While elimination of a disease occurs when the disease is no longer endemic to a particular geographical region, eradication refers to the elimination of a disease in all geographic regions—the disease is completely gone from the planet, except perhaps in vials in a couple labs. Measles and polio have been eliminated from several regions in the world. Smallpox has been eradicated from the world.
Deeper dive
The elimination of rubella in the Americas was announced in April 2015, followed by the elimination of measles across the Americas continents in September 2016. Yet measles cases still occur in North and South America, and news is still being reported on the eradication of polio, which had not been seen in the Americas in years until an imported case in New York in August 2022. The key here is that “elimination” and “eradication” are different things, though they are often confused by readers and sometimes even by journalists.
Eradication refers to a disease being completely, literally eradicated from the earth: no cases occur at all, from any source. The best-known example is the eradication of smallpox in 1980. Another lesser known disease that has been eradicated includes the livestock virus rinderpest. Campaigns to eradicate polio and Guinea worm are officially underway, and it could be argued that public health officials are — so far unofficially — working toward eradication of hookworm, measles, rubella, malaria and other diseases. (Much of the progress made in these areas was set back by the Covid-19 pandemic, however.)
Only certain conditions are able to be eradicated with current tools, and certain characteristics must be present for eradication to be possible. For example, it’s currently not possible to eradicate influenza because it has multiple non-human reservoirs—it can infect birds, dogs, cats, ferrets, pigs, bats, and other animals—and the current vaccines are not very effective. Even if a vaccine were effective enough to prevent circulation in humans—or non-pharmaceutical interventions, such as mask-wearing, prevent widespread circulation of the flu as happened in winter 2020-2021—the flu can hide out in animal populations and return at any time.
Elimination refers to a permanent interruption in indigenous transmission of a disease, making it no longer endemic, but the disease can still be introduced by a case from another geographical region. Or, as it was put in an article about the measles elimination, “Measles no longer lives in the Americas though it occasionally visits.” For example, measles has been eliminated from the U.S. since 2000, but there have been a number of measles outbreaks in the U.S. since then. All of those outbreaks, however, were introduced by a person visiting from outside the U.S. None of them began with a person already living in the U.S. because the virus no longer circulates on its own in the U.S., thanks to the effectiveness of the measles vaccine.
The distinction is important because an eliminated disease can always return if conditions allow for it, such as a sufficiently deep, sustained drop in immunization rates that allows measles to begin circulating again.