The“Herd immunity” occurs when there are enough people immune to a virus in a particular population that the virus cannot spread exponentially. The way to calculate it is very simple, but let’s say that it is the same for this number as it is for others: it has its uses, but we must also understand its limitations.

Each infectious disease spreads at its own “rate” – some germs are highly contagious, others are less infectious. Epidemiologists measure it with an indicator they call poetry R0, or “basic reproductive rate”. This is how many people a patient infects, on average, in a society where no one has immunity and no action is taken to slow the spread of the disease. For example, the R0 for measles, a highly contagious disease, is around 15 years old, which means that every person who gets measles will transmit it to an average of 15 other people (again, if no one has antibodies and if not Anything). Influenza, by comparison, is significantly less transmissible, with an R0 of about 1.5.

The famous “threshold” for herd immunity is the percentage of people who must be vaccinated for R to fall below 1 – meaning the disease loses ground naturally. For a contagious virus such as measles, nearly everyone needs a vaccination to get it. The mathematical formula for calculating it is: 1 – (1 / R0), so for measles, we get: 1 – (1/15) = 14/15, or 93%. For influenza, it is clearly much lower: 1 – (1 / 1.5) = 33%.

What about Covid-19? “It has changed over time, because of the variables,” explains Dr. Alex Karenian, an infectious disease specialist and researcher at the University of Sherbrooke. Before the variants, we worked with an R0 of about 2.5, giving a group immunity threshold of 60%. [ndlr : 1 – (1 / 2,5) = 0,6, donc 60 %], But the variants that are circulating now are much more contagious, with an R0 of around 4. That is why we are now talking about over 75%. [ndlr : 1 – (1 / 4) = 0,75]. »

However, Dr. Karenian cautions that this is still an indicator that “is still very theoretical and very simple. It gives a general idea and helps define goals, but of course there are a lot of things that are not taken into consideration.”

For example, this threshold is basically just R0, or COVID-19 transmission, regardless of its severity. However, vaccines do not appear to be as effective at preventing infection as they are in preventing disease and complications. Some work indicates an overall effectiveness of up to 90% in stopping the disease but perhaps only 55-80% in its transmission. According to the U.S. Public Health (CDC) website. If we only look at transmission, we will lead to the conclusion that the number of people who have yet to be vaccinated has yet to increase. Except that if disease does not cause more hospitalizations and deaths, it may not be as desirable as before to reduce R0 to less than 1, because the consequences are now lower – but his ‘threshold’ alone says nothing.

Also, this number assumes that social connections are the same for everyone, and this is clearly not the case. Young people, for example, have a lot more social contact than older people. From one culture to another, the generations do not have the same degree of interaction with each other. The economic fabric of a country can influence the amount and type of contact between people in the workplace – for example, factory work has been known to be linked to multiple COVID-19 outbreaks across the country. Around the world, however, some economies are more industrial than others. And so on.

In 2017, three researchers from Singapore and the United Kingdom Published in the scientific journal PLoS – Computational Biology A model that takes into account many of these factors, in order to get an idea of ​​what “infection networks” might look like in 152 countries. And in 2020, other researchers applied it to COVID-19, giving them R0s that vary widely from country to country. In some places, the normal rate of reproduction was around 1 while in other places it could exceed 4.

This demonstrates, says Dr. Karenian, that even if it does have its uses, the “herd immunity threshold” remains a somewhat rudimentary tool.