Smallpox was successfully eradicated because it fit all the requirements for an eradicable disease: It’s only transmitted person-to-person, the distinctive rash makes diagnosis easy, and the vaccine against it worked incredibly well.
“In addition, there was a great understanding of the disease burden itself,” Goodson says. “Smallpox was horrific. There was lot of death.” The political circumstances were beneficial, as well. At the end of World War II, a number of global organizations formed, which created the opportunity for a global infrastructure under the WHO, he says.
After the eradication of smallpox, efforts turned to polio. The number of cases worldwide has dropped by 99 percent since the late 1980s, and 80 percent of the world’s population, including the United States, now live in regions free of the disease. In 2018, there were only 33 cases reported worldwide. .
But getting the number of cases from a handful to zero comes with its own set of challenges, says Stephen Blount, director of Special Health Projects at the Carter Center, a human rights and public health nonprofit organization started by former president Jimmy Carter.
When the number of cases gets this small, he says, political issues become increasingly important. “It takes more time and energy and effort to find the last one, two, or 20 cases than [to] find them when there are hundreds.” People who make decisions around resource allocation might also see getting rid of the last handful of cases as a low priority, as there are always other conditions having greater impact.
However, Blount says, the last remnants of a disease won’t go away on their own. “It’ll almost certainly start to get bigger if you discontinue the human effort to drive cases down.”
Once a disease is eradicated, there’s no longer a need for a public health infrastructure to try and beat back the progress of a virus: If it’s not around, there’s no need for anyone to be vaccinated. But until the number of disease cases hits zero, researchers must invest similar effort and resources to keep the condition from spreading.
Measles is a prime example of the importance of constant management. Even though the United States’ successful elimination means that the disease is extremely uncommon, it’s important to remember that elimination is not eradication. Right now, maintaining high vaccination rates is the only way to prevent it from creeping back. “If you turn your attention away to another problem, the old problem will just come back,” Blount says.
The America’s eliminated measles in 2016, but the virus has since reemerged, especially in Venezuela, where the state vaccination programs have been interrupted after healthcare infrastructure broke down. Because the virus has been circulating continuously for more than 12 months, by definition, it’s no longer eliminated.
“If you take action for a long period of time, and get numbers down, but stop the intervention, it almost certainly will come back,” Blount says.
After polio is eradicated—which likely isn’t far off, Goodson says—measles is likely the next candidate for focused eradication efforts. It might be an easier task than eradicating polio, he says, because cases are easier to identify and the vaccine is more effective. However, notes Orenstein, the measles is highly infectious, much more so than polio, which adds an additional hurdle.