What Does Endemic Actually Mean?

When people use a word like endemic to mean whatever they want, it lets us dodge the more important questions, like what we’re going to do about it.

It’s been nearly a month and a half since California became the first state to officially adopt a policy for COVID-19 as an “endemic” disease. But even at the time, Gov. Gavin Newsom didn’t really explain how it was determined that Omicron was now endemic or even what that actually meant in practical terms, especially when Omicron BA.2 cases may be threatening another COVID bump, albeit less than those of Delta or Omicron BA.1.

Armchair epidemiologists have been saying on social media for months that COVID is or soon will be endemic, but many of them seem to think that means COVID has become less dangerous and that we stop efforts to control the spread of it.

Instead, as Kent State University epidemiologist Tara Smith told me, endemic simply means “something we’re stuck with.” That is, COVID will “remain with us for the foreseeable future, and we can (roughly) predict the number of cases and deaths it will cause in a year’s time.” But that doesn’t tell us what we should, or shouldn’t, do about it.

After all, flu has been endemic for over a century, and we still prepare for flu season every fall, starting with annual vaccination campaigns. HIV has been endemic for four decades, and scientists are still actively working on developing a vaccine because we haven’t given up and decided to just “live with it.”

“Relatively Predictable”
To make sense of what endemic actually means — and what it doesn’t mean — I reached out to Smith and a couple other real (non-armchair) epidemiologists, first asking for examples of endemic diseases in the U.S. people are familiar with. In addition to flu, whose cases fluctuate with mild or particularly deadly seasons, food-borne illnesses like E. coli or salmonella are endemic, Smith said, as are sexually transmitted infections like gonorrhea, HPV, syphilis, and chlamydia.

The most dangerous strains of HPV have declined since introduction of the HPV vaccine, but no vaccines exist for gonorrhea, syphilis, and chlamydia, however, and cases hit an all-time high for the sixth year in a row last year. Broadly speaking, then, endemic doesn’t necessarily mean a disease can’t be increasing to potentially alarming rates, even if the rise has been gradual and we’ve been living with it for a long time.

There’s not an exact threshold for determine when a disease reaches endemic status, Smith said. It’s usually just when a disease “becomes predictable,” which can involve “increases and drops, or ‘waves,’ of cases, but generally these become relatively predictable over time, such as the seasonality we see with influenza,” Smith said. Few would say right now that COVID has reached a point where we consider it predictable.

Matthew Fox, an epidemiologist at Boston University, agreed that there isn’t necessarily a fully accepted precise definition for ‘endemic.’

”Some use it to mean contained to a specific area, but I think the more common meaning that people are using with respect to COVID is controlled at an acceptable level typically at a somewhat steady state (it can be seasonal but predictable),” he said. But, he adds, “We have to define what we consider to be an acceptable level.” And therein lies the rub.

“Some think even very low levels are not acceptable if it is leading to deaths,” Fox said, while others seem to think endemic means “low enough that I can not have it involved my daily life” — hence when we can end restrictions. But it’s not that simple. “The challenge is that we are likely to see rises and falls in cases for some time and we may want to take precautions when cases are rising and relax them when they are waning,” he said, echoing Smith’s point that “we don’t know yet if it will settle into a predictable periodicity like we see with influenza.”

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