You may have heard the term “endemic” used to describe diseases, often compared to the terms “epidemic” and “pandemic.” Many health officials say that COVID-19 is likely to move from pandemic to endemic status in many countries. But what does that mean?
More generally, the term “endemic” refers to an organism found in a particular region. In ecology, it means a species that lives only in one geographical area, such as a plant or an animal confined to an island, according to US Geological Survey (opens in a new tab). However, in the context of public health, “endemic” refers to a disease with a constant presence or “usual” number of infections in a specific area, according to Centers for Disease Control and Prevention (opens in a new tab).
“The technical definition is a disease that is in a stable state. It’s there all the time,” Dr. Christopher J. Gill (opens in a new tab), a professor of global health at Boston University, told Live Science. “It doesn’t really cause outbreaks and it doesn’t die away.”
This concept, in turn, depends on the idea of an “epidemic” disease—one with higher than normal levels of infection in a population, Gypsy D’Souza (opens in a new tab), an epidemiology professor at the Johns Hopkins Bloomberg School of Public Health, told LiveScience. (An epidemic disease becomes a “pandemic” if it spreads across several countries or across the globe.)
Related: The worst epidemics and pandemics in history
Endemic status also depends on the effects a disease has on the community in a particular region, D’Souza said. “That means there’s enough immunity in the population that we’ve learned to live with that infection,” she said. “We don’t have waves of disease … disrupting daily life.”
However, not every epidemic disease becomes endemic; many simply disappear. “If you have an epidemic and you successfully contain and treat it and you’re able to drive it to zero infections, you can prevent it from becoming endemic,” D’Souza said. Examples include the 2005 H5N1 bird flu and the 2002-2004 outbreak of severe acute respiratory syndrome (SARS).
A number of diseases have become endemic in the United States, including respiratory diseases such as influenza and respiratory syncytial virus (RSV), as well as many childhood diseases. “Measlesmumps, rubella, chicken poxgroup A strep [Streptococci]pink eye — all these things are just constantly circulating,” Gill said. At the beginning of the COVID-19 pandemic, experts wondered whether the disease would rapidly increase in prevalence and disappear or eventually become endemic, although most predicted an endemic route, said Gill Nearly three years into the pandemic, many experts said that COVID-19 was endemic in the United States or soon would be, as most people had some immunity from vaccination or exposure, D’Souza said.
However, the timing of this transition could vary according to the expert, as they evaluated how far the infection and death levels had fallen. “It’s not an exact science,” she said. “We’re not going to say that this particular day is when we’ve transitioned.” Meanwhile, epidemic levels of infection may persist in other parts of the world even if a disease becomes endemic in another region.
Experts caution that becoming endemic does not necessarily mean less severe disease, although “in general, we expect pathogens to become less pathogenic over time,” Gill said. Likewise, endemicity does not mean that a disease has ceased to be a public health problem, virologist and immunologist Matt Koci said in a North Carolina State University (opens in a new tab) mail. “Cups was an endemic disease that consistently killed 1 in 3 people it infected,” he said.
Rare, endemic diseases – esp flu — could return to epidemic status, Gill said. “Sometimes the flu virus can mutate radically into something almost completely unseen by the population,” he said. “And then you can have a totally catastrophic flu pandemic,” which happened in 1918 influenza outbreakwhere a new strain of influenza killed approximately 50 million people.