Disease 'elimination' vs. 'eradication': Using the terms correctly

A child receives a polio vaccine outside of Delhi, India. The disease has been eliminated from the U.S., but is still present elsewhere in the world. Photo by Tara Haelle

When a new measles outbreak makes the news — such as the expanding one in West Texas — journalists usually note that vaccination enabled the elimination of the disease in the U.S. in 2000. But the term “eliminated” is often conflated or substituted with the term “eradicated” in discussing measles and other infectious diseases.

It may be that writers and editors are looking for ways to avoid re-using the term “eliminated” or “elimination” in a story. But it’s crucial journalists understand that “elimination” and “eradication” have very different and specific meanings in the context of infectious diseases. Using these terms incorrectly can confuse audiences, compromise your credibility, or erode audiences’ trust in infectious diseases coverage.

Elimination: It’s no longer here, but it’s still elsewhere.

Elimination refers to a disease being no longer endemic in a particular region, most often a country or a larger geographical region, such as North America, Central America or sub-Saharan Africa. An endemic disease is one that has continuous, ongoing transmission in an area. It regularly occurs in that region even if it has seasonality (such as flu season) or if the case counts ebb and flow over time.

For example, pertussis is an endemic vaccine-preventable disease in the U.S. While cases of pertussis are usually low most of the time, cyclical outbreaks occur every few years, and there has never been a time in U.S. history when there weren’t pertussis cases somewhere in the country, even if they’ve fallen as low as a couple thousand in a year.

Measles was endemic in the U.S. up until the year 2000, when it was certified as eliminated in the country. To achieve elimination status, no new cases of a disease can occur for a specified period of time. The CDC defines that time as one year, but the WHO usually requires three consecutive years with no cases to ensure no undetected transmission is occurring.

Even once a disease is eliminated from a region, however, outbreaks of it can occur if a case is imported into the country, whether by a non-resident who is infectious and visits or by a resident who travels abroad, becomes infected, and returns home while still infectious.

As long as those outbreaks are eventually contained through vaccination and other public health measures, a country retains its elimination status. After Central America and South America similarly eliminated measles, a committee at the Pan American Health Organization/WHO meeting in September 2016 declared the Americas to be the first region in the world to achieve measles elimination.

If, however, an outbreak leads to continuous transmission for long enough — typically one year — a country or region can lose its elimination status. The recirculation of measles in Venezuela in 2018, followed by outbreaks in other countries that re-established transmission in Brazil and Columbia, led the Americas to lose its measles elimination status.

Other diseases that have been eliminated from the U.S., even though they exist elsewhere in the world, include malaria, yellow fever, cholera, diphtheria, polio and rubella. Vaccination is not the only way to eliminate a disease. Malaria was eliminated from the U.S. through environmental control measures that included pesticides and mosquito repellants, getting rid of breeding sites, and using screens, windows and air conditioning. Cholera was eliminated through clean water and sanitation.

Eradication: Elimination of a disease worldwide

Eradication of a disease only occurs when it has been eliminated from every country. That means there is no ongoing, continuous transmission of the disease anywhere in the world and hasn’t been for three years, per the WHO’s definition.

Only one human disease has been completely eradicated — smallpox. Three years after the last known smallpox cases in Somalia in 1977, the WHO declared smallpox eradicated in 1980.(Although smallpox has been eradicated, it is not extinct since the pathogen still exists in some labs around the world.)

The only other disease to be eradicated is rinderpest, a disease affecting cows and other hoofed animals, in 2010.

The World Health Organization and partners of the Global Polio Eradication Initiative have been working toward polio eradication for several decades. Two subtypes of polio have been eradicated: wild poliovirus type 2 in 2015 and wild poliovirus type 3 in 2019. Only after there have been zero cases of wild poliovirus type 1 and zero cases of vaccine-derived poliovirus type 2 for three years in all regions of the world will the WHO declare polio eradicated.

Not all diseases can be eradicated. Depending on the source, a disease must have certain indicators to be eradicable. Most sources agree at a minimum on three key criteria:

An effective intervention exists to interrupt transmission, such as a vaccine or environmental control measures.
Tools exist to diagnose the disease with adequate sensitivity and specificity for sufficient surveillance of transmission.
Humans are the only reservoir for the disease, and it does not reproduce in the environment. If the disease can be transmitted between humans and another species, as influenza is, or if it occurs in the natural environment, as tetanus does, it’s impossible or too difficult to eradicate.

In addition to these criteria, however, there must be adequate economic resources and social and political factors that allow for the global cooperation needed to eradicate a disease.