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The evolution of the Covid-19 epidemic it is an interaction between infectiousness, collective immunity, and protection against infection. This interaction must be understood in order to assess the situation of the epidemic today and its possible future course.
infectivity in a population it is generally described with the basal reproduction number (Rast). This initial value is said to be around 2.5: each infected individual infects another 2-3. Of course, on an individual level, infectivity varies depending on the amount of virus and the image of symptoms (eg cough). But if a symptom-free person moves into society, many small doses of infection can also affect the spread of infection. The R₀ value will then be an average result at the population level. But R₀ can also vary widely in different communities and, for example, be high in densely populated places with many social contacts and low in sparsely populated areas. In the Stockholm area, the R₀ 2.5 value looked good at an early stage. Since epidemics are likely to take off first in areas with high R₀ values, 2.5 is probably not representative of all of Sweden. In an area with half of the social interaction, the R₀ value (1.25) is halved, that is, only a little more than one person is infected by an infected person and the epidemic increases much more slowly.
