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And thrive: the Nigerian case load grew from 202 new victims in 2002 to 355 in 2003, then jumped to 792 in 2004. And although vaccinations resumed last summer, by then it was too late to put the genie back in the bottle. Cases of polio genetically consistent with the Nigerian strain had begun popping up, in succession, in more than 10 neighboring countries, including Chad, Cameroon, Central African Republic, Côte d'Ivoire and Sudan. Last November the same virus appeared in Saudi Arabia, two months before the hajj, when 2 million Muslims from around the world descended on Mecca and then returned to their home countries, perhaps carrying more than just their memories with them. Investigators are still looking into the possibility that the outbreak last week in Indonesia was linked to the pilgrimage. Only a 2003 case in Lebanon is unconnected to either the hajj or Nigeria.
Can the spread be reversed? For now, the numbers remain relatively tiny. As of last week, the global new-case count for 2005 was still only 124. Even combined with last year's Nigerian totals, that's microscopic, epidemiologically speaking, in a world in which more than 1 million people die each year of malaria and 3 million die of AIDS. But big contagions start small. What's more, only 1 in 200 cases of polio actually causes paralysis, with the rest simply leading to fever, flu-like symptoms or no apparent illness at all. That means that for every child with paralytic polio, 199 may be carrying--and spreading--the virus. "This is a disease that can't be controlled," says Rosenbauer. "It has to be eradicated."
Indonesia is trying. With the help of WHO, Rotary and the other groups, 5 million at-risk kids will be vaccinated in the next few weeks. The immunization rate in Indonesia is already high--90% or more--but in places like Cidadap, it's less than 50%. "The challenge is to respond quickly and make sure we get vaccine to these low-coverage areas," says Robert Keegan, deputy director of the CDC's global immunization division.
The other countries in which polio has re-emerged are getting intensive attention too. In Nigeria there are six nationwide rounds of vaccinations scheduled for 2005. In other countries, such as Yemen, Egypt and India, the immunization program is getting a boost from a so-called monovalent vaccine, which more effectively knocks out the Type 1 poliovirus circulating in those areas. But even the best immunization campaign will leave a lot of poliovirus at large, at least for a while. WHO and other groups still hope to eradicate the disease this year in India, Pakistan and Afghanistan, where polio was endemic before the current outbreaks and good vaccination campaigns were in place. The other affected countries will take longer, giving the virus more time to spread elsewhere--including the U.S.
For now, U.S. officials claim not to be worried about the risk of a domestic outbreak. "Our coverage rates are at all-time highs," says Keegan. "The chance of an epidemic in the U.S. is very low."