India, ravaged by polio like no other place on the planet, has seen only a single case this year, back in January. Although the global polio eradication effort is neither celebrating nor relenting, it may have already succeeded in eliminating polio from India.

New detection of polio in India "would not be a surprise," cautions Oliver Rosenbauer, communications officer at World Health Organization, "and in fact, everyone is operating on the assumption that the likelihood of residual transmission still occurring somewhere is not unsubstantial."

However, the greatest risk for such transmission may have passed. Historically, polio cases have peaked in August, as annual rains from about June to September spread sewage- and virus-contaminated water, driving a corresponding high season of polio transmission. Even though patterns vary from year to year, and there were only a handful of cases last year, the peak still came in August.

Epidemiology also points toward elimination. The ultimate sources of polio in India, the states of Bihar and Uttar Pradesh, have been cut off. Once rife with more viruses than anywhere in the world, these neighboring states haven’t produced any cases within their borders for well over a year. Massive and increasingly clever vaccination efforts now reach more than 98 percent of children in Uttar Pradesh and over 95 percent in Bihar. In most places, 90 percent coverage is the goal. Pakistan, where polio is spreading, manages only 50 percent (cases are concentrated near the Afghan border, so risk of cross-border spread remains low).

The blanketing coverage in Bihar and Uttar Pradesh produces high levels of immunity against the remaining strains of polio. In the most recent survey, over 98 percent of children show antibodies against type 1 of the virus and 77 percent against type 3. With polio crushed at its source, cases across the country plunged from 741 in 2009 to just 42 in 2010 to just one in 2011, a type 1 case. Anywhere else, six months of no type 1 paralysis would mean that that strain had been eliminated. The last positive type 1 test occurred 10 months ago.

For type 3, the situation is a bit more complex. Polio is primarily detected via paralysis, and only about one in 2,000 type 3 infections result in paralysis (it’s one in 200 for type 1). The type 3 virus last struck in India 10 months ago, and the rough rule of thumb is that you need 12 months since the last infection to consider it eliminated.

However, India has clearly pushed the type 3 virus to the brink. There were six outbreaks in 2009; all were completely knocked out. In 2010, it took cases in June and July in one part of India to feed circulation elsewhere, leading to the final type 3 case of the year in October. But in 2011, there were no type 3 cases to feed further circulation. The type 3 virus also hasn’t been seen for more than 14 months in sewage samples taken from high risk areas.

"This is a virus which has a knack for finding susceptible groups," cautions Rosenbauer. When to give an all clear sign is unavoidably arbitrary. "[E]veryone is keeping their eye on the end of the high season at the end of the year," according to Rosenbauer. This would mark 12 months since the last case in 2010 and also align with the calendar year. Even then, "the risk will not approach zero," says Rosenbauer.

With India being a highly populous subcontinent, "gaps in detection are possible, even though [it] has extremely sensitive disease surveillance," according to Apoorva Mallya, a program officer at the Bill & Melinda Gates Foundation. "Although surveillance in Uttar Pradesh and Bihar is of amazing quality, there are [other] parts of the country where theoretically transmission could persist undetected based on current surveillance levels." Mallya knows of no recent examples of surprise "orphan" cases, seemingly coming out of nowhere. Still, based on previously established guidelines, WHO won’t certify that the disease is gone until three years from the last case, January 13, 2013, if all goes well.

The seeming absence of clear, serious threats to success is almost a source of anxiety to an eradication effort that has battled polio in India for decades, always losing. Could it now really be gone? It might be.