What does it mean for a virus to be endemic?
It means that the virus will continue to circulate in the population continuously. Exactly the same thing that happens with the flu virus.
Why was it thought at the beginning of the pandemic that it could be contained?
Because we have managed to contain other viruses. Since the mid-1970s there have been occasional outbreaks of Ebola, but they have always been contained and this pathogen does not circulate continuously among human populations (although it does remain circulating in its animal reservoirs).
The same has happened with the two great coronavirus epidemics that we have suffered during the last decades. The one baptized as SARS emerged in November 2002 and killed 770 people, but no case has been recorded again.
The coronavirus that causes MERS has also caused limited outbreaks that have killed 800 lives since 2012, but the virus does not circulate continuously among humans (although it does circulate among camels).
Why have we been able to control other coronaviruses and not SARS-CoV-2?
Because the virus that causes covid has turned out to be much more contagious than the coronaviruses that cause SARS and MERS.
In particular, SARS-CoV-2 has a characteristic that not all viruses have: it is transmitted from one human to another even during the asymptomatic phase. That is why it is practically impossible to eradicate by many social distancing measures. There will always be someone spreading that, unknowingly, infects others.
Are there other coronaviruses that are endemic?
Yes, we know of 4 coronaviruses that circulate endemically in human populations. Those viruses are called 229E, HKU1, NL63, and OC43.
All of us have been infected with these viruses at some time. They cause very mild infections, most of the time unnoticeable.
What other coronaviruses teach us about the possible future of SARS-CoV-2
The 4 coronaviruses that we have presented in the previous section could perhaps serve as a guide on a possible future of SARS-CoV-2 as an endemic virus.
Scientists who have measured long-term antibodies (called IgG) and short-term antibodies (called IgM) against the 4 endemic coronaviruses find the following distribution:
This means that, on average, the first infection we have with these viruses occurs before the age of 5 years and practically the entire population has been infected before the age of 15.
Once we have been infected as children, the presence of IgG antibodies is maintained throughout life.
This does not mean that we do not become infected again (in fact, reinfections are registered in adults), but it does mean that our immune system has mechanisms to recognize the virus.
The good news: mortality from covid among children is extremely low
Suppose that the virus that causes covid follows the pattern of the other 4 endemic coronaviruses that already exist.
In that case, once the initial pandemic is over, the virus would settle in what is called a ‘steady state’. In that case, the first infections would occur from children, as we have seen in the other 4 coronaviruses.
And here comes the good news: mortality from the COVID coronavirus is very low among young humans, unlike the MERS coronavirus.
In the following graph you have the probability of dying from an infection with different coronaviruses according to age:
Special of Public, from Spain, for Page 12