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 that the SARS-CoV-2 coronavirus will soon disappear, even when vaccines become available. A more realistic scenario is that it will be added to the (numerous and growing) family of infectious diseases that are known as “endemic” in the population.
With the global spread of the disease growing again, it seems unlikely that the measures currently available will do more than control this spread – except for countries that can effectively isolate themselves from the outside world. The fact that the vast majority of people are still susceptible to some degree means that there is enough fuel for the fire to continue to burn for quite some time.
This will be the case for a long time, even if certain areas reach what is known as herd immunity, although it would seem that it is not very effective. When a sufficient number of people become immune to a disease, either through vaccination or natural infection, its spread begins to slow down and the number of cases gradually decreases. But that doesn’t mean it will disappear instantly or completely.
Will Covid-19 disappear if we find a vaccine?
Outside of any areas with population immunity, there may be a lot of areas that still have enough susceptible individuals to continue transmission. No measure of isolation is so strong that it will completely stop human interaction between regions, within and between countries or globally.
It is also possible that the spread of an infection will eventually stabilize at a constant level so that it becomes present in the communities at any time, possibly at a relatively low rate, sometimes predictable. This is what we mean when we say that a disease is endemic.
Therefore, 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 occur when effective control has eliminated the infection elsewhere or because the conditions for effective transmission can only be found in specific locations. This is the case for malaria and many other mosquito-borne infections.
Theoretically, an infection becomes endemic if on average each infected individual transmits it to another person. In other words, when the number of reproduction (R) = 1. In comparison, during an epidemic when the spread of the disease increases, R is greater than 1 and when the spread decreases 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 have longer transmission periods, interspersed with reduced transmission periods. This could happen if seasonal factors influence how much people come in contact with each other, how sensitive they are to the disease or to other organisms that spread it, such as insects.
How many types of immunity and how can it be obtained
As long as there are enough people still susceptible to Covid-19 for each infected person to transmit it, it will continue to spread. This supply can be fed in different ways, depending on the characteristics of the disease.
In diseases that give permanent immunity after infection, every newborn baby is susceptible after the immunity obtained from the mother disappears. This is why childhood infections, such as measles, are endemic in many parts of the world, where the birth rate is high enough.
On the other hand, in diseases that give temporary immunity only through natural infections, people lose that immune protection to become susceptible again. A virus or bacterium can also steal the immune memory by mutating so that people with immunity to an older strain become susceptible to the new version of the disease. Flu is an example.
We do not yet know how long immunity will last to Covid-19 infection or how good the vaccines will be in protecting people. But other coronaviruses that are endemic to 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, either from infection or vaccination, are rarely evenly distributed throughout the community or country, let alone in 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 exempt. Without uniform distribution, there is no immunity of the population, even if enough people have been vaccinated to reach the required threshold.
Covid-19 will become like the flu or a seasonal virus
In these cases, the average R may be low enough for the infection to be under control, but in unprotected areas it will be well above 1. This leads to localized outbreaks and allows the disease to remain endemic. Covid-19 thus continues to spread from one place to another, sown in several locations where population density and interaction are high enough and protection low enough to support transmission.
How we treat Covid-19 once it becomes endemic will depend on how good our vaccines and treatments are. If I can protect people from the worst results, the infection will become manageable. Covid-19 will then become like many other diseases that we have learned to live with and that many people will experience during their lifetime.
Depending on whether the immunity – either natural infection or vaccination – is permanent or temporary, we may need annual vaccine updates to protect us (such as the flu). Or it can 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-term immunity, we can think of other scenarios, such as potential eradication of the disease. But realistically, 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.