We cannot say for sure what the future of COVID-19 is. But based on our experience with other infections, there is little reason to believe the SARS-CoV-2 coronavirus will be gone anytime soon, even when vaccines become available. A more realistic scenario is that it will be added to the (large and growing) family of infectious diseases that are called “endemic” in the human population.
With the global spread of the disease increasing again, it seems unlikely that currently available measures can do more than contain this spread – except in countries that can effectively isolate themselves from the outside world. The fact that the vast majority of people are still vulnerable to some extent means that there is enough fuel to keep the fire burning for some time.
This will be the case even if specific locations achieve what is known as population (or herd) immunity (and it is not clear to what extent this will happen). When a sufficient number of people become immune to a disease, either by vaccination or by natural infection, its spread begins to slow and the number of cases gradually decreases. But that doesn’t mean it will go away instantly or completely.
Outside of areas where the population is immune, there are likely to be many places where there are still enough susceptible individuals to maintain transmission. No isolation measure is so strong that it will completely stop human interaction between regions, within and between countries, or on a global scale.
It is also possible that the spread of an infection will eventually stabilize at a constant level so that it becomes present in communities at any time, perhaps at a relatively low, sometimes predictable rate. This is what we mean when we say that a disease is endemic.
Some infections are present and actively spread almost everywhere (such as many sexually transmitted infections and childhood infections). But most infections are endemic in some parts of the world.
This can happen when effective control has ruled out the infection elsewhere, or because the conditions necessary for effective transmission can only be found in specific places. This is the case with malaria and many other infections transmitted by mosquitoes.
Theoretically, an infection becomes endemic if, on average, each infected individual transmits it to another person. In other words, when the reproduction number (R) = 1. In comparison, during an epidemic when the spread of the disease increases, R is greater than 1, and when the spread decreases thanks to control measures or to population immunity, R is Less than 1.
In practice, a number of patterns can be observed in endemic diseases. Some may exist at low levels throughout the year, while others may exhibit periods of higher transmission interspersed with periods of low transmission. This can happen if seasonal factors influence people’s level of contact with each other, their susceptibility to the disease, or other organisms that spread it, such as insects.
As long as there are enough people still susceptible to the disease for each infected person to transmit it, it will continue to spread. This supply can be replenished in various ways, depending on the characteristics of the disease.
In diseases that confer permanent immunity after infection, each newborn is susceptible after the immunity obtained from the mother has worn off. This is why childhood infections such as measles are endemic in many parts of the world where the birth rate is high enough.
In diseases that only confer temporary immunity through natural infection, people lose that immune protection and become susceptible again. A virus or bacteria can also escape immune memory by mutation, so people immune to an older strain become susceptible to the new version of the disease. The flu is a prime example.
We do not yet know how long immunity to COVID-19 infection will last, or how effective vaccines will be in protecting people. But other coronaviruses endemic to the human population, such as those that cause the common cold, only confer temporary immunity for about a year.
Another important point is that immune people, whether infected or vaccinated, are rarely evenly distributed in a community or country, let alone the world. Certainly, in the case of COVID-19, there are areas where the infection has spread more intensely and areas that have been relatively untouched. Without uniform distribution, there is no immunity of the population even if enough people have been vaccinated to reach the expected necessary threshold.
In these cases the average R may be low enough to keep the infection under control, but in unprotected pockets it will be well above 1. This leads to localized outbreaks and allows the disease to remain endemic. It continues to spread from place to place, seeded by a few places where population density and interaction are high enough and protection low enough to support transmission.
How we respond
How we deal with COVID-19 once it becomes endemic will depend on the quality of our vaccines and treatments. If they can protect people from the most serious outcomes, the infection will become manageable. COVID-19 will then be like many other diseases that we have learned to live with and that many people will experience in their lifetime.
Depending on whether the immunity – either to a natural infection or to the vaccination – is permanent or temporary, we may need annual vaccine updates to protect ourselves (like the flu). Or it could be controlled by vaccination at an optimal age (like many childhood infections).
If vaccines not only prevent clinical disease but also greatly reduce transmission and confer lasting immunity, other scenarios can be considered, such as potential eradication of the disease. But in reality, this is unlikely. Eradication is notoriously difficult, even for diseases for which we have near-perfect vaccines and permanent immunity. Endemic disease is therefore the most likely outcome.
This article is reprinted from The Conversation by Hans Heesterbeek, Professor of Theoretical Epidemiology, University of Utrecht under a Creative Commons license. Read the original article.
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