Mohamed Ibrahim Bassyouni
2020 / 5 / 3
The coronavirus passed very quickly around the world because none of us had prior immunity from it. Do it and its spread will lead to huge loss of life. However, some politicians and epidemiologists advise that the most practical course is to manage infection while permitting so-called herd immunity.
The concept of herd immunity is usually described in the context of the vaccine. When enough people are vaccinated, a disease cannot spread easily among the population. If you have measles but everyone has been vaccinated, the transmission of the disease will be stopped in its tracks.
Vaccination levels should remain dependent on the transmissibility of the pathogen, the virus. We do not yet know exactly the extent of coronavirus transmission, but we say that on average each person infects three others. This means that approximately two thirds of the population will need immunity to grant herd immunity.
In the absence of a vaccine, developing an immunity against a disease such as Covid-19 actually requires coronavirus infection. For this to work, the previous infection must have lifelong immunity to future infection. Despite this hope, scientists are not yet sure that this is the case, and they do not know how long this immunity will last. The virus was discovered only a few months ago.
But even assuming that immunity is long-term, too many people must be infected to reach the required immunity threshold of herd. Given that current estimates indicate that between 0.5% to 1% of all injuries are fatal in most countries of the world, this means many deaths.
Perhaps the most important thing to understand is that the virus does not magically disappear when the herd immunity threshold is reached. This is not when things stop - only when you start to slow down only.
Once enough immunity is built in the population, each person will have less than another person, so the new epidemic cannot start again. But the epidemic already underway will continue to spread. If 100,000 people are infectious at their peak and each affects 0.9 people, that s still 90,000 new infections, and more after that. A runaway train does not stop the moment when the path begins to descend upwards, and the rapidly spreading virus does not stop when the herd immunity is reached.
If the epidemic goes unchecked, it may last for months after reaching the herd immunity, affecting millions of others in the process.
By the time the epidemic ends, a very large proportion of the population will become infected - well above the expected herd immunity threshold of about two-thirds. These additional infections are what epidemiologists refer to as "overtaking".
Herd immunity does not stop the virus in its tracks. Rather, the number of infections continues to rise after the herd immunity is reached.