March 04, 2011
Smallpox is the only human disease to ever be eradicated. The disease caused death in one third of its victims, and blindness and disfigurement in survivors. When WHO launched an intensified plan to eradicate smallpox in 1967, 60% of the world’s population was at risk, but by 1980, WHO had declared it officially eradicated. The nature of the disease allowed it to be eradicated relatively quickly, easily and cheaply. Because every case showed distinctive symptoms and because a victim did not become infectious until relatively long after contracting the disease, “ring vaccination” could be used to vaccinate only those local populations that were exposed to each case. Since there were no cases without symptoms, the parts of the population where no symptoms existed could be left unvaccinated with little risk. This made it possible to eliminate the disease from countries by only vaccinating a small portion of the population. It also helped that the vaccine did not require cold storage and was effective after one dose, that smallpox survivors have lifetime immunity and that there were no disease reservoirs for the disease –it can only be transferred person to person and does not exist in any other animals. Because it was so deadly but was eradicated so efficiently with a potent vaccine and resourceful planning, the vaccination campaign was extremely cost effective. The Center for Global Development determined that the United States “saves the total of all its contributions [to smallpox eradication efforts] every 26 days because it does not have to vaccinate or treat the disease.”
In contrast, the polio eradication effort has been underway since 1988, but has suffered numerous setbacks and has made slow progress in the past decade. The challenge for polio eradication is that most infected individuals do not show any symptoms, so extremely high proportions of children under the age of 5 must be vaccinated to protect against so-called “silent outbreaks.” Also, children typically need to receive the vaccine four times before they are protected, compared with just one dose of the smallpox vaccine. Any gaps in coverage will cause the eradication program to fail. However, the disease can certainly be eradicated with near universal vaccine administration: the oral polio vaccine confers lifelong immunity and the polio virus has no non-human reservoir, so, after human transmission ceases, the virus will not be able to exist on its own.
Polio is only endemic in four remaining countries — Afghanistan, India, Nigeria, and Pakistan. There were only 41 reported cases of polio in India in 2010. However, the number of yearly cases is up from its low point in 2001 primarily due to setbacks in Nigeria where the vaccine was refused by some communities for religious reasons and in India where vaccination levels also dropped.
While smallpox eradication has turned out to be incredibly cost-effective, some experts are questioning whether the resources required for polio eradication would be better spent elsewhere. The polio eradication effort “has now cost $9 billion, and each year consumes another $1 billion. By contrast, the 14-year drive to wipe out smallpox, according to Dr. Donald A. Henderson, the former World Health Organization officer who began it, cost only $500 million in today s dollars.” Due to the success of efforts so far, polio is no longer among the top causes of death and some experts are content to maintain perpetual control of the disease. The very last cases of a disease will always be the most expensive to prevent and certainly more expensive than preventing cases of numerous other deadly diseases that are not close to eradication, but the long-term payoff of eradication would be significant both in terms of costs saved from ceasing efforts to control the disease as well as the intangible effects of an enormous, collective public health achievement. Analyses have shown “substantial net benefits” to eradicating the disease relative to controlling it.
Guinea worm disease is the other serious candidate for global eradication. The Carter Center began the eradication campaign in 1986. There were only 1,797 cases reported in 2010 and with Ghana having likely eliminated the disease, it was announced that Guinea worm disease now only remains in three countries – Southern Sudan, Mali and Ethiopia.
Because its life cycle depends solely on living in and reproducing in humans as parasites (there is no animal reservoir for the disease), it could be the first disease to ever be eradicated with sanitation tools and behavior change alone (such as drinking cleaner water through filtered straws) without medicine or a vaccine.
Guinea worm meets all the criteria for eradication because it is biologically and technically feasible to eradicate and the benefits of eradication outweigh the costs. There’s no chance for the disease to return after the last human case, it’s easily diagnosable, it affects people at a predictable time each year and has been eliminated from other regions of the world. The Guinea worm eradication effort has had made steady progress with few setbacks and, now focused on just three countries, may be the most likely candidate to be the second ever successful eradication campaign.
“The last cases of any disease are the most challenging to wipe out” and there are those who are skeptical that the benefits merit the investment. But if the Guinea worm and polio eradication efforts succeed, there will be renewed confidence in the viability and value of disease eradication, and current debilitating diseases like lymphatic filariasis, measles, mumps, rubella, yaws, schistosomiasis and someday even malaria may all eventually be defeated by future campaigns.
Image credit: Flickr: Gates Foundation
