Basic reproduction number

Each infectious disease has a characteristic basic reproduction number R0. The basic reproduction number R0 is a measure of the infectivity of the disease. In other words: how many unprotected people does a diseased person infect in a population where there is no immunity against this disease ? In short, the higher this number, the more contagious the disease. For a disease to spread, R0 of a disease must be > 1. Otherwise, the disease would stop on its own. The herd immunity threshold (H) can be calculated from the "basic reproduction number". The herd immunity threshold is the number of people who must be immune to a disease for herd immunity to occur. It is: H = 1 - 1/R0 (Fine 1993). Examples of these values are:

• Diphtheria/R0=6-7/herd immunity:85%.
• Masern/R0=12-18/Herdimmunität:83-94%
• Mumps/R0=4-7/Herdimmunität:75-86%
• Chickenpox/R05-7/ hearth immunity:80-85%

For realistic considerations it is important to keep in mind that the actual infectivity is always lower, because in each population a smaller or larger proportion is already immune to each disease, e.g.

by having been through a disease, i.e. naturally acquired immunity (IN),

through vaccination (IV).

This proportion must be taken into account when calculating the reproduction number and then results in the effective reproduction number R, which is usually smaller than R0.