You may have heard the term “endemic” used to describe diseases, often compared to the terms “epidemic” and “Pandemic.” Many health officials say COVID-19 is likely transitioning 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, like a plant or animal that is confined to an island, according to the US Geological Survey (opens in 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 given area, the authorities said Centers for Disease Control and Prevention (opens in new tab).
“The technical definition is a disease that is in a stable condition. She’s there all the time.” dr Christopher J Gil (opens in new tab), professor of global health at Boston University, told Live Science. “It doesn’t actually erupt or die off.”
This concept, in turn, hinges on the idea of an “epidemic” disease – a disease with higher than average rates of infection in a population Gypsyamber D’Souza (opens in new tab), an epidemiology professor at the Johns Hopkins Bloomberg School of Public Health, told Live Science. (An epidemic becomes a “pandemic” when it spreads to multiple countries or the globe.)
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Endemic status also depends on the impact a disease has on society in a given region, D’Souza said. “That means there is enough immunity in the population that we have learned to live with this infection,” she said. “We don’t have flare-ups… disrupting daily life.”
However, not every epidemic becomes endemic; many just disappear. “If you have an epidemic and you successfully contain and treat it and you are able to get it down to zero infections, you can prevent it from becoming endemic,” D’Souza said. Examples include the H5N1 avian flu of 2005 and the severe acute respiratory syndrome (SARS) outbreaks of 2002-2004.
A variety of diseases have become endemic in the US, including respiratory diseases such as influenza and respiratory syncytial virus (RSV), as well as many childhood diseases. “measlesmumps, rubella, chickenpoxGroup A Streptococci [Streptococci]Conjunctivitis – all of these things are just floating around all the time,” Gil said. At the start of the COVID-19 pandemic, experts wondered whether the disease would rapidly increase in incidence and disappear or eventually become endemic, although most predicted an endemic route, Gill said. After nearly three years of the pandemic, many experts said COVID-19 is endemic in the United States or will become so soon because most people have some immunity to vaccination or exposure, D’Souza said.
However, the timing of that transition could vary, according to the expert, as they assessed how far infection and death rates had fallen. “It’s not an exact science,” she said. “We’re not going to say we switched on that exact day.” Meanwhile, even if a disease becomes endemic in another region, epidemic rates of infection may persist in other parts of the world.
Experts warn that endemic doesn’t necessarily mean less severe disease, although “in general, we assume that pathogens become less pathogenic over time,” Gill said. Likewise, endemicity does not mean that a disease is no longer a public health problem, said virologist and immunologist Matt Koci in a North Carolina State University (opens in new tab) Post Office. “smallpox was an endemic disease, consistently killing 1 in 3 people it infected,” he said.
Rarely endemic diseases — especially flu — may return to epidemic status, Gill said. “Occasionally, the influenza virus can radically mutate into something almost completely invisible to the general public,” he said. “And then you can have a totally catastrophic flu pandemic,” as happened in the 1918 flu outbreakin which a new strain of flu killed around 50 million people.