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Suddenly the word is collective immunity on everyone’s lips. That is, what proportion of a population must be infected (or vaccinated), so that the spread of infection begins to decrease. A problem that is on fire, especially in the Stockholm region, where the spread of infection in Sweden has gone further.
But what is meant by “enough”?
According to the most basic textbooks, this proportion is closely related to infectiousness. The higher the infectivity, the higher the proportion that must be immune before the infection spreads.
It has been estimated that the spread of the new coronavirus ranges from 2 to 3. That is, each person infected by the infection spreads between two and three people. In normal cases and if infectivity, the so-called reproduction number, is assumed to be 2.5, the proportion that must be immune before the infection spreads sounds 60%.
But this weekend, preliminary calculations by mathematician Tom Britton and colleagues at Stockholm University and the University of Nottingham showed that this ratio is likely much lower, perhaps only 40-45 percent.
The essence of the calculations is that the population is not a homogeneous group that lives and socializes in a similar way. For example, some are more social and more at risk of infection than others. In this way, immunity will not spread evenly, but those who are more motivated, the most social, are already immune.
The effect can be compared to targeted vaccines that are sometimes carried out to reduce a certain type of spread of infection. Although relatively few get vaccinated, the spread of infection can be stopped as they are at greatest risk of passing the infection on to those who get vaccinated and become immune.
– In our model, we divide people into three groups depending on how social they are, where we assume that those who are more social run the highest risk of becoming infected. In this way, it can be shown that there is a substantially smaller proportion that must be immune before a collective immunity occurs, than if they all interact with each other in the same way. We don’t know exactly how much lower it will be, but it’s not just a small percentage. That’s it, says Tom Britton, professor of mathematical statistics at Stockholm University.
Also, Tom Britton says, there are probably other differences in a population that should further amplify the effect.
– For example, it is conceivable that those who live in multi-person households are at greater risk of becoming infected, compared to those who live in individual households.
TT: Why has no one counted this way before?
“Probably because there was not a similar situation before, but regardless of us, a group in Liverpool had a similar result,” says Tom Britton.
How high is the R0 value?
Prominent epidemiologist Marc Lipsitch of Harvard University in the United States praises the two studies on Twitter, but at the same time generates a warning. The 40-45 percent ratio applies only if people don’t change their social behavior over time and if the infectivity of the virus is really 2.5, a number about which there is great uncertainty.
Recently, a US research group showed that the so-called R0 value can actually be 4-5, or even higher. In other words, the coronavirus is significantly more contagious than we have assumed so far.
– We do not say that the value is 2.5. We are just saying that herd immunity occurs earlier than previously thought. We do not know what the R0 value is for this virus. Plus, it’s probably a little different in different parts of the world, says Tom Britton.
