Words obtain meaning not just from what they refer to, but also what they do not. We can thank the nineteenth century linguist Ferdinand de Saussure for this little observation.
Sometimes words are used in multiple ways. When we forget this principle, we do so at our peril. Sometimes that peril is merely semantic. Other times the consequences are real.
We now face one such peril with the word “endemic”. We read that Covid-19 is now endemic. (Actually, it’s not Covid-19 the disease that is endemic, but SARS-CoV-2 the virus).
Politicians are making much hay with this concept, citing it as the reason to drop mask mandates and generally de-escalate state approaches to managing Covid-19. But what exactly does endemic mean? And does it justify the changes in policy?
Epidemiologists use “endemic” in two different ways — both adjectives. (So, when folks like Brit Morse use it as a noun, as in “An endemic is essentially an ever-present threat”, that’s a bit weird.)
The first usage of endemic simply distinguishes the presence of a pathogen in a population from its absence. A pathogen is endemic when it persists in the population. Some pathogens, after an initial outbreak and epidemic, die out completely. (SARS-CoV-2 almost certainly won’t be doing that.) Others settle down to the “endemic equilibrium,” wherein ongoing transmission is just high enough to keep it alive in the population, thanks to a balance of population immunity and an influx of newly susceptible people (from new, susceptible people entering the population and the gradual loss of immunity by others). It has long been clear that SARS-CoV-2 won’t be eradicated in our society and is, in this sense, already endemic. However, this does not necessarily mean it won’t continue to have periods of high transmission or cause large outbreaks.
The second usage distinguishes a pathogen that is not causing large outbreaks from one that is (i.e. an “epidemic” pathogen.) The CDC defines an endemic disease as one that is present at a consistent level within a population. In this sense, endemic and epidemic might be distinguished from one another according to some statistical criteria. For instance, we might say that we are experiencing an outbreak of Covid-19 (or any other disease) when recent cases exceed a baseline by two standard deviations or more. I think it is in this sense that the politicians want to claim that SARS-CoV-2 is endemic.
Importantly, neither concept of “endemic” implies harmlessness. Indeed, many endemic diseases cause a great deal of damage. Malaria, for example, is a devastating disease in many places where it is endemic. We don’t give up protective measures (like bednets) in places where malaria is endemic, but rather we do the opposite. We take protective measures in places where malaria is endemic exactly because that’s where the risk is highest.
Helpfully, the CDC also defines the term hyperendemic, which describes a disease that is consistently persistent at high levels.
Is Covid-19 hyperendemic? I don’t think so. At least not yet. So far, nothing about Covid-19 has been consistent. The fact that Covid-19 is (presently) going down simply reflects that it’s still prone to episodic transmission, i.e. it is not present at ANY consistent level. Covid-19 hasn’t been with us long enough to say what a consistent level is.
It may indeed be time to relinquish mask mandates (although not voluntary masking). But, please, let’s not do so just because the virus has reached a state someone calls “endemic”.
