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We cannot say with certainty what the future of COVID-19 is. But based on our experience with other infections, there is little reason to believe that the SARS-CoV-2 coronavirus will disappear soon, even when vaccines are available. A more realistic scenario is that it will be added to the (large and growing) family of infectious diseases that are known as “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 control that spread, except in countries that can effectively isolate themselves from the outside world. The fact that the vast majority of people remain susceptible to some degree means that there is enough fuel to keep the fire burning for quite some time.
This will be the case even if specific locations achieve what’s known as population (or herd) immunity (and it’s unclear how likely this is to happen). When a sufficient number of people become immune to a disease, either by vaccination or by a natural infection, its spread begins to decrease and the number of cases gradually decreases. But that does not mean that it will disappear instantly or completely.
Outside of any area with population immunity, there are likely many locations that still have enough susceptible individuals to sustain transmission. No isolation measure is strong enough to completely stop human interaction between regions, within and between countries, or globally.
It is also possible that the spread of an infection eventually stabilizes at a constant level so that it is present in communities at all times, possibly 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 (like many sexually transmitted infections and childhood infections). But most infections are endemic in specific parts of the world.
This can occur when effective control has eliminated the infection elsewhere, or because the conditions necessary for effective transmission can only be found in specific locations. This is the case with malaria and many other mosquito-borne infections.
In theory, 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 is increasing, R is more than 1, and when the spread is decreasing through control measures or immunity of the population, R is less than 1.
In practice, there are a number of patterns that can be observed in endemic diseases. Some may exist at low levels throughout the year, while others may show periods of higher transmission interspersed with periods of low transmission. This could happen if seasonal factors influence the amount of contact people have with each other, their susceptibility to the disease, or other organisms that spread it, such as insects.
As long as there is a sufficient supply of 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.
Waning immunity
In diseases that give permanent immunity after infection, each newborn is susceptible after the immunity obtained from the mother wears off. That is why childhood infections like measles are endemic in many parts of the world where the birth rate is high enough.
In diseases that only grant temporary immunity through natural infection, people lose that immune protection to become susceptible again. A virus or bacteria can also evade immunological memory by mutation, so that people with immunity to an older strain will become susceptible to the new version of the disease. The flu is a good example.
We still don’t know how long immunity against COVID-19 infection will last, or how good vaccines will be at protecting people. But other coronaviruses that are endemic in the human population, such as those that cause colds, only confer temporary immunity for about a year.
Another important point is that people with immunity, whether by infection or vaccination, are rarely evenly distributed in a community or country, much less in the world. Certainly, in the case of COVID-19, there are areas where the infection has spread most intensively and areas that have been relatively spared. Without a uniform distribution, there is no population immunity even if enough people have been vaccinated to reach the required threshold predicted.
In these cases, the average R may be low enough that the infection is under control, but in unprotected foci 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 maintain transmission.
How we respond
How we deal with COVID-19 once it becomes endemic will depend on how good our vaccines and treatments are. If they can protect people from the most serious outcomes, the infection will be manageable. COVID-19 will be like other diseases that we have learned to live with and that many people will experience during their lives.
Depending on whether immunity, whether from natural infection or vaccination, is permanent or temporary, we may need annual vaccine updates to protect ourselves (such as influenza). 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 long-lasting immunity, we can imagine other scenarios, such as possible disease eradication. But, realistically, this is unlikely. Eradication is notoriously difficult, even for diseases for which we have near-perfect vaccines and permanent immunity. Therefore, endemic disease is the most likely outcome.
Hans Heesterbeek, professor of theoretical epidemiology, Utrecht University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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