COVID-19: The Road to Endemicity

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

I’ve been blogging about this pandemic for almost two years now. One of my earliest posts on this topic was back in March of last year, in which I described the concept of herd immunity, as well as some other textbook ideas in epidemic management. Back then, these things were new to non-scientists, so that post went sort of viral (pun intended). Today, many people now have a degree of comfort with the terminology and basic concepts of epidemiology. But one idea is still relatively new to most: endemicity.

Outbreak vs Epidemic vs Pandemic vs Endemic

Let’s define some terms first. An “outbreak” is when a disease sees a sudden increase in cases within a defined area. An explosion of salmonella cases at the office picnic would qualify as an outbreak, for example.

An “epidemic” occurs when outbreaks spread beyond their immediate physical surroundings. If suddenly a bunch of office parties across the city were experiencing outbreaks, that would qualify as an epidemic. Of course there are a variety of sub-types of epidemics. For example, a “common source” epidemic is one in which all the cases result from a common source, such as a factory spewing toxins from upriver to a series of villages downriver. A “propagated” epidemic is what most people think of, in which one person gets sick and then infects the next person who then infects the next, and so on. And of course it’s possible to get mixed types, as well, For example, a person can get exposed to shigellosis at an event (common source) but then go on to infect others (propagated).

A “pandemic” is when an epidemic bleeds over into multiple countries. Note that the distinction between outbreak, epidemic, and pandemic has nothing to do with the seriousness or intensity of the crisis, but only with its geographical scope.

An endemic state, on the other hand, when a disease is pretty much always present but limited to a specific geographical region. In such a case, the disease is relatively predictable and tends not to ebb and flow in waves, as does an infectious respiratory disease in the epidemic stage, like COVID-19.

Malaria is considered an endemic in certain regions.

Another way to think about an endemic disease is that it is always present where you live, always poses a threat. Some people even use the word “hyperendemic” to describe a constant high-level presence.

Some will argue that the most important characteristic of an endemic disease is its predictability. We know how much of it is circulating. The same cannot be said of an epidemic in the throes of the rises and falls of waves.

Is it Certain That COVID Will be Endemic?

Nothing is certain. As I’m fond of telling people (much to their discontent), scientists can offer no guarantees, only probabilities. Children expect certainties and promises, whereas adults operate in the world of likelihoods. We must all be adults.

It is possible that COVID-19 can be mostly quashed through a combination of vaccination, natural recovery, and mutation. That does not seem likely to happen anytime soon, given the relatively low rates of global vaccination and the fact that a great majority of people still remain susceptible to infection. But perhaps one day it will happen.

It is possible that a combination of antigenic drift and population immunity will render SARS-Cov2 into just another common cold virus. You probably don’t remember the Asiatic Flu of 1890, which was one of the most lethal pandemics of modern times. Many researchers now believe that it was caused by the coronavirus OC43, which is now responsible for about 15% of the world’s common cold cases. That beast was mostly tamed, so SARS2 might go the same way.

Similarly, the descendants of the 1918 Spanish Flu (influenza A) still pop up to create more challenging flu seasons, but have been largely defanged due to both mutation and vaccine technology. But the inluenza A virus still circulates; it has not been driven to extinction.

As I explained a year ago, it is pretty much impossible for us to eradicate COVID-19 the same way we drove smallpox off the face of the Earth. But it’s always in the cards to strive for something close to elimination, in which we extinguish the disease locally using vaccination, dedicated testing, extreme surveillance, and border monitoring. We have done that for polio, for example.

Elimination or near-elimination locally is possible if vaccine uptake is very high, transmission among vaccinated people is very low, new variants do not challenge vaccine efficacy, and we do not make foolish policy decisions to jeopardize these possibilities. But given the amount of public resistance to the measures required to achieve full or near-elimination, I doubt that that will happen here in Canada. (But never say never!)

That leaves tolerating a level of constant infection, mostly among susceptible people, at least for a few years. And that means accepting an endemic virus.

How Will We Know When/If COVID is Endemic?

In short, we won’t know until many months or even years of the disease’s constant presence in our lives.  Frankly, the only metric we can use is the effective reproduction number (the average number of new cases produced by an existing case). If that number is stably equal to one for many months, then we are in an effective endemic state.

But it won’t be perfectly so. The unvaccinated are geographically clustered and socially linked, so when outbreaks do happen, they will explode in such clusters and drive the local reproduction number higher. And because the vaccines do not perfectly prevent transmission (at least not under the current two dose paradigm), infection will continue to be exchanged between people, even among fully vaccinated persons.

The seasonal flu is an example of an endemic respiratory virus that we manage to “live with” (though I hate that term). Many people get vaccinated, but many do not. Many people get sick, and some die. The flu has a strong seasonal component, and tends to mutate quickly, requiring a yearly vaccine update; but otherwise it is always with us to some extent.

If COVID-19 reaches flu status, then that is manageable… but not ideal. Make no mistake, the seasonal flu is a killer, and COVID more so. Even so, we tend not to wear masks, do symptom checks, mandate vaccines, or even wash our hands diligently to prevent the yearly flu. Hopefully we can do at least some of those things to keep endemic COVID in check, as it is an even bigger killer.

Frankly, it’s governments who will decide when/if COVID is endemic. Because they will determine when the level of sustained transmission is low enough to be tolerable from both a political and health systems perspective.  See, this isn’t just about the epidemiology. It’s about public tolerance.

What Will Endemic COVID Look Like?

Everyone wants to know this. And frankly the answer is that no one knows.

I’d like to think that we will soon get to the point where there is so much immunity in the population, by virtue of both vaccination and natural recovery, that the disease struggles to maintain a serious foothold in the community. So while outbreaks will continue to happen in susceptible groups (i.e., those still not immune or those for whom immunity has faded), they will not be large enough to be newsworthy or challenges to our health care system.

I would also like to think that we are rapidly approaching the point at which new cases are genuinely decoupled from hospitalization and deaths. So we would cease to worry when the case numbers mount and only take interest when the ICU beds become filled –as they should not be when sufficient numbers of people are well vaccinated.

The big confounder here, of course, is so-called “Long COVID”. We cannot afford to be blasé about mere infection, when it might lead to insidious disability down the line.

But there is a good possibility that an endemic state might mean that most citizens don’t have to think about COVID again, except to the extent that they need to remain vaccinated and remove themselves from society when experiencing symptoms. Let public health toil in the background, using testing and surveillance to contain the inevitable cases when they occur.

And do not forget that our awesome biotechnology sector is churning out actual treatments for COVID-19, not just vaccines. It is highly likely that in a year or two, our ability to further defang this beast will be appreciable.

So What’s The Bottom Line?

Endemicity is not guaranteed but does seem likely. All it means is that the disease will always be present where we are. (Though it will certainly continue to rage across the globe in fine epidemic fashion.) It says nothing about the level of transmission that must be tolerated. So it strikes me that we are best served by striving to push that level of transmission as low as possible, and to keep it down through widespread vaccination and the use of policy tools like vaccine passports, so that when when we take our foot off the brake, we can coast at a comfortable speed of tolerable disease burden.