Polio, the world's great crippler of children, will survive into the next century. But if all goes well, it will also be the new millennium's first great casualty.
Over the next 12 months, an international effort will attempt to drive polio into the most exclusive category of human disease--those eradicated from Earth. The list has one item: smallpox.
The campaign against this disease, launched in 1988 and coordinated by the World Health Organization, is the largest public health endeavor ever undertaken.
Over the last decade, the campaign has temporarily stopped wars so that whole populations of children could be immunized simultaneously. Logistical feats, such as the vaccination of 134 million Indian children on one day in 1998, have no match in history. Counting part-time volunteers, about 10 million people worked in the campaign this year.
Because polio cases must be sought out and distinguished from other forms of paralysis, the campaign has required establishment of disease surveillance systems in some of the poorest and most chaotic nations on Earth. In all, the effort will cost about $2 billion. In inflation-adjusted dollars, this is about one-third more than the cost of the campaign to eradicate smallpox, which lasted from 1967 to 1977. More than half the money will be spent during the "accelerated phase" now beginning, and in the three years of intensive surveillance that will follow the last case.
Although the Americas have been free of the disease since 1991, no child here--or anywhere else--can safely forgo polio vaccination as long as the virus exists anywhere on Earth.
Polio Can Still Be Found in 23 Countries
So far this year, 1,240 confirmed cases of polio have been found worldwide, although the actual number may be two or three times that. In 1988, there were 350,000. Although disappearing fast, the disease still exists in 23 countries and is suspected to be circulating in six others, where conditions have hampered a thorough search.
The goal of stopping transmission of the virus by the end of 2000 will be "tough," said Harry Hull, a pediatrician and epidemiologist who is the senior advisor to the program. "Not impossibly tough, but tough."
Among the many obstacles to the final push toward eradication is the supply of oral polio vaccine. About 2.5 billion doses will be used in the campaign next year, about twice as much as has ever been used in a single year. Combined with the 300 million doses or so that will be used for "routine" polio vaccination, that is all there is in the world.
"The manufacturers are running now at maximum capacity. There's probably enough, but it's very tight," said Hans Everts, the campaign's chief of vaccine supply.
Although its most serious effects are on the nervous system, poliovirus is actually an "enterovirus," a microbe that replicates in the intestinal tract. It is passed in feces for weeks after infection, and also in airborne droplets. Highly contagious, the virus moves explosively through nonimmune populations, especially where hygiene is poor. The vast majority of cases occur in children, in whom it has historically been among the leading causes of disability.
Like a few other microbes, polio has the necessary characteristics for "eradicability."
The virus has no reservoir in the natural world. It doesn't live in soil (like the bacterium that causes tetanus) and it doesn't infect animals (like the virus that causes influenza). Halt transmission in human beings, and polio dies out.
Equally necessary is a safe and effective method of preventing the infection, which came in the form of the Salk injected vaccine, introduced in 1955, and Sabin oral form, two years later. (In the United States, the vaccines drove the number of cases of paralytic polio caused by "wild" virus from about 16,000 per year in the early 1950s to zero by 1980.) Vaccination causes prolonged immunity, so people don't become reinfected. Further, polio has no chronic "carriers"--people who survive infection but continue to transmit the microbe.
Still, polio has one trait that makes eradication unusually difficult: Only one in 200 cases cause weakness or paralysis, the cardinal feature distinguishing it from other intestinal bugs. Most polio infections go undetected. This is unlike smallpox, where nearly every case gave rise to a dramatic rash.
When epidemiologists find a newly paralyzed child, it's imperative they determine whether the cause is polio. (There are numerous non-pros causes of muscle weakness in childhood, with the autoimmune disorder known as Guillain-Barre syndrome the most common.) If it is polio, investigators can be certain many dozens of undetected cases lurk nearby.
Surveillance Systems Must Be in Place
Because of this, the eradication campaign has required more than vaccination. It also has required that countries create permanent systems of disease surveillance, which is not easy in poor, rural countries with little medical infrastructure.