[fi358]

I think this based on the assumption that R0 is about 2.3. That means that on average, one infected person infects 2.3 other persons. Then you need to have about 60 percent of population to get immunity against the virus by first being infected with it. At that point you reach the threshold for herd immunity and the disease will soon stop spreading.

However, R0 is not a constant. For example, with good hygiene, avoiding human contacts etc it is possible to lower the rate of average number of persons each sick person infects. If we manage to do that, the threshold for herd immunity will be lower than 60 %.