THE WORLD witnessed only 223 polio cases last year, the lowest level in history and an impressive advance from the hundreds of thousands of children afflicted as recently as the 1980s. However, the eradication quest is not over, and the next steps look difficult. The Global Polio Eradication Initiative, an umbrella group, has unveiled a promising strategy over the next five years to reach zero cases, the elusive goal first set a quarter-century ago.

The polio virus is highly contagious, can spread rapidly and remains endemic in Pakistan, Afghanistan and Nigeria. In Pakistan, 15 health workers in the anti-polio campaign have been killed since July, and a World Health Organization official estimated that 240,000 children have missed vaccinations in the tribal areas because of security concerns. In Nigeria, nine workers have been killedthis year. In both countries, the workers were killed by militants who hold wrongheaded notions that vaccinations are some kind of Western plot. In fact, the danger is not in the vaccination but in the virus, which affects the nervous system and can lead to paralysis, largely among children age 5 and younger. Any effort to conquer polio is going to have to fully protect those carrying out the vaccination campaigns.

Wiping out a disease is an extraordinary accomplishment; it has been done only once before, with smallpox. Yet as polio case numbers plunge, the task of complete eradication grows harder. The oral polio vaccine, which contains a live, weakened virus, has been key to the dramatic progress in reducing cases. But on very rare occasions, the virus strain in the oral vaccine has reverted to a paralytic one and begun to circulate. Earlier, it was a tolerable risk, but now it must be stopped. The new global strategy involves a switch to an inactivated vaccine that can attack the circulating virus. This requires a shot, not just an oral drop. While the oral dose costs about 20 cents apiece, the inactivated one costs a dollar or more. The switch to the inactivated virus is an important firewall against a further outbreak, but it won’t be easy and must go hand in hand with strong, routine immunization to prevent the virus from returning.

The five-year strategy is estimated to cost $5.5 billion from varied sources, according to the eradication initiative, a partnership spearheaded by the WHO, Rotary International, the Centers for Disease Control and Prevention and UNICEF with support from the Bill & Melinda Gates Foundation. Instead of trying to raise money year to year, the new strategy calls for gathering up the total at the outset. This will be difficult in an era of budget austerity, but there have been some promising new private sources of funds, including $100 million recently pledged by New York Mayor and publishing magnate Michael Bloomberg.

Hopes for wiping out polio globally have been raised by the achievement of India, now free of the virus. But success is not assured and demands a fight to the finish.