Health officials say the coronavirus will likely become endemic in the next several years. What does that mean?

Even as cases continue to decline and more Americans receive their vaccines, the coronavirus isn’t likely to go away anytime soon, health officials say.

The nation’s top infectious diseases expert Dr. Anthony Fauci dismissed the idea that COVID-19 would be eradicated in the next several years at a webinar hosted by think tank Chatham House in November.

“We need to plan that this is something we may need to maintain control over chronically. It may be something that becomes endemic, that we have to just be careful about,” he said.

So, what is an endemic disease and how would COVID-19 become one? Experts say there are multiple endemic diseases in the United States that could foreshadow what the disease caused by the coronavirus may look like in the upcoming years.

What does endemic mean?

The Centers for Disease Control and Prevention defines endemic as the “constant presence and/or usual prevalence” of a disease within a population in a certain geographic area.

An endemic disease spreads at a baseline level every year without causing major disruption to people's lives, said Dr. Donald Burke, professor of epidemiology at the University of Pittsburgh Graduate School of Public Health.

“Things that are endemic are present for long periods of time without interruption, continuously circulating in the population," like the common cold, he said.

A disease can be endemic in one country but can be considered an outbreak or an epidemic in another country, explained Dr. Pritish Tosh, an infectious diseases physician and researcher at the Mayo Clinic.

For example, malaria is considered endemic in some parts of the world where mosquitos carry the parasite. However, a high number of malaria cases in the United States would be considered an epidemic if it were not contained.

What’s the difference between an epidemic and an endemic?

An epidemic is a sudden increase of a disease above what is normally expected among the population in a certain area, according to the CDC.

Epidemics aren't just caused by diseases that result from viruses or bacteria, the agency says. For example, diabetes and obesity exist in large enough proportions in the U.S. to be considered epidemics. Similarly, a sudden increase in addiction to opioids over the past several years is accurately called an “opioid epidemic.”

The part of the word “epi” means "to be upon," Burke said, and “demic” comes from “demos,” which means "people."

“Epidemic means something that comes out and is among the people,” he said. “Things that are epidemic are things that are unusual that are not there and then appear.”

Endemic means "something that's within the people," he added. Many epidemics have turned into endemics.

But an endemic disease does not necessarily mean that it will exist forever. Some endemic diseases have been eliminated in the U.S. after achieving herd immunity through vaccines and natural infection.

What are some endemic diseases?

The four common cold coronaviruses, which are considered cousins of SARS-CoV-2, the virus that causes COVID-19, are considered endemic in most parts of the world, including the U.S., Burke said.

“We don’t notice them,” he said. “They’re transmitted, they’re constant.”

Many childhood diseases also are endemic, he said. The measles, for example, used to be endemic in the U.S.

“In the old days … they were commonplace. Everybody got them,” Burke said. But childhood vaccines helped impede transmission, almost eliminating the measles from the U.S.

However, the measles is still considered endemic in some parts of the world, Tosh said. If the highly infectious virus was brought in from another country, it could cause an outbreak and possibly lead to an epidemic in the U.S.

For example, a series of outbreaks in 2019 led to more than 1,200 measles cases in the U.S. – the highest number of cases recorded in the country since 1992, according to the CDC. The agency attributed the outbreaks to travelers who got measles abroad and pockets of unvaccinated people.

Could COVID-19 become endemic?

It’s likely SARS-CoV-2 is here to stay, health experts say.

“It appears as though this virus is likely to remain endemic in populations at least for several years, possibly indefinitely,” Tosh said.

A January study found that the virus “could join the ranks of mild, cold-causing … human coronaviruses in the long run,” according to Emory University and Penn State University scientists.

The model, published in the peer-reviewed journal Science, compares SARS-CoV-2 to four common cold coronaviruses plus the SARS and MERS viruses, which surfaced in 2003 and 2012, respectively.

Researchers determined from the model that if the novel coronavirus continues to circulate in the general population and most people are exposed to it from childhood, it could be added to the list of common colds.

Listen to more details from reporter Adrianna Rodriguez in USA TODAY's 5 Things podcast below on what epidemiologists say about endemics:

However, the future of the novel coronavirus hinges on many unknowns, experts say. New variants from the United Kingdom and South Africa, which studies have shown may be more transmissible, were discovered in the U.S. Health officials are more concerned about the South Africa variant, as emerging data shows existing COVID-19 vaccines seem to be less effective against it.

Tosh expects more variants to arise as growing immunity and vaccines forces the coronavirus to mutate.

“It will be difficult to project what this will look like five years from now," he said. "But I think we can anticipate some kind of COVID endemicity over the next several years."

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: COVID-19 likely to become endemic, experts say. Here's what it means.