After Jonas Salk found a vaccine in 1953, fears of polio began to ebb in the developed world; a global eradication effort began in 1988, and the virus was gone from the Americas in 1991. By 1999, it remained only in a handful of countries in Central and South Asia and Africa.
Before the eradication campaign, India saw as many as 50,000 to 100,000 cases a year, but by 2005, it was down to just 66. But as the number waned, so did vigilance. The virus resurged the following year with 676 cases - many of them Indians who migrated, either within or without the country, taking the virus back into areas where it had long been eradicated, or abroad.
Microbiologists can "fingerprint" a virus can tell, based on its genetic makeup, where it came from - right down to the neighbourhood.
So we know that a 2007 outbreak in Angola, which had been polio-free, came from India. Then the virus spilled over the Angolan border into the Democratic Republic of Congo and then Congo-Brazzaville - two countries from which polio had been chased out, but where weak public-health systems and low vaccination coverage made it difficult to contain the new epidemics.
Meanwhile, two reservoirs of wild polio virus survived here: one in western Uttar Pradesh, the other in central Bihar. These two states in northern India are among the poorest and most populated. Some 500,000 new babies are born in Uttar Pradesh every single month, yet less than half of these are born in medical facilities where they might get the vaccine. In Bihar, the virus lurks in the Kosi River floodplains - a teeming area where people live with little no access to basic sanitation services or public health.
Dr. Jafari sends teams of vaccinators on motorbikes into the river delta, where they heave the bikes onto small boats to move between communities and then trek from each village eight or 10 kilometres out to a barsa, where people keep children in rough lean-tos to help stake a claim to land recently flooded with fertile silt. All the way, they lug ice chests with the vaccine, which quickly breaks down at room temperature. "There are more than a million children under the age of five on the Kosi River embankment alone," he said.
Teams infiltrated the most inaccessible areas, and repeatedly vaccinated more than 90 per of children. In most places, that is enough to quash polio, but not in Kosi or Uttar Pradesh. "Here you need at least 95 per cent - in an area with extremely high population density and zero public services," Dr. Aylward said. "It's like having to run the 100 meters two seconds faster than anyone else in the Olympics."
No one knows why the vaccine works only half as well in these areas. The extremely high levels of diarrhea in children? The profusion of other bugs in their gut? But they needed a new plan.
Polio comes in three distinct strains. The vaccine used in most of the world targets all three. In northern India, that vaccine wiped out Type 2, but didn't work as well on 1 or 3. So the polio campaign decided to revert to type-specific vaccines used in the early days of immunization. But whenever they drove Type 1 back, 3 flared up; when they quashed 3, then 1 came back. Dr. Jafari likened it to a vicious game of Ping-Pong.
This called for a new vaccine, one that would work against Types 1 and 3 simultaneously, without, in lay terms, cancelling each other out.
Normally the production of a new vaccine would be years in the making. But the WHO teamed up with India's Medical Research Council and local pharmaceutical firms to design a vaccine that worked on Types 1 and 3 - and they were using it on children in just six months. "The commitment of the government of India and the state governments is incredible - India is the only country that funds more than 80 per cent of its polio program," Dr. Jafari said.