis. Widespread use of poliovirus vaccine led to an increasing number of polio-free countries and to the establishment of the Global Polio Eradication Initiative (GPEI) in 1988.

As of Oct. 14, a total of 243 polio cases had been reported in 2014, with 92 percent of the cases reported from Nigeria, Afghanistan, and Pakistan, the only three countries where transmission of indigenous wild poliovirus has continued uninterrupted.

Nigeria, reservoir of more than half of all polio cases in the world three years ago, was removed from the list of polio endemic countries in September, the agency said.

Nigeria's remarkable achievement has brought the country and the African region closer than ever to being certified polio-free. In India, where thousands of children once suffered from polio-induced paralysis each year, there have been no cases in four years.

"Globally, there have been just 51 cases of wild polio virus since the beginning of 2015, compared with 242 wild polio cases for 2014," said UNICEF.

As part of its contribution to this Initiative, UNICEF delivered 1.7 billion doses of vaccine in 2014 and supported the training of tens of thousands of front-line workers in communities from Pakistan to Nigeria, helping to build trust in the vaccine among parents and communities.

Other success factors have been the integration of additional life-saving interventions for children such as routine immunization, nutrition, handwashing with soap, and breastfeeding, into polio campaigns, particularly in the most under-served and high-risk areas, the UN agency said.

Despite this progress, recent vaccine-derived poliovirus outbreaks in countries like Lao-PDR, Ukraine, Guinea and Madagascar have underscored the risks that many countries continue to face due to low routine immunization coverage.

These outbreaks serve as a reminder of the vital need for intensified efforts to strengthen routine immunization systems and address disparities in children's access to basic health services. In Ukraine, for example, fewer than 14 percent of children are immunized against polio.

"We aim to bring a global halt to polio transmission by this time next year, but the only way to do this is for countries with low vaccination dates to re-double their efforts to reach every child, wherever they are and no matter how hard this may be," said Crowley.

Polio is a highly infectious viral disease, which mainly affects young children. The virus is transmitted by person-to-person spread mainly through the faecal-oral route or, less frequently, by a common vehicle like contaminated water or food and multiplies in the intestine, from where it can invade the nervous system and can cause paralysis.

Initial symptoms of polio include fever, fatigue, headache, vomiting, stiffness in the neck, and pain in the limbs. In a small proportion of cases, the disease causes paralysis, which is often permanent. There is no cure for polio, it can only be prevented by immunization.