WASHINGTON (AP) - A new global plan aims to end most cases of polio by late next year, and essentially eradicate the paralyzing disease by 2018 - if authorities can raise the $5.5 billion needed to do the work, health officials said Tuesday. Part of the challenge will be increasing security for vaccine workers who have come under attack in two of the hardest-hit countries. And the plan calls for changing how much of the world protects against polio, phasing out the long-used oral vaccine in favor of a pricier but safer shot version. Intense vaccination campaigns have dropped cases to a historic low, a good opportunity for what's being called the "endgame" strategy for this paralyzing disease, noted Dr. Rebecca Martin of the U.S. Centers for Disease Control and Prevention. "We have a chance now, with the fewest cases in the fewest places," she said, joining officials from the World Health Organization, Gates Foundation and Rotary International to discuss the strategy that will be finalized later this month. Officials acknowledged that financing the six-year plan upfront will be difficult given tight government budgets. The money would cover vaccinations as well as the monitoring required to be sure polio really is gone. Last year, authorities counted 223 cases of polio worldwide, down from 650 the year before. Once a worldwide scourge, vaccinations began in the 1950s, and were successful enough in developed countries that a global fight for eradication began in 1988. Now polio remains endemic in parts of just three countries - Pakistan, Afghanistan and Nigeria - where all but six of last year's cases occurred. Other countries remain at risk, as travelers can spread the infectious disease to the unvaccinated. Recent killings of vaccine workers in Pakistan and Nigeria have made the anti-polio campaign more difficult, but it hasn't stopped and it won't, said WHO polio director Dr. Hamid Jafari. Despite the attacks, those countries made progress against polio last year, he said. The new strategy says countries also must deal with a smaller but real risk: polio cases caused by the vaccine itself. The highly effective oral polio vaccine contains live, weakened virus that can occasionally regain strength and cause the very disease it was intended to prevent. A shot version of the polio vaccine is made of a killed virus that poses no such risk. The polio-free U.S. switched to the shots over a decade ago, but 144 countries still use the oral vaccine. The new strategy calls for them to begin phasing out oral vaccine in 2016.