The world's largest, most intractable source of polio may be on the brink of elimination. In India the states of Bihar and Uttar Pradesh have produced more polio cases this decade—nearly 5,000—than any other location worldwide that has an active immunization campaign. Nigeria saw a handful more cases than the two Indian states because it effectively ceased immunizing in 2003 for a time due to false fears of the vaccine.

Now, even at the peak of polio season, new cases in Bihar, Uttar Pradesh and indeed all of India hover near zero—unprecedented, historic lows. In the past decade the peak months of August and September have seen an average of roughly 140 people, usually children, stricken by poliomyelitis, which attacks motor neurons in the nervous system and can cause paralysis. But for the past four weeks running, Bihar and Uttar Pradesh have hung zeros on the polio scoreboard, according to reports published as recently as October 28 by the Global Polio Eradication Initiative.

Data source: AFP Surveillance Bulletin—India Report for week 41, ending 16 October 2010; (pdf)

Members of the Initiative, a public-private partnership among agencies including the World Health Organization, U.S. Centers for Disease Control, UNICEF and Rotary International, are leery of drawing attention to the trend because it could change. "[W]e are cautious," says Sona Bari, communications officer for polio at WHO. The CDC's Steve Wassilak adds, "I would rather celebrate after the fact than before," although "we can be allowed to think that we are close."

Polio was supposed to be eradicated worldwide by 2000, per a goal set by the World Health Assembly in 1988. In fact, polio cases have been reduced by 99 percent globally from about 350,000 in 1988; sustained transmission of the virus has been eliminated from all but a handful of countries. But transmission has been uninterrupted in India, Nigeria, Afghanistan and Pakistan. Of these, India has posed the greatest scientific and logistical obstacles. Where cases are highest, in Bihar and Uttar Pradesh, efficacy of the oral vaccine has been the lowest: seven, eight or even more doses can fail to elicit immunity. Researchers conjecture that the failure owes to low levels of hygiene and immune systems taxed by multiple gastrointestinal challenges. After 2005 a trivalent vaccine, aimed at types 1, 2 and 3 of the virus, was largely replaced by monovalent and bivalent formulations that targeted types 1 and 3 separately or together, significantly improving effectiveness. Type 2 has been globally eradicated. A paper (pdf) published October 26 in The Lancet showed the bivalent vaccine is more effective than the trivalent and as effective as the monovalents.

From a logistics perspective, Uttar Pradesh and Bihar welcome more than five million newborns every year, each of whom requires multiple doses of vaccine. This enormous immunization challenge is intensified by extreme poverty, scant health infrastructure, large and frequent migration and, finally, massive annual flooding in central Bihar that renders at-risk populations very difficult to reach.

"India has been the toughest nut to crack," CDC's Wassilak says, adding that eradication efforts there have been "well executed overall, but success in all places at once has been elusive, since execution required near-perfection." Apoorva Mallya, a program officer at the Bill & Melinda Gates Foundation, described the latest hopeful statistics in India as "a special occurrence." He attributes the success not to any single factor but to exertions across the board, a host of incremental improvements yielding a qualitative shift.

Neither global eradication nor even eliminating polio from India is now a foregone conclusion. Sewage samples in Delhi continue to test positive for poliovirus, meaning that transmission is still occurring, although no polio cases have resulted. An outbreak is possible without continued vaccination efforts. Elsewhere in Asia, eradication in Pakistan has been set back by massive floods, and cases are up from last year; vaccination coverage there was already hampered by corruption. War conditions in Afghanistan severely crimp vaccination efforts, although the virus has been successfully evicted from Somalia and some other nations while they were in a state of conflict.

In Africa, Nigeria, second only to India in cumulative polio cases, looks good so far this season, having also reached a historic low of just eight cases versus 381 at this time last year. An upcoming election, however, could bring political instability and jeopardize the vaccination program. In Angola, transmission has become re-established and cases have surged because the current government has failed to prioritize anti-polio efforts.

Countries without polio remain vulnerable to outbreaks unless vaccination levels remain high. Tajikistan suffered a large outbreak earlier this year, affecting 458 people; Uganda declared an outbreak on October 27. Mop-up vaccination campaigns are highly effective in halting such occurrences, but they are expensive and challenge an already stretched eradication program.

Even if the polio case count were dropped to zero worldwide, the eradication effort would still need to wean itself from the live oral vaccine, because it is capable of mutating into virulent form, as it has in the past.

The Gates Foundation recently elevated polio eradication to the top of its agenda. Concerning whether total elimination for India is in the offing, "Ten months from now," when high season for polio returns, Mallya says, "we'll know."