MONDAY, Oct. 24, 2016 (HealthDay News) -- Polio is almost a thing of the past, but it still exists in small pockets on the planet, U.S. health officials reported Monday.

In 1988, a global effort to eradicate polio, a disease that has crippled millions of children worldwide, began. Since then, the number of cases dropped from 350,000 to just 27 this year, according to the U.S. Centers for Disease Control and Prevention.

"We are on the brink of the eradication of polio -- we are closer than ever," CDC Director Dr. Tom Frieden said during a media briefing.

"In this period [1988-2016], 2.5 billion children have been vaccinated against polio," he said. "If it were not for this effort, an estimated 15 million more children would be disabled. Every year polio eradication is delayed, the incremental cost is about $800 million."

The battle to eradicate the disease, however, continues in areas where it is still endemic, officials added.

"The new cases in Nigeria highlight the need to improve tracking of the disease," Frieden said. "We have to redouble our efforts to get over the finish line in Pakistan and Afghanistan. Despite big obstacles, both countries are making substantial progress."

Vaccinating children in some parts of the world, such as Pakistan, Afghanistan and Nigeria, can be a dangerous task. According to published reports, workers in these areas trying to vaccinate children have been killed by extremists who believe vaccinations sterilize children or that workers are Western spies.

The Global Polio Eradication Initiative is made up of five groups: the CDC, Rotary International, the World Health Organization (WHO), United Nations Children's Fund (UNICEF) and the Bill and Melinda Gates Foundation, Frieden said.

"We will get to a day when polio is history," Frieden said.

Finally eradicating polio is going to cost $1.5 billion, John Germ, president of Rotary International, said during the media briefing.

"If we don't get the funding, polio is going to spread again, and it's going to cost us billions of dollars and hundreds of thousands of lives a year for the children that we must protect against this virus," he said. "It can cost us a dream of a polio-free world."

No cure for polio exists -- it can only be prevented. The polio vaccine can protect a child for life, health officials said.

Despite the progress seen since 1988, "as long as a single child remains infected with poliovirus, children in all countries are at risk of contracting the disease," according to WHO. The virus can easily be imported into a polio-free country and can spread rapidly among unvaccinated populations.

"Failure to eradicate polio could result in as many as 200,000 new cases every year, within 10 years, all over the world," WHO says.

"Polio is almost defeated," Reza Hossaini, director of polio eradication at UNICEF, said during the briefing. But, "the recent cases in Nigeria remind us that almost is not good enough."

More information

Visit the U.S. Centers for Disease Control and Prevention for more on polio.