What does endemic mean?
Not what you think it does!
Everyone keeps talking about covid becoming endemic, but as I listen to the conversation, it’s becoming more & more clear to me that very few of you know what “endemic” means.
So here’s a thread on how pandemics end.
In the beginning of any pandemic, we have 4 options for what could happen:
1) continually occurring disease, with small or large surges
2) local elimination of disease
3) global eradication of disease
4) complete extinction of the pathogen
Option 4 (extinction) is absolutely the hardest, but long-term would mean we could all completely forget about the disease. We have basically never done this in the entire history of humans — but, if we’re being honest, a big reason we haven’t is fear & mistrust of other humans.
Option 3 (eradication) is also very hard but long-term just about everyone could forget about the disease. You & I aren’t worried about smallpox, no matter where we travel or what we do — unless you’re a researcher at one of the 2 super high biosecurity labs that have a sample.
Option 2 (elimination) is still pretty hard, and long-term not everyone can forget about the disease. For lots of the world, this is measles is — we’ve got great vaccines & most of us don’t need to worry about measles, but public health officials still monitor for it everywhere.
Now we get to Option 1 (continually occurring disease). This is easier short-term but it’s the hardest *long-term*. It’s also super vague: it could mean anything from hundreds or thousands per day to one or two per year. The other name for this is, you guessed it, “endemic”.
In more technical terms “endemic” means: “Controlled at or below an ‘acceptable’ level”. (It also has a formal mathematical definition — see the end of the article for that!)
What level is “acceptable” differs from place to place, over time & between diseases, and it may not always be explicit, but when a disease is endemic, there is a threshold!
If a disease is really bad, ‘acceptable’ is very low — potentially even indistinguishable from elimination. The plague (yes that one!) is kinda common among rodents in parts of the US, but if even ONE person shows up to a doctor with symptoms, public health jumps into action!
On the other hand, if a disease is not really that bad at all, the acceptable level can be really very high. Up to 80% of US adults are infected by the virus that causes cold sores (aka oral herpes)! But public health takes *nearly* no action against oral herpes, except to protect infants.
You might have noticed something important here, and it’s something I think MANY of the people shrugging off covid becoming endemic get wrong:
“Endemic” does NOT mean “harmless”.
Whether a disease is endemic, epidemic, eliminated, or eradicated does NOT tell us how serious it is, and does NOT tell us how many people get sick or die.
We have to CHOOSE that number.
So here’s the kicker: “endemic” doesn’t mean “never think about covid again”. It’s exactly the opposite! Endemic means someone is ALWAYS thinking about covid.
Endemic means public health is always monitoring disease & always intervening when cases cross the “acceptable” level.
Every time I write about this I get comments saying “but we don’t take precautions for the flu!”. Those people are 100% WRONG! Thousands of people work daily to monitor, prepare for, & respond to fluctuations in flu number and in the flu virus itself! It’s a HUGE task!
Sure, maybe the flu doesnt impact your life, except some years you get a flu shot, or maybe you have once or twice really actually gotten the flu & not just a bad cold you *called* the flu. (Pro-tip: if you dont feel like you’ve been hit by a train, it’s probably not the flu). But that is because PUBLIC HEALTH KNOWS WHAT IT’S DOING!!!
There are clearly defined “acceptable” levels for different strains of the flu, and we spend so so much time & energy & money working to keep the flu below those levels. So that YOU don’t get sick.
When flu cases cross the threshold & the public health lever switches from “monitor flu activity” to “take action YOU don’t notice. But not because it isnt happening — because the acceptable level is LOW ENOUGH that those actions happen before it impacts your daily life.
If you work somewhere like a daycare, or a nursing home, or (like me) a School of Public Health that shares a campus with a hospital, you probably DO sometimes notice. Because those are places where flu is most problematic & so they are also where we focus our interventions.
So what does this tell us about how pandemics end? Well, it tells us that we CAN put in the work to really get rid of it once & for all, if we have the will and the strength and the interest and the resources. It probably gets harder & harder the longer we wait, though.
Smallpox was around for a very long time before we took action & it took a long time to defeat. Similarly, polio eradication is long overdue, but we’re still trying and I’m certain we will get there.Yes, eradication is HARD but we could still choose to make it our moonshot if we wanted to.
It would definitely take time. It would probably take new science, new ideas, & new tools. And it would take both leaders & everyday people actively committing to doing what it takes. Realistically, despite New Zealand’s valiant efforts, we are not going to do that.
We don’t have the political or social will to eliminate or eradicate covid. But that means we HAVE TO continue to CONTROL covid! That’s the only other option. THAT is what endemic means!
So, yes, our only viable choice left is covid becoming endemic. It didn’t have to be this way, but our leaders MADE a choice. And now they need to make another choice: They need to choose an “acceptable” level of COVID death & disease.
Because pandemics don’t end by a disease just fading away, & pandemics don’t end with everyone able to completely forget about the disease. Pandemics end when we decide how much death and disease we’re satisfied with.
I dont know about you, but for me — this👇🏼is too much death.
Nerdy addendum: For those of you who like math, “endemic” is actually measured with something called the effective reproductive number (Re), and depends on the relative changes in protected & susceptible populations.
The formal mathematical definition of endemic is that over the long-run (a year, or a regular & predictable multi-year cycle), Re — the average number of people infected by each infectious individual — is equal to 1. Seasonality can mean that at certain times of the year Re is above 1, but at other times it must be below 1 or the disease is not considered controlled.
The value of Re isn’t completely up to us because it depends on features of the pathogen. And the level of cases and deaths we have is not determined entirely by Re either. But the purpose of infectious disease control programs is to get Re below 1 or as close to 1 as possible, and having done so, to also reduce the total amount of infected individuals.
This post began life as a tweetorial, or twitter tutorial, full of amusing gifs. To read the original tweetorial, click here.
